affect immune function and health. Other studies also have shown that early life experiences involving social stressors are related to increased alcohol consumption (Fahlke et al., 2000) and dysregulated immune responses that last well into adulthood (Coe et al., 1989). However, some studies indicate that early life stressors actually enhance certain measures of immune function in adulthood (Coe et al., 1992). Nonhuman primate studies also have shown that exposure to mild early life stressors strengthens emotional and neuroendocrine stress responses in adulthood (Parker et al., 2005). Therefore, animal and human studies are needed to further examine the downstream psychophysiological and health consequences of variations in maternal care and other aspects of early life experience and to determine why factors such as early life stressors show adaptive effects in some studies but maladaptive effects in others.

Temperament

The earliest indications that social factors might affect individual health came from clinical observations of increased vulnerability to cancer and infectious disease among “socially withdrawn” individuals. A surprisingly large number of clinical studies have shown that socially inhibited or introverted individuals are at increased risk for immune-mediated infectious diseases, allergies, and hypersensitivity responses (Kagan et al., 1991; Cole et al., 1997; Cole et al., 1999; Cohen et al., 2003; Cole et al., 2003). Studies by Cavigelli and McClintock have demonstrated the long-term health consequences in rats of differences in temperament, such as increased fear of novelty (neophobia) and stress reactivity (Cavigelli and McClintock, 2003). Neophobia was measured using a modification of the open field arena that was designed to quantify an animal’s degree of locomotion and interaction with novel objects. The authors showed that males from the same litter that demonstrate a high degree of neophobia and corticosterone stress responses to novelty during infancy maintain these characteristics as adults. They also showed that the predominant cause of death is the development of tumors in neophobic and neophilic animals, and that high neophobic males die sooner than their low neophobic brothers. The authors suggest that increased neuroendocrine reactivity of the high neophobic animals may be a mechanism that contributes to increased mortality over the life span of the animal. These studies demonstrate the usefulness of using rodent models for conducting life-span studies.

Other studies of social and behavioral development have linked socially inhibited behavior to individual differences in central nervous system information processing, brain neurotransmitter activity, and reactivity of the autonomic nervous system and HPA to social stimuli (Kagan et al., 1988; Kalin et al., 1998; Miller et al., 1999; Byrne and Suomi, 2002; Schwartz et



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