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 Psychologyportalvt
Social psychology is the scientific study of how people's thoughts, feelings and behaviors are influenced by the actual, imagined or implied presence of others.[1] In this definition, scientific refers to the empirical investigation using the scientific method. The terms thoughts, feelings and behavior refer to psychological variables that can be measured in humans. The statement that others' presence may be imagined or implied suggests that humans are malleable to social influences even when alone, such as when watching television or following internalized cultural norms. Social psychologists typically explain human behavior as a result of the interaction of mental states and social situations.
Social psychologists examine factors that cause behaviors to unfold in a given way in the presence of others. They study conditions under which certain behavior, actions, and feelings occur. Social psychology is concerned with the way these feelings, thoughts, beliefs, intentions, and goals are cognitively constructed and how these mental representations, in turn, influence our interactions with others.
Social psychology traditionally bridged the gap between psychology and sociology. During the years immediately following World War II there was frequent collaboration between psychologists and sociologists.[2] The two disciplines, however, have become increasingly specialized and isolated from each other in recent years, with sociologists focusing on "macro variables" (e.g., social structure) to a much greater extent than psychologists. Nevertheless, sociological approaches to psychology remain an important counterpart to psychological research in this area.
In addition to the split between psychology and sociology, there has been a somewhat less pronounced difference in emphasis between American social psychologists and European social psychologists. As a generalization, American researchers traditionally have focused more on the individual, whereas Europeans have paid more attention to group level phenomena (see group dynamics).[3]
History
Although there were some older writings about social psychology, such as those by Islamic philosopher Al-Farabi (Alpharabius),[4] the discipline of social psychology, as its modern-day definition, began in the United States at the beginning of the 20th century. By that time, though, the discipline had already developed a significant foundation. Following the 18th century, those in the emerging field of social psychology were concerned with developing concrete explanations for different aspects of human nature. They attempted to discover concrete cause and effect relationships that explained the social interactions in the world around them. In order to do so, they believed that the scientific method, an empirically based scientific measure, could be applied to human behavior.[5]
The first published study in this area was an experiment in 1898 by Norman Triplett, on the phenomenon of social facilitation.[6] During the 1930s, many Gestalt psychologists, most notably Kurt Lewin, fled to the United States from Nazi Germany. They were instrumental in developing the field as something separate from the behavioral and psychoanalytic schools that were dominant during that time, and social psychology has always maintained the legacy of their interests in perception and cognition. Attitudes and small group phenomena were the most commonly studied topics in this era.
During World War II, social psychologists studied persuasion and propaganda for the U.S. military. After the war, researchers became interested in a variety of social problems, including gender issues and racial prejudice.  Most notable, revealing, and contentious of these were the Stanley Milgram shock experiments on obedience to authority. In the sixties, there was growing interest in new topics, such as cognitive dissonance, bystander intervention, and aggression. By the 1970s, however, social psychology in America had reached a crisis. There was heated debate over the ethics of laboratory experimentation, whether or not attitudes really predicted behavior, and how much science could be done in a cultural context.[7] This was also the time when a radical situationist approach challenged the relevance of self and personality in psychology.[8]
Throughout the 1980s and 1990s social psychology reached a more mature level. Two of the areas social psychology matured in were theories and methods.[8] Careful ethical standards now regulate research. Pluralistic and multicultural perspectives have emerged. Modern researchers are interested in many phenomena, but attribution, social cognition, and the self-concept are perhaps the greatest areas of growth in recent years. Social psychologists have also maintained their applied interests with contributions in the social psychology of health, education, law, and the workplace.[9]
Intrapersonal phenomena
Attitudes

In social psychology, attitudes are defined as learned, global evaluations of a person, object, place, or issue that influence thought and action.[10] Put more simply, attitudes are basic expressions of approval or disapproval, favorability or unfavorability, or as Bem put it, likes and dislikes.[11] Examples would include liking chocolate ice cream, or endorsing the values of a particular political party.
Social psychologists have studied attitude formation, the structure of attitudes, attitude change, the function of attitudes, and the relationship between attitudes and behavior. Because people are influenced by the situation, general attitudes are not always good predictors of specific behavior. For example, for a variety of reasons, a person may value the environment but not recycle a can on a particular day.
In recent times, research on attitudes has examined the distinction between traditional, self-reported attitude measures and "implicit" or unconscious attitudes. For example, experiments[12] using the Implicit Association Test have found that people often demonstrate implicit bias against other races, even when their explicit responses reveal equal mindedness. One study found that explicit attitudes correlate with verbal behavior in interracial interactions, whereas implicit attitudes correlate with nonverbal behavior.[13]
One hypothesis on how attitudes are formed, first advanced by Abraham Tesser in 1983, is that strong likes and dislikes are ingrained in our genetic make-up. Tesser speculates that individuals are disposed to hold certain strong attitudes as a result of inborn physical, sensory, and cognitive skills, temperament, and personality traits. Whatever disposition nature elects to give us, our most treasured attitudes are often formed as a result of exposure to attitude objects; our history of rewards and punishments; the attitude that our parents, friends, and enemies express; the social and cultural context in which we live; and other types of experiences we have. Obviously, attitudes are formed through the basic process of learning. Numerous studies have shown that people can form strong positive and negative attitudes toward neutral objects that are in some way linked to emotionally charged stimuli.[14]:185–186
Attitudes are also involved in several other areas of the discipline, such as conformity, interpersonal attraction, social perception, and prejudice.
Persuasion

The topic of persuasion has received a great deal of attention in recent years. Persuasion is an active method of influence that attempts to guide people toward the adoption of an attitude, idea, or behavior by rational or emotive means. Persuasion relies on "appeals" rather than strong pressure or coercion. Numerous variables have been found to influence the persuasion process; these are normally presented in five major categories: who said what to whom and how.[15]
The Communicator, including credibility, expertise, trustworthiness, and attractiveness.
The Message, including varying degrees of reason, emotion (such as fear), one-sided or two sided arguments, and other types of informational content.
The Audience, including a variety of demographics, personality traits, and preferences.
The Channel or Medium, including the printed word, radio, television, the internet, or face-to-face interactions.
The Context, including the environment, group dynamics, and preamble to the message.
Dual-process theories of persuasion (such as the elaboration likelihood model) maintain that the persuasive process is mediated by two separate routes; central and peripheral. The central route of persuasion is more fact-based and results in longer lasting change, but requires motivation to process. The peripheral route is more superficial and results in shorter lasting change, but does not require as much motivation to process. An example of a peripheral route of persuasion might be a politician using a flag lapel pin, smiling, and wearing a crisp, clean shirt. Notice that this does not require motivation to be persuasive, but should not last as long as persuasion based on the central route. If that politician were to outline exactly what they believed, and their previous voting record, this would be using the central route, and would result in longer lasting change, but would require a good deal of motivation to process.
Social cognition

Social cognition is a growing area of social psychology that studies how people perceive, think about, and remember information about others. Much research rests on the assertion that people think about (other) people differently from non-social targets.[16]  This assertion is supported by the social cognitive deficits exhibited by people with Williams syndrome and autism.[17] Person perception is the study of how people form impressions of others. The study of how people form beliefs about each other while interacting is known as interpersonal perception.
A major research topic in social cognition is attribution.[18] Attributions are the explanations we make for people's behavior, either our own behavior or the behavior of others. One element of attribution ascribes the locus of a behavior to either internal or external factors. An internal, or dispositional, attribution assigns behavior to causes related to inner traits such as personality, disposition, character or ability. An external, or situational, attribution involves situational elements, such as the weather.[19]:111  A second element of attribution ascribes the cause of behavior to either stable or unstable factors (whether the behavior will be repeated or changed under similar circumstances). Finally, we also attribute causes of behavior to either controllable or uncontrollable factors: how much control one has over the situation at hand.
Numerous biases in the attribution process have been discovered. For instance, the fundamental attribution error is the tendency to make dispositional attributions for behavior, overestimating the influence of personality and underestimating the influence of situations.[20]:724 The actor-observer difference is a refinement of this bias, the tendency to make dispositional attributions for other people's behavior and situational attributions for our own.[19]:107 The self-serving bias is the tendency to attribute dispositional causes for successes, and situational causes for failure, particularly when self-esteem is threatened. This leads to assuming one's successes are from innate traits, and one's failures are due to situations, including other people.[19]:109 Other ways people protect their self-esteem are by believing in a just world, blaming victims for their suffering, and making defensive attributions, which explain our behavior in ways which defend us from feelings of vulnerability and mortality.[19]:111 Researchers have found that mildly depressed individuals often lack this bias and actually have more realistic perceptions of reality (as measured by the opinions of others).[21]
Heuristics are cognitive short cuts. Instead of weighing all the evidence when making a decision, people rely on heuristics to save time and energy. The availability heuristic occurs when people estimate the probability of an outcome based on how easy that outcome is to imagine. As such, vivid or highly memorable possibilities will be perceived as more likely than those that are harder to picture or are difficult to understand, resulting in a corresponding cognitive bias. The representativeness heuristic is a shortcut people use to categorize something based on how similar it is to a prototype they know of.[19]:63 Numerous other biases have been found by social cognition researchers. The hindsight bias is a false memory of having predicted events, or an exaggeration of actual predictions, after becoming aware of the outcome. The confirmation bias is a type of bias leading to the tendency to search for, or interpret information in a way that confirms one's preconceptions.
Another key concept in social cognition is the assumption that reality is too complex to easily discern. As a result, we tend to see the world according to simplified schemas or images of reality. Schemas are generalized mental representations that organize knowledge and guide information processing. Schemas often operate automatically and unintentionally, and can lead to biases in perception and memory. Expectations from schemas may lead us to see something that is not there. One experiment found that people are more likely to misperceive a weapon in the hands of a black man than a white man.[22] This type of schema is actually a stereotype, a generalized set of beliefs about a particular group of people (when incorrect, an ultimate attribution error). Stereotypes are often related to negative or preferential attitudes (prejudice) and behavior (discrimination). Schemas for behaviors (e.g., going to a restaurant, doing laundry) are known as scripts.
Self-concept

Self-concept is a term referring to the whole sum of beliefs that people have about themselves. However, what specifically does self-concept consist of? According to Hazel Markus (1977), the self-concept is made up of cognitive molecules called self-schemas– beliefs that people have about themselves that guide the processing of self-reliant information. For example, an athlete at a university would have multiple selves that would process different information pertinent to each self: the student would be one "self," who would process information pertinent to a student (taking notes in class, completing a homework assignment, etc.); the athlete would be the "self" who processes information about things related to being an athlete (recognizing an incoming pass, aiming a shot, etc.). These "selves" are part of one's identity and the self-reliant information is the information that relies on the proper "self" to process and react on it. If a "self" is not part of one's identity, then it is much more difficult for one to react. For example, a civilian may not know how to handle a hostile threat as a trained Marine would. The Marine contains a "self" that would enable him/her to process the information about the hostile threat and react accordingly, whereas a civilian may not contain that self, disabling them from properly processing the information from the hostile threat and, furthermore, debilitating them from acting accordingly. Self-schemas are to an individual’s total self–concept as a hypothesis is to a theory, or a book is to a library. A good example is the body weight self-schema; people who regard themselves as over or underweight, or for those whom body image is a significant self-concept aspect, are considered schematics with respect to weight. For these people a range of otherwise mundane events– grocery shopping, new clothes, eating out, or going to the beach– can trigger thoughts about the self. In contrast, people who do not regard their weight as an important part of their lives are a-schematic on that attribute.[23]
It is rather clear that the self is a special object of our attention. Whether one is mentally focused on a memory, a conversation, a foul smell, the song that is stuck in one's head, or this sentence, consciousness is like a spotlight. This spotlight can shine on only one object at a time, but it can switch rapidly from one object to another and process the information out of awareness. In this spotlight the self is front and center: things relating to the self have the spotlight more often.
The self's ABCs are affect, behavior, and cognition. An affective (or emotional) question: How do people evaluate themselves, enhance their self-image, and maintain a secure sense of identity? A behavioral question: How do people regulate their own actions and present themselves to others according to interpersonal demands? A cognitive question: How do individuals become themselves, build a self-concept, and uphold a stable sense of identity?[14]:53
Affective forecasting is the process of predicting how one would feel in response to future emotional events. Studies done by Timothy Wilson and Daniel Gilbert in 2003 have shown that people overestimate the strength of reaction to anticipated positive and negative life events that they actually feel when the event does occur.[24]
There are many theories on the perception of our own behavior. Daryl Bem's (1972) self-perception theory claims that when internal cues are difficult to interpret, people gain self-insight by observing their own behavior.[25] Leon Festinger's 1954 social comparison theory is that people evaluate their own abilities and opinions by comparing themselves to others when they are uncertain of their own ability or opinions.[26] There is also the facial feedback hypothesis: that changes in facial expression can lead to corresponding changes in emotion.[14]:56
The fields of social psychology and personality have merged over the years, and social psychologists have developed an interest in self-related phenomena. In contrast with traditional personality theory, however, social psychologists place a greater emphasis on cognitions than on traits. Much research focuses on the self-concept, which is a person's understanding of their self. The self-concept is often divided into a cognitive component, known as the self-schema, and an evaluative component, the self-esteem. The need to maintain a healthy self-esteem is recognized as a central human motivation in the field of social psychology.
Self-efficacy beliefs are associated with the self-schema. These are expectations that performance on some task will be effective and successful. Social psychologists also study such self-related processes as self-control and self-presentation.
People develop their self-concepts by varied means, including introspection, feedback from others, self-perception, and social comparison. By comparing themselves to relevant others, people gain information about themselves, and they make inferences that are relevant to self-esteem. Social comparisons can be either "upward" or "downward," that is, comparisons to people who are either higher in status or ability, or lower in status or ability.[27] Downward comparisons are often made in order to elevate self-esteem.
Self-perception is a specialized form of attribution that involves making inferences about oneself after observing one's own behavior. Psychologists have found that too many extrinsic rewards (e.g. money) tend to reduce intrinsic motivation through the self-perception process, a phenomenon known as overjustification. People's attention is directed to the reward and they lose interest in the task when the reward is no longer offered.[28] This is an important exception to reinforcement theory.
Interpersonal phenomena
Social influence

Social influence is an overarching term given to describe the persuasive effects people have on each other. It is seen as a fundamental value in social psychology and overlaps considerably with research on attitudes and persuasion. The three main areas of social influence include: conformity, compliance, and obedience. Social influence is also closely related to the study of group dynamics, as most principles of influence are strongest when they take place in social groups.
The first major area of social influence is conformity. Conformity is defined as the tendency to act or think like other members of a group. The identity of members within a group, i.e. status, similarity, expertise, as well as cohesion, prior commitment, and accountability to the group help to determine the level of conformity of an individual. Individual variation among group members plays a key role in the dynamic of how willing people will be to conform.[29]:27 Conformity is usually viewed as a negative tendency in American culture, but a certain amount of conformity is adaptive in some situations, as is nonconformity in other situations.[29]:15

The second major area of social influence research is compliance. Compliance refers to any change in behavior that is due to a request or suggestion from another person. The foot-in-the-door technique is a compliance method in which the persuader requests a small favor and then follows up with requesting a larger favor, e.g., asking for the time and then asking for ten dollars. A related trick is the bait and switch.[30]
The third major form of social influence is obedience; this is a change in behavior that is the result of a direct order or command from another person. Obedience as a form of compliance was dramatically highlighted by the Milgram study, wherein people were ready to administer shocks to a person in distress on a researcher's command.[29]:41
An unusual kind of social influence is the self-fulfilling prophecy. This is a prediction that, in being made, actually causes itself to become true. For example, in the stock market, if it is widely believed that a crash is imminent, investors may lose confidence, sell most of their stock, and thus actually cause the crash. Similarly, people may expect hostility in others and actually induce this hostility by their own behavior.[19]:18
Group dynamics

A group can be defined as two or more individuals that are connected to each another by social relationships.[31] Groups tend to interact, influence each other, and share a common identity. They have a number of emergent qualities that distinguish them from aggregates:
Norms: Implicit rules and expectations for group members to follow, e.g. saying thank you, shaking hands.
Roles: Implicit rules and expectations for specific members within the group, e.g. the oldest sibling, who may have additional responsibilities in the family.
Relations: Patterns of liking within the group, and also differences in prestige or status, e.g., leaders, popular people.
Temporary groups and aggregates share few or none of these features, and do not qualify as true social groups. People waiting in line to get on a bus, for example, do not constitute a group.
Groups are important not only because they offer social support, resources, and a feeling of belonging, but because they supplement an individual's self-concept. To a large extent, humans define themselves by the group memberships which form their social identity. The shared social identity of individuals within a group influences intergroup behavior, the way in which groups behave towards and perceive each other. These perceptions and behaviors in turn define the social identity of individuals within the interacting groups. The tendency to define oneself by membership in a group may lead to intergroup discrimination, which involves favorable perceptions and behaviors directed towards the in-group, but negative perceptions and behaviors directed towards the out-group.[32] On the other hand, such discrimination and segregation may sometimes exist partly to facilitate a diversity which strengthens society.[33] Intergroup discrimination leads to prejudice and stereotyping, while the processes of social facilitation and group polarization encourage extreme behaviors towards the out-group.
Groups often moderate and improve decision making, and are frequently relied upon for these benefits, such as in committees and juries. A number of group biases, however, can interfere with effective decision making. For example, group polarization, formerly known as the "risky shift," occurs when people polarize their views in a more extreme direction after group discussion. More problematic is the phenomenon of groupthink. This is a collective thinking defect that is characterized by a premature consensus or an incorrect assumption of consensus, caused by members of a group failing to promote views which are not consistent with the views of other members. Groupthink occurs in a variety of situations, including isolation of a group and the presence of a highly directive leader. Janis offered the 1961 Bay of Pigs Invasion as a historical case of groupthink.[34]
Groups also affect performance and productivity. Social facilitation, for example, is a tendency to work harder and faster in the presence of others. Social facilitation increases the dominant response's likelihood, which tends to improve performance on simple tasks and reduce it on complex tasks. In contrast, social loafing is the tendency of individuals to slack off when working in a group. Social loafing is common when the task is considered unimportant and individual contributions are not easy to see.[35]
Social psychologists study group-related (collective) phenomena such as the behavior of crowds. An important concept in this area is deindividuation, a reduced state of self-awareness that can be caused by feelings of anonymity. Deindividuation is associated with uninhibited and sometimes dangerous behavior. It is common in crowds and mobs, but it can also be caused by a disguise, a uniform, alcohol, dark environments, or online anonymity.[36][37]

Interpersonal attraction

A major area in the study of people's relations to each other is interpersonal attraction. This refers to all forces that lead people to like each other, establish relationships, and (in some cases) fall in love. Several general principles of attraction have been discovered by social psychologists, but many still continue to experiment and do research to find out more.  One of the most important factors in interpersonal attraction is how similar two particular people are.  The more similar two people are in general attitudes, backgrounds, environments, worldviews, and other traits, the more probable an attraction is possible.[38]
Physical attractiveness is an important element of romantic relationships, particularly in the early stages characterized by high levels of passion. Later on, similarity and other compatibility factors become more important, and the type of love people experience shifts from passionate to companionate. Robert Sternberg has suggested that there are actually three components of love: intimacy, passion, and commitment.[39] When two (or more) people experience all three, they are said to be in a state of consummate love.
According to social exchange theory, relationships are based on rational choice and cost-benefit analysis. If one partner's costs begin to outweigh their benefits, that person may leave the relationship, especially if there are good alternatives available. This theory is similar to the minimax principle proposed by mathematicians and economists (despite the fact that human relationships are not zero-sum games). With time, long term relationships tend to become communal rather than simply based on exchange.
Research
Methods
Social psychology is an empirical science that attempts to answer questions about human behavior by testing hypotheses, both in the laboratory and in the field. Careful attention to sampling, research design, and statistical analysis is important; results are published in peer reviewed journals such as the Journal of Experimental Social Psychology, Personality and Social Psychology Bulletin and the Journal of Personality and Social Psychology. Social psychology studies also appear in general science journals such as Psychological Science and Science.
Experimental methods involve the researcher altering a variable in the environment and measuring the effect on another variable. An example would be allowing two groups of children to play violent or nonviolent videogames, and then observing their subsequent level of aggression during free-play period. A valid experiment is controlled and uses random assignment.
Correlational methods examine the statistical association between two naturally occurring variables. For example, one could correlate the amount of violent television children watch at home with the number of violent incidents the children participate in at school. Note that this study would not prove that violent TV causes aggression in children: it is quite possible that aggressive children choose to watch more violent TV.
Observational methods are purely descriptive and include naturalistic observation, "contrived" observation, participant observation, and archival analysis. These are less common in social psychology but are sometimes used when first investigating a phenomenon. An example would be to unobtrusively observe children on a playground (with a videocamera, perhaps) and record the number and types of aggressive actions displayed.
Whenever possible, social psychologists rely on controlled experimentation. Controlled experiments require the manipulation of one or more independent variables in order to examine the effect on a dependent variable. Experiments are useful in social psychology because they are high in internal validity, meaning that they are free from the influence of confounding or extraneous variables, and so are more likely to accurately indicate a causal relationship. However, the small samples used in controlled experiments are typically low in external validity, or the degree to which the results can be generalized to the larger population. There is usually a trade-off between experimental control (internal validity) and being able to generalize to the population (external validity).
Because it is usually impossible to test everyone, research tends to be conducted on a sample of persons from the wider population. Social psychologists frequently use survey research when they are interested in results that are high in external validity. Surveys use various forms of random sampling to obtain a sample of respondents that are representative of a population. This type of research is usually descriptive or correlational because there is no experimental control over variables. However, new statistical methods like structural equation modeling are being used to test for potential causal relationships in this type of data. Some psychologists, including Dr. David O. Sears, have criticized social psychological research for relying too heavily on studies conducted on university undergraduates in academic settings. Over 70% of experiments in Sears' study used North American undergraduates as subjects, a subset of the population that may not be representative of the population as a whole.[40]
Regardless of which method has been chosen to be used, the results are of high importance. Results need to be used to evaluate the hypothesis of the research that is done. These results should either confirm or reject the original hypothesis that was predicted.There are two different types of testing social psychologists use in order to test their results. Statistics and probability testing define a significant finding that can be as low as 5% or less, likely to be due to chance.[41] Replications are important, to ensure that the result is valid and not due to chance, or some feature of a particular sample. False positive conclusions, often resulting from the pressure to publish or the author's own confirmation bias, are a hazard in the field.[42]
Ethics
The goal of social psychology is to understand cognition and behavior as they naturally occur in a social context, but the very act of observing people can influence and alter their behavior. For this reason, many social psychology experiments utilize deception to conceal or distort certain aspects of the study. Deception may include false cover stories, false participants (known as confederates or stooges), false feedback given to the participants, and so on.
The practice of deception has been challenged by some psychologists who maintain that deception under any circumstances is unethical, and that other research strategies (e.g., role-playing) should be used instead. Unfortunately, research has shown that role-playing studies do not produce the same results as deception studies and this has cast doubt on their validity. In addition to deception, experimenters have at times put people into potentially uncomfortable or embarrassing situations (e.g., the Milgram experiment and Stanford prison experiment), and this has also been criticized for ethical reasons.
To protect the rights and well-being of research participants, and at the same time discover meaningful results and insights into human behavior, virtually all social psychology research must pass an ethical review process. At most colleges and universities, this is conducted by an ethics committee or Institutional Review Board. This group examines the proposed research to make sure that no harm is likely to be done to the participants, and that the study's benefits outweigh any possible risks or discomforts to people taking part in the study.
Furthermore, a process of informed consent is often used to make sure that volunteers know what will happen in the experiment and understand that they are allowed to quit the experiment at any time. A debriefing is typically done at the experiment's conclusion in order to reveal any deceptions used and generally make sure that the participants are unharmed by the procedures. Today, most research in social psychology involves no more risk of harm than can be expected from routine psychological testing or normal daily activities.[43]
Replication crisis

Social psychology has recently found itself at the center of a "replication crisis" due to some research findings proving difficult to replicate.  Replication failures are not unique to social psychology and are found in all fields of science.  However, several factors have combined to put social psychology at the center of the current controversy.
Firstly, questionable research practices (QRP) have been identified as common in the field.  Such practices, while not necessarily intentionally fraudulent, involve converting undesired statistical outcomes into desired outcomes via the manipulation of statistical analyses, sample size or data management, typically to convert non-significant findings into significant ones.[42]  Some studies have suggested that at least mild versions of QRP are highly prevalent.[44] One of the critics of Daryl Bem in the feeling the future controversy has suggested that the evidence for precognition in this study could (at least in part) be attributed to QRP.
Secondly, social psychology has found itself at the center of several recent scandals involving outright fraudulent research.  Most notably the admitted data fabrication by Diederik Stapel[45] as well as allegations against others.  However, most scholars acknowledge that fraud is, perhaps, the lesser contribution to replication crises.
Third, several effects in social psychology have been found to be difficult to replicate even before the current replication crisis. For example, the scientific journal Judgment and Decision Making has published several studies over the years that fail to provide support for the unconscious thought theory. Replications appear particularly difficult when research trials are pre-registered and conducted by research groups not highly invested in the theory under questioning.
These three elements together have resulted in renewed attention for replication supported by Daniel Kahneman. Scrutiny of many effects have shown that several core beliefs are hard to replicate. A recent special edition of the journal Social Psychology focused on replication studies and a number of previously held beliefs were found to be difficult to replicate.[46]  A 2012 special edition of the journal Perspectives on Psychological Science also focused on issues ranging from publication bias to null-aversion that contribute to the replication crises in psychology[47]
It is important to note that this replication crisis does not mean that social psychology is unscientific. Rather this process is a healthy if sometimes acrimonious part of the scientific process in which old ideas or those that cannot withstand careful scrutiny are pruned.[48]  The consequence is that some areas of social psychology once considered solid, such as social priming, have come under increased scrutiny due to failed replications[49]
Famous experiments

The Asch conformity experiments demonstrated the power of conformity in small groups with a line length estimation task that was designed to be extremely easy.[50] In well over a third of the trials, participants conformed to the majority, who had been instructed to provide incorrect answers, even though the majority judgment was clearly wrong. Seventy-five percent of the participants conformed at least once during the experiment.  Additional manipulations to the experiment showed participant conformity decreased when at least one other individual failed to conform, but increased when the individual began conforming or withdrew from the experiment.[50] Also, participant conformity increased substantially as the number of incorrect individuals increased from one to three, and remained high as the incorrect majority grew.  Participants with three incorrect opponents made mistakes 31.8% of the time, while those with one or two incorrect opponents made mistakes only 3.6% and 13.6% of the time, respectively.[50]
Muzafer Sherif's  Robbers' Cave Experiment divided boys into two competing groups to explore how much hostility and aggression would emerge. Sherif's explanation of the results became known as realistic group conflict theory, because the intergroup conflict was induced through competition over resources.[51] Inducing cooperation and superordinate goals later reversed this effect.
In Leon Festinger's cognitive dissonance experiment, participants were asked to perform a boring task. They were divided into 2 groups and given two different pay scales. At the study's end, some participants were paid $1 to say that they enjoyed the task and another group of participants was paid $20 to say the same lie. The first group ($1) later reported liking the task better than the second group ($20). Festinger's explanation was that for people in the first group being paid only $1 is not sufficient incentive for lying and those who were paid $1 experienced dissonance. They could only overcome that dissonance by justifying their lies by changing their previously unfavorable attitudes about the task. Being paid $20 provides a reason for doing the boring task, therefore no dissonance.[52][53]
One of the most notable experiments in social psychology was the Milgram experiment, which studied how far people would go to obey an authority figure. Following the events of The Holocaust in World War II, the experiment showed that (most) normal American citizens were capable of following orders from an authority even when they believed they were causing an innocent person to suffer.[54]
Albert Bandura's Bobo doll experiment demonstrated how aggression is learned by imitation.[55] This set of studies fueled debates regarding media violence which continue to be waged among scholars.
In the Stanford prison study, by Philip Zimbardo, a simulated exercise between student prisoners and guards showed how far people would follow an adopted role. In just a few days, the "guards" became brutal and cruel, and the prisoners became miserable and compliant. This was initially argued to be an important demonstration of the power of the immediate social situation and its capacity to overwhelm normal personality traits.[56] However, to this day, it remains a matter of contention what conclusions may be drawn from this study. For example, it has been pointed out that participant self-selection may have affected the participants' behaviour,[57] and that the participants' personality influenced their reactions in a variety of ways, including how long they chose to remain in the study. One of the most concerted empirical revisitations of the themes raised by Zimbardo came with the 2002 BBC prison study.[58]
Academic journals

Asian Journal of Social Psychology
Basic and Applied Social Psychology
British Journal of Social Psychology
European Journal of Social Psychology
Journal of Applied Social Psychology
Journal of Experimental Social Psychology
Journal of Personality and Social Psychology
Journal of Social Psychology
Personality and Social Psychology Bulletin
Personality and Social Psychology Review
Social Psychology

See also

Association of Psychological and Social Studies
Crowd psychology
Intergroup relations
European Association of Social Psychology
Fuzzy-trace theory
List of biases in judgment and decision making
List of publications in social psychology
List of social psychologists
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A plough (UK) or plow (US; both /plaʊ/) is a tool or farm implement used in farming for initial cultivation of soil in preparation for sowing seed or planting to loosen or turn the soil.[1] Ploughs were traditionally drawn by working animals such as oxen and horses, but in modern times are mostly drawn by tractors. A plough may be made of wood, iron, or steel frame with an attached blade or stick used to cut the soil and loosen it. It has been a basic instrument for most of recorded history, although despite archeological evidence for its use[2] written references to the plough do not appear in the English language before c. 1100, after which point it is referenced frequently. The plough represents one of the major agricultural inventions in human history. The earliest ploughs were wheelless, and the Romans used a wheelless plough called the aratrum, but Celtic peoples began using wheeled ploughs during the Roman era.[3]
The primary purpose of ploughing is to turn over the upper layer of the soil,[4] bringing fresh nutrients to the surface,[5] while burying weeds and the remains of previous crops and allowing them to break down. As the plough is drawn through the soil it creates long trenches of fertile soil called furrows. In modern use, a ploughed field is typically left to dry out, and is then harrowed before planting. Ploughing and cultivating a soil homogenises and modifies the upper 12 to 25 centimetres (5 to 10in) of the soil to form a plough layer. In many soils, the majority of fine plant feeder roots can be found in the topsoil or plough layer.
Ploughs were initially human-powered, but the process became considerably more efficient once animals were pressed into service. The first animal-powered ploughs were undoubtedly pulled by oxen, and later in many areas by horses (generally draft horses) and mules, although various other animals have been used for this purpose. In industrialised countries, the first mechanical means of pulling a plough were steam-powered (ploughing engines or steam tractors), but these were gradually superseded by internal-combustion-powered tractors.
Modern competitions take place for ploughing enthusiasts like the National Ploughing Championships in Ireland. Use of the plough has decreased in some areas, often those significantly threatened by soil damage and erosion, in favour of shallower ploughing and other less-invasive conservation tillage techniques.
Etymology

In older English, as in other Germanic languages, the plough was traditionally known by other names, e.g. Old English sulh, Old High German medela, geiza, huohilī(n), Old Norse arðr (Swedish årder), and Gothic hōha, all presumably referring to the ard (scratch plough). The term plough, as used today, was not common until 1700.
The modern word plough comes from Old Norse plógr, and therefore Germanic, but it appears relatively late (it is not attested in Gothic), and is thought to be a loanword from one of the north Italic languages. Words with the same root appeared with related meanings: in Raetic plaumorati "wheeled heavy plough" (Pliny, Nat. Hist. 18, 172), and in Latin plaustrum "farm cart", plōstrum, plōstellum "cart", and plōxenum, plōximum "cart box".[6][7] The word must have originally referred to the wheeled heavy plough, which was common in Roman northwestern Europe by the a.d. 5th century.[8]
Orel (2003)[9] tentatively attaches plough to a PIE stem *blōkó-, which gave Armenian peɫem "to dig" and Welsh bwlch "crack", though the word may not be of Indo-European origin.
Parts

The diagram (right) shows the basic parts of the modern plough:
beam
hitch (Brit: hake)
vertical regulator
coulter (knife coulter pictured, but disk coulter common)
chisel (foreshare)
share (mainshare)
mouldboard
Other parts not shown or labelled include the frog (or frame), runner, landside, shin, trashboard, and stilts (handles).
On modern ploughs and some older ploughs, the mouldboard is separate from the share and runner, so these parts can be replaced without replacing the mouldboard. Abrasion eventually destroys all parts of a plough that come into contact with the soil.
History

Hoeing

When agriculture was first developed, soil was turned using simple hand-held digging sticks and hoes.[4] These were used in highly fertile areas, such as the banks of the Nile where the annual flood rejuvenates the soil, to create drills (furrows) to plant seeds in. Digging sticks, hoes, and mattocks were not invented in any one place, and hoe-cultivation must have been common everywhere agriculture was practiced. Hoe-farming is the traditional tillage method in tropical or sub-tropical regions, which are characterised by stony soils, steep slope gradients, predominant root crops, and coarse grains grown at wide distances apart. While hoe-agriculture is best suited to these regions, it is used in some fashion everywhere. Instead of hoeing, some cultures use pigs to trample the soil and grub the earth.
Ard

Some ancient hoes, like the Egyptian mr, were pointed and strong enough to clear rocky soil and make seed drills, which is why they are called hand-ards. However, the domestication of oxen in Mesopotamia and the Indus valley civilization, perhaps as early as the 6th millennium BC, provided mankind with the draft power necessary to develop the larger, animal-drawn true ard (or scratch plough). The earliest evidence of a ploughed field in the world was found at the Indus Valley Civilization site of Kalibangan (c. 2800 B.C.).[10] Archeological finds in Prague, Czech Republic, push oldest known ploughed field even further, to 3500 - 3800 B.C.  A terracotta model of the early ards was found at Banawali, giving historians insight into the form of the tool.[11]
The earliest was the bow ard, which consists of a draft-pole (or beam) pierced by a thinner vertical pointed stick called the head (or body), with one end being the stilt (handle) and the other a share (cutting blade) that was dragged through the topsoil to cut a shallow furrow suitable for most cereal crops. The ard does not clear new land well, so hoes or mattocks must be used to pull up grass and undergrowth, and a hand-held, coulter-like ristle could be used to cut deeper furrows ahead of the share. Because the ard leaves a strip of undisturbed earth between the furrows, the fields are often cross-ploughed lengthwise and widthwise, and this tends to form squarish fields (Celtic fields).[12] The ard is best suited to loamy or sandy soils that are naturally fertilised by annual flooding, as in the Nile Delta and Fertile Crescent, and to a lesser extent any other cereal-growing region with light or thin soil. By the late Iron Age, ards in Europe were commonly fitted with coulters.
Mouldboard plough

To grow crops regularly in less-fertile areas, it was once believed that the soil must be turned to bring nutrients to the surface. A major advance for this type of farming was the turnplough, also known as the mouldboard plough (UK), moldboard plow (US), or frame-plough. A coulter (or skeith) could be added to cut vertically into the ground just ahead of the share (in front of the frog), a wedge-shaped cutting edge at the bottom front of the mouldboard with the landside of the frame supporting the undershare (below-ground component). The mouldboard plough was introduced in the 18th century and was a major advance in plough technology.[4]
The upper parts of the frame carry (from the front) the coupling for the motive power (horses), the coulter and the landside frame. Depending on the size of the implement, and the number of furrows it is designed to plough at one time, a forecarriage with a wheel or wheels (known as a furrow wheel and support wheel) may be added to support the frame (wheeled plough). In the case of a single-furrow plough there is only one wheel at the front and handles at the rear for the ploughman to steer and manoeuvre it.
When dragged through a field the coulter cuts down into the soil and the share cuts horizontally from the previous furrow to the vertical cut. This releases a rectangular strip of sod that is then lifted by the share and carried by the mouldboard up and over, so that the strip of sod (slice of the topsoil) that is being cut lifts and rolls over as the plough moves forward, dropping back to the ground upside down into the furrow and onto the turned soil from the previous run down the field. Each gap in the ground where the soil has been lifted and moved across (usually to the right) is called a furrow. The sod that has been lifted from it rests at about a 45 degree angle in the next-door furrow and lies up the back of the sod from the previous run.
In this way, a series of ploughing runs down a field leaves a row of sods that lie partly in the furrows and partly on the ground lifted earlier. Visually, across the rows, there is the land (unploughed part) on the left, a furrow (half the width of the removed strip of soil) and the removed strip almost upside-down lying on about half of the previous strip of inverted soil, and so on across the field. Each layer of soil and the gutter it came from forms the classic furrow.
The mouldboard plough greatly reduced the amount of time needed to prepare a field, and as a consequence, allowed a farmer to work a larger area of land. In addition, the resulting pattern of low (under the mouldboard) and high (beside it) ridges in the soil forms water channels, allowing the soil to drain. In areas where snow buildup causes difficulties, this lets farmers plant the soil earlier, as the snow runoff drains away more quickly.

There are five major parts of a mouldboard plough:
Mouldboard
Share
Landside (short or long)
Frog (sometimes called a standard)
Tailpiece
Share, landside, mouldboard are bolted to the frog which is an irregular piece of cast iron. The base of a plough body is called the frog and the soil wearing parts are bolted to it.
The share is the cutting edge of the mouldboard plough. The share makes the horizontal cut that separates the furrow slice from the soil below and when it is worn it is important to fit a new one. Conventional shares have points that are shaped. This shape, which was developed through years of field experience, penetrates ordinary soil efficiently and stands rough use. The share is shaped so that it pulls itself into the ground. As the share wears away, it becomes blunt and the plough will require more power to pull it through the soil. A plough body with a worn share will not have enough 'suck' to ensure that it penetrates the ground to its full working depth. The tip of the share is pointed downward, causing the plough to run into the ground. This is called “suction or down suck”; it literally sucks the plough into the ground, to a regulated depth. The clearance, also usually referred to as “suction or down suck”, varies with different makes and types of ploughs (the related clearance at the side is called “land suck”). Its configuration is related to soil type, particularly in the down suction, or concavity, of its lower surface. Generally, three degrees of clearance or down suction are recognized: regular for light soil, deep for ordinary dry soil, and double-deep for clay and gravelly soils. In addition, the share has horizontal suction, which is related to the amount its point is bent out of line with the landside. Down suction causes the plough to penetrate to proper depth when pulled forward, while horizontal suction causes the plough to create the desired width of furrow. The share is a plane part with trapezoidal shape. It cuts the soil horizontally and lifts it. Common types are regular, winged-plane, bar-point, and share with mounted or welded point. The regular share conserves a good cut but is recommended on stone-free soils. The winged-plane share is used on heavy soil with a moderate amount of stones. The bar-point share can be used in extreme conditions (hard and stony soils). The use of the share with mounted point is somewhere between the last two types. Manufacturers have designed shares of various shapes (trapezium, diamond, etc.) with bolted point and wings, often separately renewable. Sometimes the share cutting edge is placed well in advance of the mouldboard to reduce the pulverizing action of the soil.
The mouldboard is that part of the plough which receives the furrow slice from the share.[4] The mouldboard is responsible for lifting and turning the furrow slice and sometimes for shattering it, depending on the type of mouldboard, ploughing depth, and soil conditions. The intensity of this action depends on the type of the mouldboard.  To suit different soil conditions and crop requirements, mouldboards have been designed in different shapes, each producing its own furrow profile and surface finish, but they still basically conform to the original plough body classification. Traditionally, the various types of plough body have been classified as general purpose, digger, and semi-digger, as described below.
1. The general-purpose mouldboard. This is a low draft body with a gently curved and a cross-sectional convex curve from top to bottom, which turns a furrow three parts wide by two parts deep, e.g. 300mm wide by 200mm deep. It turns the furrow slice slowly almost without breaking it, and is normally used for shallow ploughing (maximum 200mm depth). It is useful for grassland ploughing and sets up the land for weathering by winter frosts, which reduces the time taken to prepare a seedbed for spring sown crops.
2. The digger mouldboard has a short, abruptly curved with a concave cross-section both from top to bottom and from shin to tail. It turns the furrow slice rapidly, giving maximum shatter and deeper than its width. It is normally used for very deep ploughing (300mm deep or more). It has a higher power requirement and leaves a very broken surface. Digger ploughs are mainly used for land to be or planted with potatoes and other root crops.
3. The semi-digger mouldboard. This is a bit shorter than the general-purpose mouldboard, but with a concave cross-section and a more abrupt curve. Being intermediate between the two mouldboards above described, it has a performance that comes in between (approximately 250mm deep) and with less shattering than the digger mouldboard. It turns an almost square sectioned furrow and leaves a more broken surface finish. Semi-digger mouldboards can be used at different depths and speeds, making them suitable for most of the general ploughing on the farm.
(In addition, slatted mouldboards are preferred by some farmers, though they are a less common type. They consist of a number of curved steel slats bolted to the frog along the length of the mouldboard, so that there are gaps between the slats. They tend to break up the soil more than a full mouldboard and improve soil movement across the mouldboard when working in sticky soils where the solid mouldboard does not scour well.)
The landside is the flat plate which presses against and transmits the lateral thrust of the plough bottom to the furrow wall. It helps to resist the side pressure exerted by the furrow slice on the mouldboard. It also helps in stabilizing the plough while it is in operations. The rear bottom end of the landslide, which rubs against the furrow sole, is known as heel . A heel iron is bolted to the end of the rear of the landside and helps to carry the back of the plough. The landside and share are arranged to give a ‘'lead” towards the unploughed land, thus helping to maintain the correct furrow width. The landside is usually made of solid medium carbon steel, and is very short except on the rear bottom of the plough. The heel or rear end of the rear landside may be subject to excessive wear if the rear wheel is out of adjustment, therefore, a chilled iron heel piece is frequently used. This heel is inexpensive and can be easily replaced. The land side is fastened to the frog with the help of plough bolts.
The frog (standard) is the central part of the plough bottom to which the other components of the bottom are attached. It is an irregular piece of metal, which may be made of cast iron for cast iron ploughs or welded steel for steel ploughs. The frog is the foundation of the plough bottom. It takes the shock resulting from hitting rocks, and therefore should be tough and strong. The frog is in turn fastened to the plough frame.
A runner extending from behind the share to the rear of the plough controls the direction of the plough, because it is held against the bottom land-side corner of the new furrow being formed. The holding force is the weight of the sod, as it is raised and rotated, on the curved surface of the mouldboard. Because of this runner, the mouldboard plough is harder to turn around than the scratch plough, and its introduction brought about a change in the shape of fields– from mostly square fields into longer rectangular "strips" (hence the introduction of the furlong).
An advance on the basic design was the iron ploughshare, a replaceable horizontal cutting surface mounted on the tip of the share. The earliest ploughs with a detachable and replaceable share date from around 1000 BC in the Ancient Near East,[13] and the earliest iron ploughshares from ca. 500 BC in China.[14] Early mouldboards were basically wedges that sat inside the cut formed by the coulter, turning over the soil to the side. The ploughshare spread the cut horizontally below the surface, so when the mouldboard lifted it, a wider area of soil was turned over. Mouldboards are known in Britain from the late 6th century[15] on.

Mouldboard plough type is usually determined by the method in which the plough is attached to the tractor and by the way it is lifted and carried.
The basic types are:
1.Three wheel trailing type– attached to the standard tractor drawbar and carried on its own three wheels.
2.Mounted or integral– most use a three-point hitch and have a rear wheel in use only when ploughing. Some also have a gauge wheel to regulate maximum depth.
3.Semi-mounted– used principally for larger ploughs. These have a rear wheel which usually carries weight and side thrust when ploughing and sometimes the weight of the rear end of the plough when lifted. The front end of the plough is carried on the tractor lower or draft links.
Plough wheel
Gauge wheel– It is an auxiliary wheel of an implement to maintain uniform depth of working. Gauge wheel helps to maintain uniformity in respect of depth of ploughing in different soil conditions. It is usually placed in hanging position.
Land wheel– It is the wheel of the plough, which runs on the ploughed land.
Front furrow wheel– It is the front wheel of the plough, which runs in the furrow.
Rear furrow wheel– It is the rear wheel of the plough, which runs in the furrow.
Plough protective devices
When a plough hits a rock or other solid obstruction, serious damage may result unless the plough is equipped with some safety device. The damage may be bent or broken shares, bent standards, beams or braces.
Three basic types of safety devices are used on mouldboard ploughs as follows:
A spring release device in the plough drawbar.
Trip beam construction on each bottom
Automatic reset design on each bottom.
The spring release device was used in the past almost universally on trailing type ploughs with one to three or four bottoms. It is not practical on larger ploughs. When an obstruction is encountered the spring release mechanism in the hitch permits the plough to uncouple from the tractor. When a hydraulic lift is used on the plough, the hydraulic hoses will also usually uncouple automatically when the plough uncouples. Most plough makers offer an automatic reset system for tough conditions or rocky soils. The re-set mechanism allows each body to move rearward and upward to pass over obstacle (e.g. rocks hidden below soil surface) without damage. A heavy leaf or coil spring mechanism, which holds the body in its working position under normal conditions, resets the plough after the obstruction is passed.
Another type of auto-reset mechanism uses an oil (hydraulic) and gas accumulator. Shock loads cause the oil to compress the gas, when the gas expands again the leg returns to its working ploughing position after passing over the obstacle. The most simple mechanism is a breaking (shear) bolt that needs replacement. Shear bolts which break when a plough body hits an obstruction are a cheaper overload protection device. It is important to use the correct replacement bolt.
Trip beam ploughs are constructed with a hinge point in the beam. This point is usually located some distance above the top of the plough bottom. The bottom is held in normal ploughing position by a spring operated latch. When an obstruction is encountered the entire bottom is released and hinges back and up so as to pass over the obstruction. It is necessary to back up the tractor and plough to reset the bottom This construction is used to protect the individual bottoms. The automatic reset design has only recently been introduced on American ploughs, but has been used extensively on European and Australian ploughs. In this construction the beam is hinged at a point nearly above the point of the share. The bottom is held in the normal position by a set of springs or by a hydraulic cylinder on each bottom.
When an obstruction is encountered, the plough bottom hinges back and up in such a way as to pass over the obstruction, without stopping the tractor and plough. The bottom automatically returns to normal ploughing position as soon as the obstruction is passed, without any interruption of forward motion. The automatic reset design permits higher field efficiencies since stopping for stones is practically eliminated. It also reduces costs for broken shares, beams and other parts. The fast resetting action also helps produce a better job of ploughing since large areas of unploughed land are not left as when lifting a plough over a stone.
Loy ploughing

Loy ploughing was a form of manual ploughing in Ireland, on very small farms– or on very hilly ground, where horses could not work or where farmers could not afford them.[16] It was used up until the 1960s in poorer land.[17] This suited the moist climate of Ireland, as the trenches formed by turning in the sods provided drainage. It also allowed the growing of potatoes in bogs as well as on mountain slopes where no other cultivation could take place.[18][19]
Heavy ploughs

In the basic mouldboard plough the depth of the cut is adjusted by lifting against the runner in the furrow, which limited the weight of the plough to what the ploughman could easily lift. This limited the construction to a small amount of wood (although metal edges were possible). These ploughs were fairly fragile, and were not suitable for breaking up the heavier soils of northern Europe. The introduction of wheels to replace the runner allowed the weight of the plough to increase, and in turn allowed the use of a much larger mouldboard faced in metal. These heavy ploughs led to greater food production and eventually a significant population increase beginning around 1000 AD.[20]
Before the Han Dynasty (202 BC– AD 220), Chinese ploughs were made almost entirely of wood, except the iron blade of the ploughshare. By the Han period, the entire ploughshare was made of cast iron; these are the first known heavy mouldboard iron ploughs.[14][21]
The Romans achieved the heavy wheeled mouldboard plough in the late 3rd and 4th century AD, when archaeological evidence appears, inter alia, in Roman Britain.[22] The first indisputable appearance after the Roman period is from 643, in a northern Italian document.[23] Old words connected with the heavy plough and its use appear in Slavic, suggesting possible early use in this region.[24] The general adoption of the carruca heavy plough in Europe appears to have accompanied the adoption of the three-field system in the later eighth and early ninth centuries, leading to an improvement of the agricultural productivity per unit of land in northern Europe.[25] This was accompanied by larger fields as well, known variously as carucates, ploughlands, and ploughgates.
Improved designs

The basic plough, with coulter, ploughshare and mouldboard, remained in use for a millennium. Major changes in design did not become common until the Age of Enlightenment, when there was rapid progress in design. Joseph Foljambe in Rotherham, England, in 1730 used new shapes as the basis for the Rotherham plough, which also covered the mouldboard with iron.[26] Unlike the heavy plough, the Rotherham (or Rotherham swing) plough consisted entirely of the coulter, mouldboard and handles. It was much lighter than conventional designs and became very popular in England. It may have been the first plough to be widely built in factories and the first to be commercially successful.[27]
In 1789 Robert Ransome, an iron founder in Ipswich, started casting ploughshares in a disused malting at St Margaret's Ditches. As a result of a mishap in his foundry, a broken mould caused molten metal to come into contact with cold metal, making the metal surface extremely hard. This process, chilled casting, resulted in what Ransome advertised as "self-sharpening" ploughs. He received patents for his discovery.
James Small further advanced the design. Using mathematical methods he experimented with various designs until he arrived at a shape cast from a single piece of iron, an improvement on the Scots plough of James Anderson of Hermiston.[28] A single-piece cast iron plough was also developed and patented by Charles Newbold in the United States. This was again improved on by Jethro Wood, a blacksmith of Scipio, New York, who made a three-part Scots Plough that allowed a broken piece to be replaced. In 1837 John Deere introduced the first steel plough; it was so much stronger than iron designs that it could work soil in areas of the US that had previously been considered unsuitable for farming.
Improvements on this followed developments in metallurgy: steel coulters and shares with softer iron mouldboards to prevent breakage, the chilled plough (an early example of surface-hardened steel),[29] and eventually mouldboards with faces strong enough to dispense with the coulter.
Single-sided ploughing

The first mouldboard ploughs could only turn the soil over in one direction (conventionally always to the right), as dictated by the shape of the mouldboard, and so the field had to be ploughed in long strips, or lands. The plough was usually worked clockwise around each land, ploughing the long sides and being dragged across the short sides without ploughing. The length of the strip was limited by the distance oxen (or later horses) could comfortably work without a rest, and their width by the distance the plough could conveniently be dragged. These distances determined the traditional size of the strips: a furlong, (or "furrow's length", 220 yards (200m)) by a chain (22 yards (20m))– an area of one acre (about 0.4 hectares); this is the origin of the acre. The one-sided action gradually moved soil from the sides to the centre line of the strip. If the strip was in the same place each year, the soil built up into a ridge, creating the ridge and furrow topography still seen in some ancient fields.
Turnwrest plough
The turnwrest plough allows ploughing to be done to either side. The mouldboard is removable, turning to the right for one furrow, then being moved to the other side of the plough to turn to the left (the coulter and ploughshare are fixed). In this way adjacent furrows can be ploughed in opposite directions, allowing ploughing to proceed continuously along the field and thus avoiding the ridge and furrow topography.
Reversible plough

The reversible plough (or "rollover plow") has two mouldboard ploughs mounted back-to-back, one turning to the right, the other to the left. While one is working the land, the other is carried upside-down in the air. At the end of each row, the paired ploughs are turned over, so the other can be used. This returns along the next furrow, again working the field in a consistent direction.
These ploughs date back to the days of the steam engine and the horse. In almost universal use on farms, they have right- and left-handed mouldboards enabling them to work up and down the same furrow. Reversible ploughs may either be mounted or semi-mounted and are heavier and more expensive than right-handed models, but they have the great advantage of leaving a level surface which makes seedbed preparation and harvesting easier. Very little marking out is necessary before ploughing can start and idle running on the headland is minimal compared with conventional ploughs.
Driving the tractor with the furrow-side wheels in the furrow bottom provides the most efficient line of draft between tractor and plough. It is also easier to steer the tractor and driving with the front wheel against the furrow wall will keep the front furrow at the correct width. This is less satisfactory when using a tractor with very wide front tyres, for although they make better use of the tractor power, the tyres may compact part of the last furrow slice turned on the previous run. The use of furrow widener or a longer mouldboard on the rear body will overcome the problem. The latter moves the soil further towards the ploughed land leaving more room for the tractor wheels on the next run.
Driving with all four wheels on unploughed land is another solution to the problem of wide tyres. Semi-mounted ploughs can be hitched in a way that allows the tractor to run on unbroken land and pull the plough in correct alignment without any sideways movement (crabbing).
Riding and multiple-furrow ploughs

Early steel ploughs, like those for thousands of years prior, were walking ploughs, directed by the ploughman holding onto handles on either side of the plough. The steel ploughs were so much easier to draw through the soil that the constant adjustments of the blade to react to roots or clods was no longer necessary, as the plough could easily cut through them. Consequently, it was not long after that the first riding ploughs appeared. On these, wheels kept the plough at an adjustable level above the ground, while the ploughman sat on a seat; whereas, with earlier ploughs the ploughman would have had to walk. Direction was now controlled mostly through the draught team, with levers allowing fine adjustments. This led very quickly to riding ploughs with multiple mouldboards, dramatically increasing ploughing performance.
A single draught horse can normally pull a single-furrow plough in clean light soil, but in heavier soils two horses are needed, one walking on the land and one in the furrow. For ploughs with two or more furrows more than two horses are needed and, usually, one or more horses have to walk on the loose ploughed sod– and that makes hard going for them, and the horse treads the newly ploughed land down. It is usual to rest such horses every half-hour for about ten minutes.
Heavy volcanic loam soils, such as are found in New Zealand, require the use of four heavy draught horses to pull a double-furrow plough. Where paddocks are more square than long-rectangular it is more economical to have horses four wide in harness than two-by-two ahead, thus one horse is always on the ploughed land (the sod). The limits of strength and endurance of horses made greater than two-furrow ploughs uneconomic to use on one farm.
Amish farmers tend to use a team of about seven horses or mules when spring ploughing and as Amish farmers often help each other plough, teams are sometimes changed at noon. Using this method about 10 acres (4.0ha) can be ploughed per day in light soils and about 2 acres (0.81ha) in heavy soils.
Improvement in metallurgy and design
John Deere, an Illinois, US blacksmith, noted that the ploughing of many sticky, non-sandy soils might benefit from modifications in the design of the mouldboard and in the metals used. He noted that a polished needle would enter leather and fabric with greater ease, and a polished pitchfork required less effort as well. In the pursuit of a polished and thus slicker surface for a plough, he experimented with portions of saw blades and by 1837, he was making polished, cast steel ploughs. The energy effort required was lessened, which enabled the use of larger ploughs, making more effective use of horse power.
Balance plough

The advent of the mobile steam engine allowed steam power to be applied to ploughing from about 1850. In Europe, soil conditions were often too soft to support the weight of heavy traction engines. Instead, counterbalanced, wheeled ploughs, known as balance ploughs, were drawn by cables across the fields by pairs of ploughing engines on opposite field edges, or by a single engine drawing directly towards it at one end and drawing away from it via a pulley at the other end. The balance plough had two sets of ploughs facing each other, arranged so when one was in the ground, the other set was lifted into the air. When pulled in one direction, the trailing ploughs were lowered onto the ground by the tension on the cable. When the plough reached the edge of the field, the other engine pulled the opposite cable, and the plough tilted (balanced), putting the other set of shares into the ground, and the plough worked back across the field.
One set of ploughs was right-handed, and the other left-handed, allowing continuous ploughing along the field, as with the turnwrest and reversible ploughs. The man credited with the invention of the ploughing engine and the associated balance plough, in the mid nineteenth century, was John Fowler, an English agricultural engineer and inventor.[30]
In America the firm soil of the Plains allowed direct pulling with steam tractors, such as the big Case, Reeves or Sawyer-Massey breaking engines. Gang ploughs of up to fourteen bottoms were used. Often these big ploughs were used in regiments of engines, so that in a single field there might be ten steam tractors each drawing a plough. In this way hundreds of acres could be turned over in a day. Only steam engines had the power to draw the big units. When internal combustion engines appeared, they had neither the strength nor the ruggedness compared to the big steam tractors. Only by reducing the number of shares could the work be completed.
Stump-jump plough

The stump-jump plough was an Australian invention of the 1870s, designed to cope with the breaking up of new farming land, that contains many tree stumps and rocks that would be very expensive to remove. The plough uses a moveable weight to hold the ploughshare in position. When a tree stump or other obstruction such as a rock is encountered, the ploughshare is thrown upwards, clear of the obstacle, to avoid breaking the plough's harness or linkage; ploughing can be continued when the weight is returned to the earth after the obstacle is passed.
A simpler system, developed later, uses a concave disc (or a pair of them) set at a large angle to the direction of progress, that uses the concave shape to hold the disc into the soil– unless something hard strikes the circumference of the disk, causing it to roll up and over the obstruction. As the arrangement is dragged forward, the sharp edge of the disc cuts the soil, and the concave surface of the rotating disc lifts and throws the soil to the side. It doesn't make as good a job as the mouldboard plough (but this is not considered a disadvantage, because it helps fight wind erosion), but it does lift and break up the soil (see disc harrow).
Modern ploughs

Modern ploughs are usually multiple reversible ploughs, mounted on a tractor via a three-point linkage.[31] These commonly have between two and as many as seven mouldboards– and semi-mounted ploughs (the lifting of which is supplemented by a wheel about halfway along their length) can have as many as eighteen mouldboards. The hydraulic system of the tractor is used to lift and reverse the implement, as well as to adjust furrow width and depth. The ploughman still has to set the draughting linkage from the tractor so that the plough is carried at the proper angle in the soil. This angle and depth can be controlled automatically by modern tractors. As a complement to the rear plough a two or three mouldboards-plough can be mounted on the front of the tractor if it is equipped with front three-point linkage.
Specialist ploughs
Chisel plough
The chisel plough is a common tool to get deep tillage (prepared land) with limited soil disruption. The main function of this plough is to loosen and aerate the soils while leaving crop residue at the top of the soil. This plough can be used to reduce the effects of compaction and to help break up ploughpan and hardpan. Unlike many other ploughs the chisel will not invert or turn the soil. This characteristic has made it a useful addition to no-till and low-till farming practices that attempt to maximise the erosion-prevention benefits of keeping organic matter and farming residues present on the soil surface through the year. Because of these attributes, the use of a chisel plough is considered by some to be more sustainable than other types of plough, such as the mouldboard plough.

Chisel ploughs are becoming more popular as a primary tillage tool in row crop farming areas. Basically the chisel plough is a very heavy duty field cultivator intended to operate at depths from 15cm [6 in] to as much as 46cm [18 in]. However some models may run much deeper. Each of the individual ploughs, or shanks, are typically set from nine inches (229mm) to twelve inches (305mm) apart. Such a plough can encounter significant soil drag, consequently a tractor of sufficient power and good traction is required. When planning to plough with a chisel plough it is important to bear in mind that 10 to 20 horsepower (7.5 to 15kW) per shank will be required, depending on depth.
Pull type chisel ploughs are made in working widths from about 2.5 m [8ft] up to 13.7 m [45ft], they are tractor mounted and working depth is hydraulically controlled. Those more than about 4 m [13ft] wide may be equipped with folding wings to reduce transport width. Wider machines may have the wings supported by individual wheels and have hinge joints to allow flexing of the machine over uneven ground. The wider models usually have a wheel at each side to control working depth. Three-point hitch mounted units are made in widths from about 1.5 m [5ft] to 9 m [30ft].
Cultivators are often similar in form to chisel ploughs, but their goals are different. Cultivator teeth work near the surface, usually for weed control, whereas chisel plough shanks work deep beneath the surface. Consequently, cultivating also takes much less power per shank than does chisel ploughing.
Ridging plough
A ridging plough is used for crops, such as potatoes or scallions, which are grown buried in ridges of soil using a technique called ridging or hilling. A ridging plough has two mouldboards facing away from each other, cutting a deep furrow on each pass, with high ridges either side. The same plough may be used to split the ridges to harvest the crop.
Scottish hand plough
This is a variety of ridge plough notable in that the blade points towards the operator. It is used solely by human effort rather than with animal or machine assistance, and is pulled backwards by the operator, requiring great physical effort. It is particularly used for second breaking of ground, and for potato planting. It is found in Shetland, some western crofts and more rarely Central Scotland. The tool is typically found on small holdings too small or poor to merit use of animals.
Mole plough
The mole plough allows underdrainage to be installed without trenches, or it breaks up deep impermeable soil layers that impede drainage. It is a very deep plough, with a torpedo-shaped or wedge-shaped tip, and a narrow blade connecting this to the body. When dragged through the ground, it leaves a channel deep under the ground, and this acts as a drain. Modern mole ploughs may also bury a flexible perforated plastic drain pipe as they go, making a more permanent drain– or they may be used to lay pipes for water supply or other purposes. Similar machines, so called pipe-and-cable-laying ploughs, are even used under the sea, for the laying of cables, as well as preparing the earth for side-scan sonar in a process used in oil exploration.
Heavy land requires draining to reduce its water content to a level satisfactory for efficient plant growth. Heavy soils usually have a system of permanent drains using either perforated plastic or clay pipes which discharge into a ditch. Mole ploughs are used to form small tunnels (mole drains) in the soil at a depth of up to 950mm (30in) at an angle to the pipe drains. Water from the mole drains seeps into the pipes and runs along them into a ditch.
The mole plough allows underdrainage to be installed without trenches, or it breaks up deep impermeable soil layers which impede drainage. It is a very deep plough, with a torpedo-shaped or wedge-shaped tip, and a narrow blade connecting this to the body. When dragged through the ground, it leaves a channel deep under the ground, and this acts as a drain. Modern mole ploughs may also bury a flexible perforated plastic drain pipe as they go, making a more permanent drain– or they may be used to lay pipes for water supply or other purposes. A simple check can be made to find if the subsoil is in the right condition for mole ploughing. Compact a tennis ball sized sample taken at moling depth by hand, then push a pencil through. If the hole remains intact without splitting the ball the soil is in an ideal condition for the mole plough.
Mole ploughs are usually trailed and pulled by a crawler tractor, but lighter models for use on the three-point linkage of powerful four-wheel drive tractors are also made. A mole plough has a very strong frame which slides along the ground when the machine is in work. A heavy leg, similar to a subsoiler leg, is attached to the frame and a circular section share with a larger diameter expander on a flexible link is bolted to the leg. The bullet shaped share forms a tunnel, about 75mm diameter, in the soil and the expander presses the soil outwards to form a long-lasting drainage channel.
Paraplough
The paraplough, or paraplow, is a tool for loosening compacted soil layers 12 to 16 inches deep and still maintain high surface residue levels.[32]
Spade plough
The spade plough is designed to cut the soil and turn it on its side, minimising the damage to the earthworms, soil microorganism, and fungi. This helps maximise the sustainability and long term fertility of the soils.
Switch plough
Using a bar with square shares mounted perpendicularly and a pivot point to change the bar's angle, the switch plough allows ploughing in either direction. It is best in previously-worked soils, as the ploughshares are designed more to turn the soil over than for deep tillage. At the headland, the operator pivots the bar (and thus the ploughshares) to turn the soil to the opposite side of the direction of travel. Switch ploughs are usually lighter than rollover ploughs, requiring less horsepower to operate.
Effects of mouldboard ploughing
Mouldboard ploughing, in cold and temperate climates, no deeper than 20cm, aerates the soil by loosening it. It incorporates crop residues, solid manures, limestone and commercial fertilisers along with oxygen. By doing so, it reduces nitrogen losses by denitrification, accelerates mineralization and increases short-term nitrogen availability for transformation of organic matter into humus. It erases wheel tracks and ruts caused by harvesting equipment. It controls many perennial weeds and pushes back the growth of other weeds until the following spring. It accelerates soil warming and water evaporation in spring because of the lesser quantity of residues on the soil surface. It facilitates seeding with a lighter seeder. It controls many enemies of crops (slugs, crane flies, seedcorn maggots-bean seed flies, borers). It increases the number of "soil-eating" earthworms (endogea) but is detrimental to vertical-dwelling earthworms (anecic).
Ploughing leaves very little crop residue on the surface, which otherwise could reduce both wind and water erosion. Over-ploughing can lead to the formation of hardpan. Typically farmers break up hardpan up with a subsoiler, which acts as a long, sharp knife to slice through the hardened layer of soil deep below the surface. Soil erosion due to improper land and plough utilisation is possible. Contour ploughing mitigates soil erosion by ploughing across a slope, along elevation lines. Alternatives to ploughing, such as the no till method, have the potential to actually build soil levels and humus. These may be suitable to smaller, more intensively cultivated plots, and to farming on poor, shallow or degraded soils that ploughing would further degrade.

Ploughs in art
Back side of a 100 Mark banknote issued 1908
1975 Italian Lira coin
Robert Burns statue, Schenley Park, Pittsburgh
The Gefion Fountain in Copenhagen
Henry Herbert La Thangue, The Last Furrow, 1895
Ploughing in the Nivernais by Rosa Bonheur (1849)

See also

Boustrophedon (Greek: "ox-turning")— an ancient way of writing, each line being read in the opposite direction like reversible ploughing.
Conduit current collection
Foot plough
Headland (agriculture)
History of agriculture
Railroad plough
Ransome Victory Plough
Silviculture has a technique for preparing soil for seeding in forests called scarification, which is explained in that article.
Snowplough
Whippletree
Notes

Further reading
Brunt, Liam. "Mechanical Innovation in the Industrial Revolution: The Case of Plough Design". Economic History Review (2003) 56#3, pp.444–477. JSTOR.
Hill, P. and Kucharski, K. "Early Medieval Ploughing at Whithorn and the Chronology of Plough Pebbles", Transactions of the Dumfriesshire and Galloway Natural History and Antiquarian Society, Vol. LXV, 1990, pp 73–83.
Nair, V. Sankaran. Nanchinadu: Harbinger of Rice and Plough Culture in the Ancient World.

Steven Stoll, Larding the Lean Earth: Soil and Society in Nineteenth-Century America (New York: Hill and Wang, 2002)

— the first commercially successful iron plough
— as developed by John Deere in the United States

, Popular Mechanics, December 1934.

What are the most common materials used for making a plow?