In some respects, it should not be surprising that putative vulnerability genes may actually function more like plasticity genes, resulting in certain individuals being more responsive than others to both positive and negative environmental experiences, including the simple absence of contextual adversity. Not only has plasticity been found to be heritable in many species,48, 49 functioning perhaps as a selectable character in and of itself,50 but recent computer simulations show that individual differences in responsiveness to the environment could most certainly evolve.51 In fact, one wild bird population shows evidence that selection favoring individuals who are highly plastic with regard to the timing of reproduction has intensified over the past three decades, perhaps in response to climate change causing a mismatch between the breeding times of the birds and their caterpillar prey.52 Of note too is Suomi's53 observation that only two species of primates fill diverse ecological niches around the world, humans and rhesus macaques, and that what distinguishes both of these ‘weed species', as he calls them, from all other primates is the presence of 5-HTTLPR short allele in
