ductive hormone levels is common. Large numbers of women take exogenous hormones in the form of contraceptives, and even greater numbers of women are given estrogen replacement therapy in the postmenopausal period. A more limited but rapidly expanding subset of individuals consume steroid hormones to regulate body strength and endurance, particularly individuals who participate in competitive sports. And with the more widespread consumption of foods that contain phytoestrogens, environmental exposure to hormones is becoming an issue of greater importance.

A variety of events that occur over the course of a normal life can also significantly influence activity of the reproductive axis. Pregnancy and lactation are associated with profound changes in the functioning of the reproductive axis, fertility, sexual behavior, and maternal behavior. Common life stresses, including metabolic stresses associated with undernutrition or the increased energy expenditure of participation in chronic vigorous exercise can suppress the activity of the reproductive axis. And psychosocial stresses provide an even more common inhibition to the reproductive axis. Even if reproductive hormone secretion is maintained, these life events can markedly alter circulating levels of reproductive hormones and thus influence fertility and sexual behavior. Biodemographic studies need to track lifestyle choices and life events to allow an accurate conceptualization of factors influencing fertility outcomes in human relationships.

Lastly, it is important to keep in mind that there are dramatic individual differences in normal circulating levels of reproductive hormones, the amount of hormone needed to maintain normal reproductive physiology and sexual behavior, and the sensitivity of individuals to the various forms of stress-induced reproductive dysfunction. We are just beginning the daunting task of elucidating the systems in the brain that underlie these individual differences. However, it is likely that the task of understanding the role that individual differences play in contributing to fertility outcomes will be even more complex.

HORMONES INFLUENCING REPRODUCTIVE FUNCTION AND BEHAVIOR

Physiological Regulation by Reproductive Hormones

This section provides an overview of the hormones that comprise the reproductive axis, how they are regulated and secreted, and their physiological actions in the body (for more detailed information see Steiner and Cameron, 1989; and Griffin and Ojeda, 2000). Particular attention is given to issues that influence the types of measurements made in the field of biodemography.

Although many people think of reproductive function as a bodily func-



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