This report examines the scientific evidence behind the recent controversial claim by Lea and colleagues (see below for a link to the on-line abstract) that the low activity variant of the monoamine oxidase-A (MAO-A) gene was strongly associated with risk-taking and aggressive behaviour in Mäori. The MAO-A gene is believed to be important for the correct regulation of dopamine and serotonin levels, via the production of the MAO-A enzyme which breaks down both neurotransmitters. The high activity gene variant (found in 65% of Caucasians) has up to a 10-fold greater activity than the low activity variant, and may therefore be more effective in removing excess dopamine and serotonin. In MAO-A deficient mice (which lack the enzyme entirely), dopamine and serotonin levels are increased and aggressive behaviours have been observed. The term “warrior gene” was first coined following a small, un-replicated experiment using Rhesus macaque monkeys. No evidence for an association between genotype alone and aggressive behavior was found in this study, and aggressive behaviour could be predicted by both low and high-activity gene variants under different environmental conditions. In the three largest population-based studies in humans (all in Caucasians) the results were all similar and suggested the high-activity MAO-A variant may be protective against adult anti-social behavior for children who were abused or neglected. The low-activity variant did not predict aggressive behaviour unless these additional environmental factors were also considered. The study presented by Lea et al. was conducted in a very small (n = 17) sample of Mäori males. They found the low-activity MAO-A variant present in 60% of subjects, and called it the “warrior allele”, suggesting it was strongly associated with risk taking and aggressive behaviour in Mäori males. The authors of the current report, dispute this finding, citing a lack of scientific rigour and an absence of appropriate genetic epidemiological experiments testing for an association. They also point to evidence of racial variation between genetic associations indicating that results from a Caucasian population should not be extrapolated to Mäori.
