During our lives, the brain changes as part of a dynamic developmental system that includes everything from nerve cells to interpersonal interactions. In principle, we can apply similar concepts to gonads and genitals. The gonads and genitals developed during fetal development continue to grow and change shape during childhood, affected by such things as nutrition, health status, and random accidents. At puberty anatomic sex expands to include not only genital differentiation but also secondary sex characteristics, which in turn depend not only on nutrition and general health but also on levels of physical activity. For example, women who train for long-distance events lose body fat, and below a certain fat-to-protein ratio, the menstrual cycle shuts down. Thus, gonadal structure and function respond to exercise and nutrition levels, and of course they also change during the life cycle.

Not only does sexual physiology change with age—so, too, does sexual anatomy. I don’t mean that a penis drops off or an ovary dissolves, but that one’s physique, one’s anatomical function, and how one experiences one’s sexual body change over time. We take for granted that the bodies of a new­born, a twenty-year-old, and an eighty-year-old differ. Yet we persist in a static vision of anatomical sex. The changes that occur throughout the life cycle all happen as part of a biocultural system in which cells and culture mutually construct each other. For example, competitive athletics leads both athletes, and a larger public who emulate them, to reshape bodies through a process that is at once natural and artificial. Natural, because changing patterns of diet and exercise change our physiology and anatomy. Artificial, because cultural practices help us decide what look to aim for and how best to achieve it. Fur­thermore, disease, accident, or surgery—from the transformations under­gone by surgical transsexuals, to the array of procedures (applied to secondary sexual characteristics) that include breast reduction or enlargement and pe­nile enlargement—can modify our anatomic sex. We think of anatomy as constant, but it isn’t; neither, then, are those aspects of human sexuality that derive from our body’s structure, function, and inward and outward image.

Reproduction also changes throughout the life cycle. As we grow, we move from a period of reproductive immaturity into one during which procreation is possible. We may or may not actually have children (or actually be fertile, for that matter), and when and how we choose to do so will profoundly affect the experience. Motherhood at twenty and at forty, in a heterosexual couple, as a single parent, or in a lesbian partnership is not a singular, biological expe­rience. It will differ emotionally and physiologically according to one’s age, social circumstance, general health, and financial resources. The body and the circumstances in which it reproduces are not separable entities. Here again something that we often think of as static changes across the life cycle and can be understood only in terms of a biocultural system.43

In their book Rethinking Innateness, the psychologist Jeffrey Elman and his colleagues ask why animals with complex social lives go through long periods of postnatal immaturity, which would seem to present big dangers: ‘‘vulnera­bility, dependence, consumption of parental and social resources.’’ ‘‘Of all primates,’’ they note, ‘‘humans take the longest to mature.’’44 Their answer: long periods of development allow more time for the environment (historical, cultural, and physical) to shape the developing organism. Indeed, develop­ment within a social system is the sine qua non of human sexual complexity. Form and behavior emerge only via a dynamic system of development. Our psyches connect the outside to the inside (and vice versa) because our multi­year development occurs integrated within a social system.45