The effect of genotype on outcomes of interest may have been missed had we not examined the susceptibility hypothesis. Belsky and Pluess (2009) point to the susceptibility hypothesis as a compelling account of the manner in which genetic variability may exert its influence, one that does not require genotypic main effects and fits with broader evolutionary theorizing (Belsky, 1997, 2005). In the case of DRD4, increased susceptibility to variation in parenting behavior and decreased responsiveness of the dopaminergic reward pathway may have been particularly adaptive in contexts of deprivation (Campbell & Eisenberg, 2007), which characterized much of human evolutionary history. At a minimum, some advantages of the DRD4 7-repeat allele seem likely because it appears to have arisen as a rare variant that increased substantially through positive selection over the past 40,000-50,000 years (Ding et al., 2002).
