Sunday, December 10, 2006

MAO-A and personality   posted by agnostic @ 12/10/2006 09:40:00 PM

Human Genetics has an article (Rosenberg et al 2006) on variation at the monoamine oxidase A gene -- the warrior gene -- and phenotypic personality variation in normal males. Presumably only males were chosen since previous studies (reviewed at OMIM) found a sex-by-genotype interaction, such that associations were noted in males but not females. Unlike previous studies, this one sought to uncover links between genetic variants and the full spectrum of personality traits measured by the NEO-PI (the Big Five questionnaire). In brief, no associations were found with any facets of Openness or Neuroticism; an impressive link was found between the VNTR and the facet of Agreeableness called "Straightforwardness," as well as with the facet of Extraversion called "Activity;" and a less robust link was found between haplotype and the facet of Conscientiousness called "Order."

If you read the article, I would treat as significant any result to which they've applied a Bonferroni correction to reflect 5 independent hypotheses being tested (1 per trait investigated). I assume they had their arms twisted by a referee or someone to also report corrections to reflect 30 hypotheses being tested -- 6 lower-level facets per 5 personality traits -- but the way that questionnaires are designed is so that the facets of traits like Agreeableness all intercorrelate. For example, the hypothesis that an allele correlates with being antagonistic is not independent of the hypothesis that it correlates with mistrusting the motives of others, since these facets of Agreeableness correlate with each other.

It is known that the MAO-A locus has been under positive selection*, and because its effect on phenotype is to generally make people less Agreeable, one assumes there are alleles at modifying loci that have co-evolved to keep the individual from becoming too much of a nasty SOB. In the comments to a recent post on interracial offspring at my personal blog, the subject of hybrid depression came up. Here would be an interesting case to test: if a child were born to parents from population A, in which the "warrior" variant has high frequency, and population B, in which the variant has low frequency, the kid might inherit the "warrior" variant but none of the modifying alleles. It would be like a bad cop paired with -- not a team of "good cops," in the sense of seasoned partners who serve to check the loose cannon -- but with a team of Alaskan mall cops, over whom the bad cop would run roughshod. Alternatively, the kid might inherit the "docile" variant but would also inherit the modifying alleles from the parent with the warrior variant. Thus, there would be a team of restraining cops with no bad cop to contain, perhaps resulting in an overly meek and credulous phenotype. Technically, this latter scenario should fall under hybrid vigor, since the value would be above expectation based on additive factors; I just find it hard to infuse "born sucker" with a positive connotation.

*For free full text of the article, click on "Full text (PDF)" at the link here. A replication of the finding of positive selection is here.