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FEW WORDS ARE MORE LOADED WITH meaning than the word “sex.” Sexual activity is a biological imperative and a major human preoccupation. The physical differences between men and women that underlie partner recognition and reproduction are obvious to all of us, and their developmental origins are well understood. In contrast, our understanding of behavioral differences between the sexes is primitive. In many cases, their very existence remains controversial, and the origins of those that have been clearly demonstrated remain unclear.

In this chapter, we first briefly summarize the embryological basis of sexual differentiation. We then discuss at greater length the behavioral differences between the two sexes, focusing on those differences or dimorphisms for which some neurobiological basis has been found. These dimorphisms include physiological responses (erection, lactation), drives (maternal behavior), and even more complex behaviors (gender identity). In analyzing these dimorphisms, we will discuss three issues.

First, what are the genetic origins of sexual differences? Human males and females have a complement of 23 chromosomal pairs, and only one differs between the sexes. Females have a pair of X chromosomes (and are therefore XX), whereas males have one copy of the X chromosome paired with a Y chromosome (XY). The other 22 chromosome pairs, called autosomes, are shared between males and females. We will see that the initial genetic determinants arise from a single gene on the Y chromosome, while later ones arise indirectly from sex-specific patterns of expression imposed upon other genes as development proceeds.

Second, how are sexual differences initiated by the Y chromosome translated into differences between the brains of men and women? We will see that key intermediates are the sex hormones, a set of steroids that includes testosterone and estrogens. These hormones act during embryogenesis as well as postnatally, first organizing the physical development of both genitalia and brain regions, and later activating particular physiological and behavioral responses. Hormonal regulation is especially complex because the nervous system, which is profoundly influenced by sex steroids, also controls their synthesis. This feedback loop may help to explain how the external environment, including social and cultural factors, can ultimately shape sexual dimorphism at a neural level.

Third, what are the crucial neural differences that underlie sexually dimorphic behaviors? Clear physical and molecular differences between the brains of men and women have been found. These differences reflect differences in neural circuitry between the sexes, and in a few cases, these distinctions in connectivity are directly related to behavioral differences. In other cases, however, sexually dimorphic behaviors appear to result from differential usage of the same basic circuits.

Before proceeding, we must define two words that are commonly used in many ways and sometimes confused with each other: sex and gender. As a descriptor of biological differences between men and women, the word sex is used in three ways. First, anatomical sex refers to ...

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