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14-3 How do sex hormones influence prenatal and adolescent sexual development, and what is a disorder of sexual development?

In many physical ways—regulating heat with sweat, preferring energy-rich foods, growing calluses where the skin meets friction—men and women are similar. When looking for a mate, men and women also prize many of the same traits. They prefer having a mate who is kind, honest, and intelligent. But, say evolutionary psychologists, in mating-related domains, guys act like guys whether they’re chimpanzees or elephants, rural peasants or corporate presidents (Geary, 2010).

Biology does not dictate gender. But in two ways, biology influences gender:

These two influences began to form you long before you were born.

Six weeks after you were conceived, you and someone of the other sex looked much the same. Then, as your genes kicked in, your biological sex—determined by your twenty-third pair of chromosomes (the two sex chromosomes)—became more apparent. Whether you are male or female, your mother’s contribution to that chromosome pair was an X chromosome. From your father, you received the one chromosome out of 46 that is not unisex—either another X chromosome, making you female, or a Y chromosome, making you male.

About seven weeks after conception, a single gene on the Y chromosome throws a master switch, which triggers the testes to develop and to produce testosterone, the principal male hormone that promotes development of male sex organs. (Females also have testosterone, but less of it.)

Later, during the fourth and fifth prenatal months, sex hormones bathe the fetal brain and influence its wiring. Different patterns for males and females develop under the influence of the male’s greater testosterone and the female’s ovarian hormones (Hines, 2004; Udry, 2000).

A flood of hormones triggers another period of dramatic physical change during adolescence, when boys and girls enter puberty. In this two-year period of rapid sexual maturation, pronounced male-female differences emerge. A variety of changes begin at about age 11 in girls and at about age 12 in boys, though the subtle beginnings of puberty, such as enlarging testes, appear earlier (Herman-Giddens et al., 2012). A year or two before the physical changes are visible, boys and girls often feel the first stirrings of sexual attraction (McClintock & Herdt, 1996).

Girls’ slightly earlier entry into puberty can at first propel them to greater height than boys of the same age (FIGURE 14.2 below). But boys catch up when they begin puberty, and by age 14, they are usually taller than girls. During these growth spurts, the primary sex characteristics—the reproductive organs and external genitalia—develop dramatically. So do secondary sex characteristics. Girls develop breasts and larger hips. Boys’ facial hair begins growing and their voices deepen. Pubic and underarm hair emerges in both girls and boys (FIGURE 14.3 below).

For boys, puberty’s landmark is the first ejaculation, which often occurs first during sleep (as a “wet dream”). This event, called spermarche, usually happens by about age 14.

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In girls, the landmark is the first menstrual period, menarche, usually within a year of age 12½ (Anderson et al., 2003). Genes play a major role in predicting when girls experience menarche (Perry et al., 2014). But environment matters, too. Early menarche is more likely following stresses related to father absence, sexual abuse, insecure attachments, or a history of a mother’s smoking during pregnancy (DelPriore & Hill, 2013; Rickard et al., 2014; Shrestha et al., 2011). In various countries, girls are developing breasts earlier (sometimes before age 10) and reaching puberty earlier than in the past. Suspected triggers include increased body fat, diets filled with hormone-mimicking chemicals, and possibly greater stress due to family disruption (Biro et al., 2010, 2012; Herman-Giddens, 2012).

Girls prepared for menarche usually experience it positively (Chang et al., 2009). Most women recall their first menstrual period with mixed emotions—pride, excitement, embarrassment, and apprehension (Greif & Ulman, 1982; Woods et al., 1983). Men report mostly positive emotional reactions to spermarche (Fuller & Downs, 1990).

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Sometimes nature blurs the biological line between males and females. When a fetus is exposed to unusual levels of sex hormones, or is especially sensitive to those hormones, the individual may develop a disorder of sexual development, with chromosomes or anatomy not typically male or female. For example, a genetic male may be born with normal male hormones and testes but no penis or a very small one.

In the past, medical professionals often recommended sex-reassignment surgery to create an unambiguous identity for some children with this condition. One study reviewed 14 cases of boys who had undergone early surgery and had been raised as girls. Of those cases, 6 had later declared themselves male, 5 were living as females, and 3 reported an unclear male or female identity (Reiner & Gearhart, 2004).

Sex-reassignment surgery can create distress among those not born with a disorder of sexual development. In one famous case, a little boy lost his penis during a botched circumcision. His parents followed a psychiatrist’s advice to raise him as a girl rather than as a damaged boy. Alas, “Brenda” Reimer was not like most other girls. “She” didn’t like dolls. She tore her dresses with rough-and-tumble play. At puberty she wanted no part of kissing boys. Finally, Brenda’s parents explained what had happened, whereupon “Brenda” immediately rejected the assigned female identity. He cut his hair and chose a male name, David. He eventually married a woman and became a stepfather. And, sadly, he later committed suicide (Colapinto, 2000).

The bottom line: “Sex matters,” concluded the National Academy of Sciences (2001). Sex-related genes and physiology “result in behavioral and cognitive differences between males and females.” Yet environmental factors matter too, as we will see next. Nature and nurture work together.