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In the second week of August 2006, the media were gripped with the announcement that Maori men were genetically predisposed to "violence, criminal acts, and risky behaviour." (Christchurch Press, 9 August 2006). The basis for this release was the report of a study that purported to show that a genetic variant of the monoamine oxidase (MAO)-A gene, dubbed the "warrior gene" by the media, had previously been "strongly associated with risk taking and aggressive behaviour" and was "strikingly over-represented" in Maori men.1
Because these findings were released at an international conference by a government-funded researcher, there has generally been an assumption that the link between the MAO-A variant and aggression in Maori is based on robust scientific evidence. Here we place the scientific evidence under closer scrutiny.
The regulatory region of the human MAO-A gene has a genetic variation consisting of a small stretch of DNA repeated a variable number of times.2 The 3-repeat form of this 'variable number of tandem repeats (VTNR)' polymorphism (present on 33% of Caucasian chromosomes) exhibits up to 10-fold less activity than the 4-repeat form (present in 65% of Caucasian), and has been commonly termed the 'low-activity' variant.2
The MAO-A gene controls the production of the MAO-A enzyme, which is involved in the breakdown of several neurotransmitters in the brain, such as dopamine and serotonin. It is believed that this enzyme is important in preventing the build-up of an excess of these transmitters. In MAO-A deficient mice that lack the enzyme altogether, increased levels of neurotransmitters were documented and the mice were observed to exhibit aggressive behaviour.3
In a Dutch family, a dysfunctional MAO-A gene has been linked to antisocial behaviour and increased aggression amongst humans.4 Based on this observation, the MAO-A VNTR has been tested for association with antisocial behaviour in population-based male cohorts. The three largest studies (all in groups of Caucasian ethnicity) published remarkably consistent findings; no main effect was found for the relationship between the MAO-A VNTR genotype...