Genetic Factors in Aggression

Twin studies show compelling evidence for genetic influences on aggression. Research by Coccaro found that identical twins (MZ) have higher concordance rates for both physical assault (50% vs 28%) and verbal aggression (28% vs 7%) compared to fraternal twins (DZ). This significant difference suggests our genes play a substantial role in aggressive tendencies.

The MAOA gene, which regulates serotonin metabolism in the brain, has been directly linked to aggression. Brunner's study of a Dutch family found males with abnormally low MAOA levels engaged in repeated aggressive criminal behaviours. Further research identified two variants: MAOA-H and MAOA-L. Caspi discovered boys with MAOA-L were more likely to develop antisocial behaviour, but interestingly, only if they'd been maltreated as children. Those with MAOA-H remained resilient despite similar maltreatment.

Adoption studies help separate nature from nurture in understanding aggression. Hutchings and Mednick's large Danish study found adoptees most likely became criminals when both biological and adoptive parents were criminals, with biological parents having greater influence. Similarly, Miles and Carey's meta-analysis of 24 studies concluded aggressive behaviour is largely genetic, with this influence increasing with age.

Did you know? The MAOA gene might explain gender differences in aggression! The gene is linked to the X chromosome - women have two X chromosomes, so a healthy version can compensate for an abnormal one. Men have only one X chromosome, making them more vulnerable to MAOA-L expression and potentially explaining why males typically show more aggressive behaviour.

Measuring aggression presents challenges that complicate research. Studies using self-report methods tend to show stronger genetic influences, while observational studies show greater environmental contributions. This inconsistency, along with the fact that genes and environment interact (as in Caspi's study), suggests an interactionist approach is most appropriate for understanding aggression – neither nature nor nurture alone tells the complete story.