Chapter 4 Summary

• Sex is usually determined by the sex chromosomes: The XX pattern causes female development, and the XY pattern causes male development. The key player in male development is the gene SRY, on the Y chromosome, which induces the embryo’s genital ridges to become testes. In the absence of SRY, other genes induce the genital ridges to become ovaries.

• The male and female internal reproductive tracts develop from different precursors—the Wolffian and Müllerian ducts. In XY embryos, the testes secrete anti-Müllerian hormone (AMH), which causes the Müllerian ducts to regress, as well as androgens, which cause the Wolffian ducts to develop further and produce the male internal anatomy. In XY embryos lacking functional androgen receptors (a condition called androgen insensitivity syndrome), neither the male nor the female reproductive tract develops. In XX embryos, the lack of AMH allows the Müllerian ducts to develop further, and the lack of androgens allows the Wolffian ducts to regress, producing the female internal anatomy.

• The external genitalia of the two sexes develop from common precursors. The urethral folds give rise to the inner labia in females and to the shaft of the penis in males. The genital swellings give rise to the outer labia in females and the scrotum in males. The genital tubercle forms the glans of the clitoris in females and the glans of the penis in males. Male-typical development of the external genitalia requires the presence of testosterone and its conversion to 5α-dihydrotestosterone (DHT). In female fetuses that are exposed to high levels of androgens (as in congenital adrenal hyperplasia), the external genitalia are partially masculinized.

• Male and female brains differ in structure, chemistry, and function. Some sexual differentiation of the brain occurs prenatally—high levels of androgens drive male-typical brain development, and low levels permit female-typical development. At puberty and thereafter, estrogens become important in establishing and maintaining female-typical body structure and function and also influence the brain.

• Examples of atypical sex development include chromosomal anomalies such as Klinefelter syndrome (XXY or XXXY) and Turner syndrome (XO), as well as genetic conditions that affect sex hormone production (e.g., congenital adrenal hyperplasia) or the body’s sensitivity to sex hormones (e.g., androgen insensitivity syndrome). The proper treatment of children with ambiguous genitalia is a subject of controversy.

• Many psychological characteristics differ to a great or lesser extent between females and males. Gender identity is a person’s core sense of being a female, a male, or a combination of the two. Sex roles are the sets of behaviors that represent a person as male, female, or non-binary.

• On average, women outperform men in fine movements, verbal fluency, and some aspects of memory. Men outperform women in some cognitive traits, such as visuospatial skills. Personality differences include greater aggressiveness in men.

• In the area of sexuality, men and women differ in the strength of sex drive, interest in casual sex, interest in visual sexual stimuli, styles of jealousy, sexual orientation, interest in unusual forms of sexual expression, likelihood of engaging in coercive sex, sexual risk taking, willingness to pay for sex, frequency of masturbation, sexual response cycles, and the duration of reproductive capacity over the lifespan. Most psychological sex differences show considerable overlap between the sexes, and their significance is debated.

• Many gender differences arise early in life. Boys are typically more active and aggressive; girls are more interested in socializing. Boys and girls prefer different toys, and both prefer to associate with children of their own sex. Sex-specific interaction styles develop within these same-sex groups. Differences in other cognitive traits emerge gradually during childhood.

• Biological factors help create these sex differences. These include genes that have evolved to help men and women improve their reproductive success. A role for sex hormones, especially during prenatal life, is illustrated by experiments on animals, by observation of humans affected by endocrinological disorders, and by the study of anatomical markers (such as finger length ratios) that are correlated with gender traits.

• Socialization influences sex differences. This can happen through the innumerable rewards and punishments that children receive from parents and others. Imitation also has an important influence on sex roles.

• Several cognitive developmental models stress the importance of children’s thought processes in the development of gender identity. In sexual script theory, the learning of sex roles involves the social negotiation of roles, such as those to be played by the man and woman in heterosexual relationships.

• Transgender people are those whose gender identity does not fully correspond to their natal sex. Transexuals are transgender people who seek to change their anatomical sex: They may transition from male to female (transexual women) or from female to male (transexual men). The change may involve hormone treatment and sex-reassignment (gender confirmation) surgery, or just hormone treatment. All transexual women and some transexual men have a childhood history of strong gender nonconformity. They dislike the bodily changes induced by puberty and may attempt to conceal them. They are usually homosexual in the sense that they are sexually attracted to persons of the same natal sex as themselves. They usually do not identify as gay, however, but rather as heterosexual individuals of the sex with which they identify. Some transexual women are sexually attracted to women. Some or most of these individuals have a different developmental history, in which their desire to change sex develops from a wish to incorporate the sex characteristics of their preferred sexual partners (women) into their own bodies (autogynephilia).

• Transitioning is a multistage process. It includes assessment and psychotherapy, living for some period in the identity of the other sex, hormonal treatments, and, often, sex-reassignment surgery. Genitals can be transformed into those of the other sex, but the procedure is expensive and, particularly in the case of female-to-male reassignment, can yield inadequate results. Not all people who transition undergo genital surgery. Many transexual women and men are satisfied with the results of sex reassignment and are able to surmount the social and sexual challenges of post-transition life.

• Many transgender people do not seek sex reassignment, for a variety of reasons. They may not fully identify with either sex. Some believe that sex reassignment would be unnecessary if society could be persuaded to abandon its obsession with the binary nature of gender. All transgender people face the possibility of discrimination victimization, and many states fail to offer them specific protections.