(2 of 4)
This cultural disparity indicates that there are factors in American society -- such as the availability of guns, economic inequity and a violence- saturated culture -- that are not rooted in human biology. Nevertheless, a susceptibility to violence might partly be genetic. Errant genes play a role in many behavioral disorders, including schizophrenia and manic depression. "In virtually every behavior we look at, genes have an influence -- one person will behave one way, another person will behave another way," observes Gregory Carey, assistant professor at the University of Colorado's Institute for Behavioral Genetics. It stands to reason that genes might contribute to violent activity as well.
Some studies of identical twins who have been reared apart suggest that when one twin has a criminal conviction, the other twin is more likely to have committed a crime than is the case with fraternal twins. Other research with adopted children indicates that those whose biological parents broke the law are more likely to become criminals than are adoptees whose natural parents were law-abiding.
No one believes there is a single "criminal gene" that programs people to maim or murder. Rather, a person's genetic makeup may give a subtle nudge toward violent actions. For one thing, genes help control production of behavior-regulating chemicals. One suspect substance is the neurotransmitter serotonin. Experiments at the Bowman Gray School of Medicine in North Carolina suggest that extremely aggressive monkeys have lower levels of serotonin than do more passive peers. Animals with low serotonin are more likely to bite, slap or chase other monkeys. Such animals also seem less social: they spend more time alone and less in close body contact with peers.
A similar chemical variation appears to exist in humans. Studies at the National Institute on Alcohol Abuse and Alcoholism conclude that men who commit impulsive crimes, such as murdering strangers, have low amounts of serotonin. Men convicted of premeditated violence, however, show normal levels. As for aggressive behavior in women, some researchers speculate that it might be tied to a drop in serotonin level that normally occurs just before the menstrual period. Drugs that increase serotonin, researchers suggest, may make people less violent.
Scientists are also trying to find inborn personality traits that might make people more physically aggressive. The tendency to be a thrill seeker may be one such characteristic. So might "a restless impulsiveness, an inability to defer gratification," says psychologist Richard Herrnstein of Harvard, whose theories about the hereditary nature of intelligence stirred up a political storm in the 1970s. A high threshold for anxiety or fear may be another key trait. According to psychologist Jerome Kagan, also of Harvard, such people tend to have a "special biology," with lower-than-average heart rates and blood pressure.