reproductive axis is crucial to sexual behavior, whereas development of the adrenal axis is crucial to social behavior and attachment and hence the expression of sexual behavior, as fertility and family formation. It is only by understanding the separate roles of these two important domains of human behavior and their interaction during maturation that we can get a differentiated account of the development of male reproductive behavior.

Of course, any such model represents a vast simplification of all that goes into male reproductive maturation. At the same time, it is intended to be a human model in the essential sense of the word, a model that integrates factors we encounter as part of our everyday existence, without necessarily experiencing them fully or in isolation. As such it should be applicable to variation across societies, including variation in social structures, and biological differences in the timing of adolescent maturation, as well as cultural practices that may have independent effects on behavior.

Finally, much of what will be discussed here is necessarily speculative. Thus, I conclude with recommendations for research that might elaborate and test how these hormones mediate the interaction of biological, psychological, and social levels of fertility behavior in human males.

THE SCOPE OF MALE REPRODUCTIVE MATURATION

Most biological analyses of male reproductive maturation focus exclusively on adolescence because of the obvious connection between pubertal maturation and the onset of sexual behavior. However, the onset of reproductive capacity and sexual behavior may be considered a necessary, but not sufficient, condition for reproduction among human males for whom pair bonding and paternal investment play an additional role (Lancaster and Lancaster, 1987). Thus, changes in familial relationships (Bee, 1997), cognition (Nelson, 1996), and the subjective sense of self (Herdt and McClintock, 2000) that begin prepubertally may be considered as part of the reproductive transition.

In addition, reproduction among males is generally associated with young adulthood and family formation, which do not take place until the early 20s. For instance, in the United States only 3 percent of births are fathered by men under the age of 20, while traditional Kikuyu men marry and become fathers around the age of 25 (Bogin, 2001). Thus, the establishment of productive skills and the development of social position can also be considered part of the process of male reproductive maturation (see Kaplan and Lancaster, this volume).

Furthermore, juvenile, adolescent, and young adult stages appear to be linked as a coherent biological process by increases in the production of DHEA/S by the adrenal gland (because of the close relationship of the two hormones, I will refer to them here collectively as DHEA/S unless discussing



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