The Genetics of Aggression
Growing evidence suggests that genes, as well as environmental factors, play a key role in aggressive behavior. A new study just published in the Proceedings of the National Academy of Sciences (PNAS) by myself and colleagues (Rose McDermott, Dustin Tingley, Jonathan Cowden, and Giovanni Frazetto), provides the first experimental evidence—in a controlled laboratory setting—that a single gene influences people’s level of aggression.
Previous studies had found a correlation between observed and self-reported aggression and a particular gene called Monoamine oxidase A (MAOA). This gene regulates an enzyme that breaks down key neurotransmitters in the brain, including dopamine, norepinephrine, and serotonin. Humans have various forms of the MAOA gene, resulting in different levels of enzymatic activity: people with the low activity form produce less of the enzyme, while those with the high activity form produce more of the enzyme. Only about 1/3 of people in western populations have the low activity form of MAOA. By comparison, low activity MAOA has been reported to be much more frequent (approaching 2/3 of people) in some populations that had a history of warfare, which led to the “warrior gene” controversy.
Our paper is the first experimental test of whether low activity MAOA individuals display higher levels of actual, behavioral aggression in response to provocation. Subjects genotyped for MAOA played a “power-to-take” game over networked computers. Each subject first performed a vocabulary task in which they earned real money. Then they were told that an anonymous partner, linked over the network, could choose to take some of their earnings away from them. The original subject could then punish the taker by forcing them to eat unpleasantly hot (spicy) sauce—but they had to pay to do so, so punishment was costly. In fact, the “partner” who took money away was a computer, which allowed us to standardize provocation treatments and no one had to burn their mouths out.
The results showed strong evidence for a gene-by-environment interaction, such that MAOA was not associated with the occurrence of aggression when provocation was low (that is, when 20% of their money was taken from them), but MAOA signiﬁcantly predicted aggression when provocation was high (when 80% of their money was taken from them).
Our study therefore supports previous research suggesting that MAOA influences aggressive behavior, with potentially important implications for interpersonal aggression, violence, political decision-making, and crime. A genetic influence on costly aggression also questions the recently proposed idea that humans are ‘‘altruistic’’ punishers (who willingly punish free-riders for the good of the group). Instead, our results support theories of cooperation that propose there are mixed strategies in the population—some people may punish more than others, and there may be an underlying evolutionary logic for doing so.
The full paper “Monoamine oxidase A gene (MAOA) predicts behavioral aggression following provocation” is available at the “early edition” of Proceedings of the National Academy of Sciences (PNAS), and will shortly be available on my website.