the animal is no longer accustomed to being around or near other animals. Therefore, rather than or in addition to being a stressor itself, social isolation may increase stress reactivity or stress responsivity, which may be a potential confounder if it is not the focus of study.

Studies using different species of voles have begun to elucidate genetic and hormonal mechanisms mediating complex social behaviors such as those involving monogamy versus polygamy (Young et al., 1998; Young et al., 2001). Male prairie voles show increased partner preference for a female with whom they are paired following stressful conditions that result in elevations of plasma corticosterone or following pharmacologically induced increases in plasma corticosterone, with females showing the opposite effect of exposure to stress (DeVries et al., 1996). Vasopressin-1a receptor (V1aR) gene transfer into the ventral forebrain region of male prairie voles (a monogamous species) increases affiliative behavior and strengthens partner preference (Pitkow et al., 2001). Interestingly, similar gene transfer into the ventral forebrain region of meadow voles significantly increased partner preference formation in this polygamous species (Lim et al., 2004), and transfer of vole V1aR in the rat septum increased social discrimination and social behavior in rats (Landgraf et al., 2003). In contrast, V1aR gene knockout mice show deficits in social recognition and anxiety-related behavior (Bielsky et al., 2004). Moreover, variations in microsatellite segments in the 5’ region of the transcription start site for the V1aR gene differs in terms of length and regulatory control of gene expression among different individuals and is associated with individual differences in receptor expression and behavioral characteristics (Hammock and Young, 2005).

These studies suggest that some complex social and behavioral traits may be strongly modulated by changes in gene expression in critical areas of the brain. Such differences in regulation and expression of genes, their effects on social behavior, and ultimately on health, need to be investigated further. Moreover, more complex models of social affiliation may come from nonhuman primates that have been shown to demonstrate reconciliatory behavior after aggressive encounters, which are thought to be important for maintaining cooperative social hierarchies (de Waal, 2000).

Evidence suggests that three contributing factors result in susceptibility to inflammatory and autoimmune disorders (Mason, 1991; Tsigos and Chrousos, 1994; Sternberg, 1995; Wick et al., 1998; Ermann and Fathman,