understood (Bachevalier, 1998; Vazquez, 1998). The maturity of these systems relative to parturition also differs markedly among species. In general, however, as shown in Figure 1, the developing stress- and threat-response systems develop earlier than systems involved in behavioral and emotional regulation. There is also only emerging data on the impact that disruptions in each of these systems have on one another. For example, there is some evidence from work with Rhesus monkeys that lesions to amygdalar, hippocampal, or orbital frontal regions altered behavioral responses to social threat; however, the effect on HPA axis functioning requires further study (Machado and Bachevalier, 2006, 2008). Regardless, it is clear that damage to these stress-defense systems early in life has profound effects on socioemotional development (e.g. decreased social interaction; Bachevalier, 1998; Malkova et al., 1997).
