Monkeys and humans share the same two variants of the serotonin gene (known as the short and long serotonin transporter alleles). In humans the short allele has been associated with impulsivity, aggression, sensation seeking, suicide attempts, and severe depression. Suomi showed that, at least in monkeys, the environment shapes how these genes affect behavior. Monkeys with the short allele that were raised by an adequate mother behaved normally and had no deficit in their serotonin metabolism. Those who were raised with their peers became aggressive risk takers.
