Testosterone has a profound effect on the developing brain of a young male infant. Most guys equate testosterone with muscles and erections, but don't realize how much it affects that "muscle between their ears", especially to their expectant male child.
In the last five to ten years, researchers have made one groundbreaking discovery after another in this area. In 2001 researchers found that fetal testosterone levels were negatively correlated with eye contact even amount genders.  The latter study literally "opened the eyes" of the research world and demonstrated how testosterone affected how the brain interacts with the world. Other related traits such as empathy and lowered social skills were also found to be linked. 
The importance of this cannot be emphasized enough, because it is so critical to the health and development of your child. Subsequent studies, for example, found that (in females) a number of autistic traits correlated with fetal testosterone.  Finally, in 2009 a team of researchers concluded that fetal testosterone levels were very likely one of the key contributing factors to autism.  A young male needs testosterone desperately, but not too much of it!
So what can increase fetal testosterone levels? Smoking is a definite risk factor and has been shown in animal studies to increase fetal testosterone.  One study even showed a weak association between autistism and smoking.  There simply is no reason to expose your child to nicotine and smoke. By the way, smoking is also a risk factor for ADHD  and behavior problems  as well! (See my link on Smoking for what it does to adult males as well.)
Low testosterone can be just as devastating for a male infant. Researchers have found evidence that phthalates, a common family of chemicals in plastics that lower testosterone  are linked to effeminizing behavior in boys. (See number 10 in my link on Child IQ for more details.) Animal studies have shown nasty effects from phthalates as well, including permanent reproductive disorders  and a recent human study showed lowered sperm counts as verification.  In other words, the damage seems to be permanent.
Even more frightening is the fact that initial studies show a correlation between phthalate exposure and autism  and schizophrenia. 
What can you do? Tell anyone you know who is pregnant to avoid smoking and plastics for starters. Recommended cooking and drinking materials are glass and stainless steel whenever possible, especially around children.
1) Infant Behavior and Development, 2002, 25(3):327-335, "Foetal testosterone eye contact in 12-month-old infants"
2) Hormones and Behavior, May 2007, 51(5):597-604, "Elevated rates of testosterone-related disorders in women with autism spectrum conditions"
3) Brit J of Psychology, 2009, 100:1-22, "Fetal Testosterone and autistic traits"
5) J Child Psychol Psychiatry, Feb 2005, 46(2):198-210, "Foetal testosterone, social relationships, and restricted interests in children"
6) Nicotine & Tobacco Research, 2003, 5(3):369-374, "Prenatal nicotine increases testosterone levels in the fetus and female offspring"
7) Amer J of Epidemiology, 2005, 161(10):916-925, "Risk Factors for Autism: Perinatal Factors, Parental Psychiatric History, and Socioeconomic Status"
8) Am J Psychiatry, 1996, 153:1138-1142, "Is maternal smoking during pregnancy a risk factor for attention deficit hyperactivity disorder in children?"
9) PEDIATRICS, Dec 1993, 92(6):815-822, "Maternal Smoking Before and After Pregnancy: Effects on Behavioral Outcomes in Middle Childhood"
10) Toxicology, 1 June 2006,223(1-2):144-155, "Mechanisms underlying the anti-androgenic effects of diethylhexyl phthalate in fetal rat testis"
11) Toxicological sciences, 2008, 105:(1)153-65, "A Mixture of Five Phthalate Esters Inhibits Fetal Testicular Testosterone Production in the Sprague-Dawley Rat in a Cumulative, Dose-Additive Manner"
12) Neurotoxicology, Sep 2009, 30(5):822-31, Epub 2009 Feb 10, "Associations between indoor environmental factors and parental-reported autistic spectrum disorders in children 6-8 years of age"
13) Progress in Neuro-Psychopharmacology and Biological Psychiatry, January 2004
14) J of Environmental Health Perspectives, 2005, "Phthalate Exposure during Pregnancy and Lower Anogenital Index in Boys: Wider Implications for the General Population?"