Human Sexuality

III

Sexual Development

There are two periods of marked sexual differentiation in human life. The first occurs prenatally and the second occurs at puberty. Although adult women and men may differ greatly in genital appearance and secondary sexual characteristics, they are almost identical during prenatal development. When an egg and a sperm unite during fertilization, they each bring to the new cell half the number of chromosomes (threadlike structures that contain genetic material) present in other cells. From fertilization through about the first six weeks of development, male and female embryos differ only in the pair of sex chromosomes they have in each cell—two X chromosomes (XX) in females and one X and one Y chromosome (XY) in males. At this stage, both male and female embryos have undifferentiated gonads (ovaries or testes), two sets of ducts (one set capable of developing into male internal organs and the other into female organs), and undifferentiated external genital folds and swellings. See Embryology.

A

Prenatal Sexual Development

About six weeks after conception, if a Y chromosome is present in the embryo's cells (as it is in normal males), a gene on the chromosome directs the undifferentiated gonads to become testes. If the Y chromosome is not present (as in normal females), the undifferentiated gonads will become ovaries.

If the gonads become testes, they begin to produce androgens (male hormones, primarily testosterone) by about eight weeks after conception. These androgens stimulate development of the one set of the genital ducts into the epididymes, vas deferens, and ejaculatory ducts. The presence of androgens also stimulates development of the penis and the scrotum. The testes later descend into the scrotum. Males also produce a substance that inhibits the development of the second set of ducts into female organs. In the absence of such hormonal stimulation, female structures develop.

Prenatal hormones also play a role in the sexual differentiation of the brain. For example, prenatal hormones direct the development of sex differences in some cells and the neural pathways in the hypothalamus (the part of the brain that controls the endocrine system). Beginning at puberty, based on prenatal sexual differentiation, the hypothalamus directs either the cyclic secretion of sex hormones that controls the female menstrual cycle or the relatively continuous production of male sex hormones. Other brain differences may be related to differences in sexual and aggressive behavior or in cognitive and perceptual characteristics. Most of the research on sexual differentiation of the brain has been performed with animals or with biased human samples, and there is much debate about the nature and behavioral relevance of these differences in humans.

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B

Childhood

After birth, the process of sex-role socialization begins immediately. There may be small, physiologically-based differences present at birth that lead girls and boys to perceive the world or behave in slightly different ways. There are also well-documented differences in the ways that boys and girls are treated from birth onward. The behavioral differences between the sexes, such as differences in toy and play preference and in the degree of aggressive behavior, are most likely the product of complex interactions between the way that the child perceives the world and the ways that parents, siblings, and others react to the child. The messages about appropriate behavior for girls and boys intensify differences between the sexes as the child grows older.

It is not uncommon for children to touch or play with their genitals or to play games, such as “doctor” or “house,” that include sexual exploration. Such experiences are usually not labeled sexual by the children. Adults will often discourage such behavior and respond negatively to it. Generally by the age of six or seven, children develop a sense of privacy and are aware of social restrictions on sexual expression.

As the first bodily changes of puberty begin, sometime from the age of 8 to the age of 12, the child may become self-conscious and more private. During this period, more children gain experience with masturbation (self-stimulation of genitals). Surveys indicate that about one-third of all girls and about half of all boys have masturbated to orgasm by the time they reach the age of 13, boys generally starting earlier than girls. Because preadolescents tend to play with others of their own sex, it is not at all uncommon that early sexual exploration and experience may happen with other members of the same sex.

C

Puberty

Puberty marks the second stage of physical sexual differentiation—the time when both primary and secondary sexual characteristics as well as adult reproductive capacity develop, and when sexual interest surges. Puberty typically begins in girls from 8 to 12 years of age, whereas boys start about two years later. The hypothalamus initiates pubertal changes by directing pituitary growth hormones and gonadotropins (hormones that control the ovaries and testes).

A girl's breasts grow, her pubic hair develops, and her body grows and takes on the rounded contours of an adult woman. This is followed by the first menstrual period (menarche) at about age 12 or 13 (although ages of onset range from 10 to 16.5), underarm-hair growth, and increased secretions from oil- and sweat-producing glands. It may take a year or two before menstruation and ovulation occur regularly. The hormones primarily responsible for these changes in young girls are the adrenal androgens, estrogens, progesterone, and growth hormone.

During puberty, a boy's testes and scrotal sac grow, his pubic hair develops, his body grows and develops, his penis grows, his voice deepens, facial and underarm hair appear, and secretions from his oil- and sweat-producing glands increase. Penile erections increase in frequency, and first ejaculation (thorarche) typically occurs sometime from the age of 11 to the age of 15. For a boy who has not masturbated, a nocturnal emission, or so-called wet dream, may be his first ejaculation. The ability to produce sperm may take another year or two and typically begins at about age 14. Growth hormone and androgens, particularly testosterone, are responsible for these pubertal changes in boys.

The fact that boys tend to develop more slowly than girls can cause some social awkwardness. Girls who have grown earlier may find themselves much taller than their dates, for example, and they may be more physically and psychologically mature than their male peers.

The first menstruation and first ejaculation are often considered the most important events of puberty, particularly for the individual. However, it is the development of the secondary sexual characteristics that serve as more apparent signals to others that the person is becoming a man or a woman. These signals lead to increasingly differential treatment of adolescent girls and boys by parents or other adults. The changes in hormone levels that occur during puberty may cause boys and girls to perceive the world in different ways, leading them to react differently to situations. Thus, puberty augments behavioral sex differences between young men and women. In some cultures and religions, puberty is recognized with rituals that mark the transition into adulthood.

D

Adolescence

Whereas the term puberty refers to the period of physical maturation, the term adolescence typically refers to the socially defined period during which a person adjusts to the physical, emotional, and social changes associated with the transition from childhood to adulthood. Adolescence, which occurs from about the age of 12 to the age of 17 or older, is a period marked by increased sexual behavior. By the end of adolescence, two-thirds of young women and almost all young men have masturbated to orgasm. In recent decades, surveys indicate that more adolescents have begun engaging in intercourse at a younger age. However, studies of college students often find that 20 to 30 percent of these students have not had sexual intercourse. Adolescence can be particularly difficult for teens who feel different from their peers. Sexually active adolescents may wonder if their peers are abstinent, while sexually inactive adolescents may believe that their peers are sexually active. Others may struggle with same-sex attractions.

Sexual orientation may become a question during puberty or adolescence. The term sexual orientation refers to a person's erotic, romantic, or affectional attraction to the other sex, the same sex, or both. A person who is attracted to the other sex is labeled heterosexual, or sometimes straight. A person attracted to the same sex is labeled homosexual. The word gay may be used to describe homosexuals and is most often applied to men, whereas the term lesbian is applied to homosexual women. A person who is attracted to both men and women is labeled bisexual. A transsexual is a person whose sense of self is not consistent with his or her anatomical sex—for example, a person whose sense of self is female but who has male genitals. Homosexuality is not synonymous with transsexuality. Homosexual men's sense of self is male and lesbian women's sense of self is female.

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