The so-called “warrior gene” is the reason babies have tantrums. And you thought it was creamed spinach. Okay, I lied. Actually, the first study on the effects of monoamine oxidase A (MAOA) on infants determined that the 3-repeat allele decreases behavioral regulation in Chinese baby girls. Crying was a controlled factor because it could represent fear rather than baby rage. Behavioral regulation was measured by gaze aversion from a menacing toy gorilla. Believe it or not, the inability to do this is associated with childhood externalizing behaviors like fighting and hitting.The only significant main effect of MAOA was in the girls, which was fodder for interesting discussion of the gender differences and developmental component of the gene’s expression. MAOA is subject to epigenetic methylation, but mostly just in women. The vast majority of that methylation seems to have already occurred by age 5, according to Wong et al. Increased methylation gives women more symptoms of alcohol and nicotine dependence but not antisocial personality disorder, according to Philibert et al. The warrior gene concerns a VNTR (variable number tandem repeat) promoter, but another study by Philibert et al recently discovered a second VNTR that seems to have a greater effect on antisocial personality disorder in women (but not men) than the heavily studied VNTR, and both VNTR influence MAOA methylation in women. The fact that the sex hormones testosterone and estradiol affect MAOA expression should also enter any thorough discussion of sex differences in the warrior gene. Such increasingly complex factors involved in MAOA expression in women are modifying a long-held view that MAOA does not affect women even when it is completely shut off in Brunner syndrome. Or as Dr. Phil put it, “[the warrior gene] is more rare in women, of course” which is not actually true, but at least he is trying.
Speaking of hormones, over the past decade many studies have examined the effects of testosterone and cortisol on aggression. One study determined that testosterone and cortisol even affect the militancy and aggression of Palestinians. Oddly, this research has never seemed to cross paths with research on the proven effects of testosterone and cortisol on MAOA expression. However, scientists have identified a gene-gene interaction between the androgen receptor and glucocorticoid receptor genes, so these might be violence genes just like MAOA, DAT1, DRD2, DRD4, and 5-HTTLPR.
Getting back to the study on babies, this new data also helps clarify the allele frequencies in Asians. I now count a nearly 1,500 cumulative allele sample size among those Asian subjects, predominantly Chinese, for whom selection bias does not apply, and 54% are the warrior gene, which I consider the three-repeat allele (although the much less common 2-repeat allele is also included under this label). This is not much different from that of African Americans, but nearly five percent of African American men have the more violent 2-repeat allele, compared to only one allele of Asian control subjects in the seven studies that I counted. A 2009 editorial that was included in a series of attacks on MAOA research as a reaction to the Rod Lea/Maori controversy claimed Chinese people have the highest warrior gene allele frequency of any ethnic group. That was based on an uncorrected error in The New Zealand Medical Journal that switched the number of subjects (77) with the percentage with the three-repeat allele (55%) from the study by Lu et al. One would think that researchers who study warriors would be more careful.
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