Twin Studies and Anti-Social Behavior

Genes most likely influence everything about us from the color of our eyes to whether we like to eat vegetables or not. The extent to which genetics or environment (nature vs. nurture) influence our behavior is still unclear. But, a recent study of twins suggests that antisocial behavior, such as lack of empathy and remorse, may be heritable. (Life Style Extra, May 25, 2005)

Twin studies are often used to discern the contribution of genetics and environment to a trait. While both identical and non-identical twin pairs share the same environment starting in-utero, identical twins share all of the same genes while non-identical twins share half, just like any other sibling pair. So, if more identical than non-identical twin pairs share the same trait, the trait is believed to be highly influenced by genetics.

Monitor, published by the American Psychological Association, points out some limitations to twin studies:

• Random mating. Twin researchers assume that people are as likely to choose partners who are different from themselves as they are to choose partners who are similar for a particular trait. If, instead, people tend to choose mates like themselves, then fraternal twins could share more than 50 percent of their genes–and hence more similarities on genetically influenced traits–because they would receive similar genes from their mothers and fathers.
• Equal environments. Twin researchers also assume that fraternal and identical twins raised in the same homes experience equally similar environments. But some research suggests that parents, teachers, peers and others may treat identical twins more similarly than fraternal twins.
• Gene-environment interaction. Some researchers think that interactions between genes and environment, rather than genes and environment separately, may influence many traits. A recent study from Science (Vol. 297, No. 5582) by Avshalom Caspi, PhD, of King’s College London, for example, suggests that a gene might moderate propensity for violence, particularly in people who are severely maltreated as children. Many twin study designs don’t take this type of complication into account.
• Genetic mechanisms. Traits can be inherited through different genetic mechanisms. For traits governed by dominant genetic mechanisms, a dominant gene inherited from one parent trumps a recessive gene inherited from the other parent: If a person inherits a recessive gene for blue eyes from one parent and a dominant gene for brown eyes from the other parent, then the dominant brown gene wins, and the person’s eyes are brown.

Despite these limitations, twins and their unique situation should still provide some valuable and interesting insights. Not the least of which is this finding that psychopathic tendencies in grade school kids may be strongly influenced by genetics. But as Professor Marian Kent, visiting professor of criminology points out (BBC News, May 24, 2005),

Most people who get involved in crime and antisocial behaviour are not genetically predisposed. A lot of kids get swept up in their teens and there are many social, economic and environmental factors involved. Most grow out of it. Obviously, individuals with genetic factors that predispose them to this behaviour will be more at risk. But tackling this small number of people will not make a big difference for society. What is important is to look at early interventions – family support, economics, improving neighbourhoods, preschool education and so on. These are critically important.

Another reminder that genes are not destiny.