Aggression, digit ratio, and variation in the androgen receptor, serotonin transporter, and dopamine D4 receptor genes in African foragers: the Hadza.

The role of genes in the expression of aggression and masculinity traits in humans has been a focus of recent behavioral genetic studies. This is the first study on the variation in aggression, the digit ratio (the ratio between the second and the fourth digits, 2D:4D), the directional asymmetry in 2D:4D (D(R-L)) and polymorphisms of the AR, DRD4, and 5-HTTL genes in simple hunter-gatherers, namely the Hadza of Tanzania (142 adult men). The distribution of AR, DRD4E3, and 5-HTTLPR genotypes and allele frequencies in Hadza was compared to other African populations on which the data were available. Hadza and Ariaal differed significantly in the distributions of frequencies of AR alleles with different numbers of CAG repeats. Hadza population was similar to other African populations in the distribution of allelic frequencies of the DRD4E3 locus, and to Afro-Americans in the distribution of allelic types of the 5-HTTLPR locus. We found no influence of AR gene on the right hand 2D:4D ratio, D(R-L), and any of aggression subscales of the Buss-Perry Aggression Questionnaire (AQ). Although, a weak positive correlation between CAG repeats and the left hand 2D:4D was found. The multiple regression analysis with digit ratios, D(R-L) and aggression subscales of AQ as dependent variables and the three gene candidates (AR, DRD4E3, and 5-HTTLPR) as independent variables revealed the following: men with lower number of CAG repeats had significantly lower left hand 2D:4D ratio; men with higher numbers of 48-bp unit copies in exon 3 of a VNTR polymorphism in the DRD4 gene had significantly lower digit ratios on both hands; no effect of the 5-HTTLPR gene on either the digit ratio or aggressive behavior. These findings demonstrate the complexity of gene effects on digit ratios and aggression and call for simultaneous analysis of more candidate genes. It is noteworthy that these results were obtained for a human population that is still practicing foraging and has been subjected to a high selective pressure due to harsh environments and practically has no access to modern medical care. Hadza are highly egalitarian, and their culture does not favor persons with a dominant or aggressive behavior. It is still to be found to what extent the relationships observed in this study are similar to those in other human populations.