The Identity Dance

Moffitt and Caspi have found a similar relationship between another gene and antisocial behavior. Abused and neglected children with a gene responsible for low levels of monoamine oxidase in the brain were nine times more likely to engage in violent or other antisocial behavior as adults than were people with the same gene who were not mistreated. Finnish scientists have since found similar effects on genes for novelty seeking—a trait associated with attention deficit hyperactivity disorder. Children who had the genes and who were also raised by strict, emotionally distant parents were much more likely to engage in risky behavior and make impulsive decisions as adults than children with the same genes who were raised in more tolerant and accepting environments.

While scientists don't exactly know how genes are influenced by environment at the molecular level, there are clues that genes have the equivalent of molecular "switches" and can be programmed—turned on or off, up or down—very early. Both Lesch and Suomi have shown that the level of biochemicals such as the serotonin transporter molecule can be "set" as early as in the womb, at least in mice and monkeys.

The prenatal environment also has a major influence on differences between identical twins. Mothers of multiples will tell you that their babies were distinct the moment they were born, and research backs them up. Twins experience different environments even in the womb, as they compete with each other for nutrients. One can beat out the other, which is why they often have different birth weights: Marisa Pena was born first and is a bit taller and heavier than her sister.

Prenatal experiences are just the first in a lifetime of differentiating factors. Only about 50 percent of the characteristics twins have in common are due to genes alone. Researchers now believe that an illness suffered by only one twin, or different amounts of attention from peers or parents, can set the stage for personality differences. This makes it easier to understand why the Pena sisters reacted as they did: By the time their parents died, "these twins had had a lifetime of experiences which might have made them react differently," says Moffitt. "In addition, some pairs of identical twins individuate themselves in early childhood. They seem to take on the roles of 'the shy one' and 'the outgoing one' and then live up to those roles." In other words, they customize their environment, and the world treats them accordingly.

The new science of nature and nurture isn't as straightforward as the DNA-is-destiny mantra, but it is more accurate. "People have a really hard time understanding the probabilistic nature of how genes impact traits like depression," says Kenneth Kendler, director of the Virginia Institute for Psychiatric and Behavioral Genetics at Virginia Commonwealth University, who heads a major twin registry. "They think that if something is heritable, then it can't be modified by the environment." The knowledge that the traits we inherit are also contingent on what the world does to us promises more insight into why people act and feel differently—even when they look exactly the same.