reproductive opportunities. There is no evidence for such reproductive suppression in humans and the role of male-male competition during adolescence seems secondary to the role of energetic factors. However, the fact that the presence/absence of the father during early childhood has been linked to hormonal profiles (Flinn et al., 1996) suggests that male parental care may play a role as a social cue about later reproductive opportunities (see section on Cultural Variation).

Nonetheless, male-male competition would make adolescence a stressful time. In addition to a potential role in suppressing the onset of reproductive maturation, increased DHEA/S production during reproductive maturation may have been selected for because of its protective role in the stress response. Under conditions of stress such as those associated with major life transitions (Dorn and Chrousos, 1997), elevated cortisol is associated with anxiety (Frankenhaeuser, 1980), which may in turn interfere with social interaction and learning. As Fredrickson (2000) has put it, activation of the hypothalamic-pituitary-adrenol (HPA) axis may be associated with execution of narrow behavioral programs mediated by the limbic system. In contrast, DHEA/S has been associated with improved mood among HIV-positive men (Cruess et al., 1999) and improved mood and cognition in aging men (van Niekerk et al., 2001). Furthermore Boudarene and Legos (2002) report that low anxiety levels are associated with increased levels of DHEA/S during cognitive stress tests.

Importantly, DHEA/S is a cortisol antagonist, meaning that it acts to block the effects of cortisol on neurons in the brain (Kimonides et al., 1999; Yoo et al., 1996). Given these actions of cortisol and DHEA on the hippocampus for learning and memory (McEwen, 1999), increased levels of DHEA would alleviate the effects of cortisol in narrowing concentration (Stansbury and Gunnar, 1994) and lead to increased inputs to the hippocampus from the cortex allowing for greater possibility of flexible behavior and increased learning.

In the case of adolescent males for whom increased testosterone levels are associated with increases in sexual behavior (Halpern et al., 1998), learning may be very much focused on interactions with the opposite sex as well as competitive interactions with other males (Weisfield, 1999). Increasing levels of testosterone can be thought of as making such interactions increasingly salient and promoting impulses toward sex and aggression. Cortisol and DHEA/S may be thought of as having contrasting effects on the execution of these impulses. Increased levels of DHEA/S would promote their expression and relate them to a wider arrange of social inputs through its effects on the prefrontal cortex. Cortisol, on the other hand, would interfere with the broader associations of such behavior by blocking the formation of new memories by the hippocampus (see Figure 9-3 for a diagrammatic representation).

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