Individuals with the so-called warrior gene display higher levels of aggression in response to provocation, according to new research co-authored by Rose McDermott, professor of political science at Brown University. In the experiment, which is the first to examine a behavioral measure of aggression in response to provocation, subjects were asked to cause physical pain to an opponent they believed had taken money from them by administering varying amounts of hot sauce. The findings are published in the Proceedings of the National Academy of Sciences. Monoamine oxidase A is an enzyme that breaks down important neurotransmitters in the brain, including dopamine, norepinephrine, and serotonin. The enzyme is regulated by monoamine oxidase A gene (MAOA). Humans have various forms of the gene, resulting in different levels of enzymatic activity. People with the low-activity form (MAOA-L) produce less of the enzyme, while the high-activity form (MAOA-H) produces more of the enzyme. Several studies have found a correlation between the low-activity form of MAOA and aggression in observational and survey-based studies. Only about a third 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 two-thirds of people) in some populations that had a history of warfare. This led to a controversy over MAOA being dubbed the warrior gene. The results demonstrate that low-activity MAOA subjects displayed slightly higher levels of aggression overall than high-activity MAOA subjects. There was strong evidence for a gene-by-environment interaction, such that MAOA is less associated with the occurrence of aggression in the low-provocation condition (when the amount of money taken was low), but significantly predicted aggression in a high-provocation situation (when the amount of money taken was high). The results support previous research suggesting that MAOA influences aggressive behavior, with potentially important implications for interpersonal aggression, violence, political decision-making, and crime. The finding of genetic influences on aggression and punishment behavior also questions the recently proposed idea that humans are "altruistic" punishers, who willingly punish free-riders for the good of the group. These 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.
MAOA, aggression and behavioral economics