How we change what others think, feel, believe and do
Anger happens when I am frustrated from achieving my goals. This may be a repeated thing, with each occurrence stoking the anger I am repressing until I cannot hold it in any more.
Anger is often an response to an insult to me or my friends. It is thus a defending action against an attack on my identity. Other reasons for anger include its use in aggression and defense.
Anger is generally past-focused. That is, we are more often angry at things that have already happened. Even when I am angry about the future, it often is triggered by my putting myself into further into the future and looking back at a 'future perfect' event that 'has happened'.
Anger can be a very powerful way of getting what you want, especially in the short term. Indeed, this is one of the key reasons why it exists. When a chimpanzee displays anger, it is signalling to others that the subject in hand is very important to it, and it is ready to fight for it. Unless other chimpanzees are prepared to duel, maybe to the death, then they are more likely to back down.
Anger is also the last response of a cornered victim, and many a mild-mannered person has turned into a raging hulk when they feel there is no alternative. The surprise they create in their oppressors is often so great it gives them the winning hand.
Anger also gives a general power signal that does not need to be used that often, as other peace-loving people will generally tend to back down before a person they know has a tendency to anger. A 'look' is often all it takes.
Anger can also be a positive long-term energiser, such as when a teenager decides to 'show' his parents that he is not the lazy slob they seem to think he is.
Although there are short-term gains to be had with anger, there are also a number of hazards. First, your bluff may be called and you can get into losing battles. A reason for this is that as anger boils over into rage you can easily lose all judgement and control and hence take on an opponent who can easily give you a good pasting (or hurt someone you do not want to hurt).
Anger and the effective attack (physical or psychological) can also gain you enemies. Even someone you have beaten and who now seems to be your friend is probably waiting for the moment when they can safely take their revenge.
As anger is so hazardous, we often find other ways to vent it, displaced either in time or location. For example a person who is frustrated at work may be angry with their family, or perhaps will avoid this by going for a run immediately when they get home.
Displacement is a huge source of continued human strife, as we pass on our anger to other people, who do likewise.
One method of displacement of anger towards another person is to vent it on a punchbag or another inanimate object. This is seldom useful in cathartic 'releasing' of the anger -- research has shown that it just makes you angry for longer.
Avoid arousing the anger of people who can damage you. You can, however, use their anger. One way is to redirect it at other people. Another approach is to let them get angry and then play the hurt party. When they cool down they will need to apologies and you can ask for what you really want from them as a condition for forgiveness.
Lazarus, R. and Lazarus, B. (1994), Passion and Reason, Oxford University Press, New York