Timing of reproductive maturation by itself seems unlikely to play a major role in fertility behavior in males. In many societies reproduction does not start until the mid-20s long after reproductive maturation is complete and late maturers have had a chance to catch up with early maturers in terms of the impact of testosterone and its behavioral and somatic effects. For instance, early maturers start the pubertal growth spurt earlier and go through it with greater intensity; they are not taller than late maturers at the end of puberty (Bourguignon, 1988; Vizmanos et al., 2001). Thus, although height has been related to male fertility (Pawlowski et al., 2000) as well as sperm count (Manning et al., 1998), variation in height is not established at puberty.
Instead, pubertal timing in males is likely a proxy for individual variation in the activity of the hypothalamic-pituitary-testicular axis established early in development and persisting into adulthood (Bribiescas, 2001). Variations in testosterone levels as well as production rates in Western populations are quite heritable (Handelsman, 1997; Meikle et al., 1997), suggesting that individual differences in testosterone and its effects on behavior go back at least to prenatal development. For instance, testosterone has been related to aggression in both preschool boys (Sanchez-Martin et al., 2000) and prepubertal adolescent males (Gerra et al., 1998). Scerbo and Kolko (1994) report that testosterone was related to aggression in a sample of disruptive prepubertal children, although Constantino et al. (1993) did not find elevated testosterone and aggression in 18 highly aggressive prepubertal boys compared to normal controls.
Such a developmental model may help explain recent findings of genetic influences on the timing of first intercourse among adolescent boys (Miller et al., 1999; Rodgers et al., 1999). Rodgers et al.’s findings fail to account for the effects of pubertal timing, which presumably has genetic elements in boys, as it does in girls (Campbell and Udry, 1995). On the other hand, Miller et al.’s findings that variation in genes for dopamine receptor type helps account for variation in age at first sexual intercourse in a sample of non-Hispanic European American men and women, even controlling for a variety of psychosocial factors, is compatible with individual variation in risk taking, given evidence for dopamine in impulsivity among adolescents with attention-deficit disorder with hyperactivity (Rosa Neto et al., 2002).
However, Rodger et al.’s genetic findings hold for European American males, but not females and not African American males, suggesting an important interaction of genetic factors with the social context in which behavior is expressed. We will come back to this point in our discussion of cultural variation in the development of male fertility behavior.