4

Genes and Hormones Guide Sex Development
Testes and ovaries form from the precursor gonads
The gene that instructs the ridges to differentiate into testes is called SRY
This gene is located on the Y chromosome

Figure 4.1 Human chromosomes

Male and Female Reproductive Tracts Develop from Different
Precursors
Embryos of both sexes possess both Wolffian and
Müllerian ducts
• Wolffian ducts are precursors of the male reproductive tract
• Müllerian ducts are precursors of the female reproductive tract
In males, testes produce antiMüllerian hormone (AMH) to eliminate the female ducts
In females, the absence of AMH allows Müllerian ducts to persist and to develop into the oviducts, uterus, and deeper parts of the vagina

Figure 4.2 Development of the male and female reproductive tracts

Male and Female External Genitalia Develop from the Same
Precursors
Female and male external genitalia have the same developmental origin
Most prenatal sex development occurs during weeks 8 through 24 of gestation, when testosterone levels are high in male fetuses
The testicles descend during development

Figure 4.3 Development of the male and female external genitalia (Part 1)

Figure 4.3 Development of the male and female external genitalia (Part 2)

Figure 4.3 Development of the male and female external genitalia (Part 3)

Figure 4.4 Descent of the testicles (Part 1)

Figure 4.4 Descent of the testicles (Part 2)

The Brain Also Differentiates Sexually
There are differences in brain structure, function, and chemistry between the two sexes
• These differences are brought about by higher levels of androgens in males than in females during development

Sex Development May Go Awry
Chromosomal anomalies that do not fall under typical female (46,XX) or male (46,XY) chromosomal complements affect growth and fertility
• Klinefelter syndrome (XXY, XXXY)
• Turner syndrome (XO)
• XYY syndrome
• Triple-X syndrome
The gonads or genitals may be sexually ambiguous
• Gonadal intersexuality
• Congenital adrenal hyperplasia (CAH)
• Androgen Insensitivity Syndrome (AIS)

Figure 4.6 Partial masculinization of genitalia in a girl with congenital adrenal hyperplasia

Box 4.1 Personal Points of View: My Life With Androgen Insensitivity Syndrome

Gender Is a Central Aspect of Personhood
Gender identity is the sense of which sex one belongs to.
For some people, gender identity does not match anatomical sex
This identity is expressed via gender role behavior
Sexual orientation, cognition, and personality traits differ between men and women
Differences in sexuality include attitudes toward casual sex, jealousy, and frequency of masturbation

Figure 4.7 Mental rotation task

Figure 4.8A Test of object location memory

Figure 4.8B Test of object location memory

Gender Is a Central Aspect of Personhood
Many gender differences arise at a young age
• By about 3 years of age, most children show evidence of gender constancy
• By about 1 year of age, most children can distinguish between men and women
• Differences in toy preferences

Figure 4.9 Gender constancy

Figure 4.10 Toy preference test

Biological Factors Influence Gender
Evolutionary factors influence gender development
• Evolutionary factors:
 Cognitive differences between the sexes due to long-standing division of labor between men and women
 Difference in interest in casual sex between men and women
 Jealousy

Figure 4.11 Monkeys show humanlike toy preferences

Experiments Demonstrate a Role for Sex Hormones
Girls with CAH have behavioral traits shifted in the masculine direction
Prenatal androgens influence gender
The 2D:4D ratio correlates with many gendered characteristics

Box 4.2 Biology of Sex: “Eggs at Twelve”

Box 4.3 Personal Points of View: The Boy Who Was Raised as a Girl

Figure 4.12 Hormones and play

Figure 4.13 Finger-length ratio and gender

Life Experiences Influence Gender
Life experiences influence gender development beginning early in life
• Observing socialization
• Rewards and punishments
• Imitation
Cognitive models focus on thought processes
• Gender schemas
• Sexual scripts

Figure 4.14 Babies enter a gendered world

Figure 4.15 Influence of siblings on gender

Figure 4.16 The media influence gender

Figure 4.17 Influence of gender stereotypes on children’s memories

Transgendered People Cross Society’s Deepest Divide
Unhappiness caused by discordance between anatomical sex and gender identity is called gender dysphoria
• F-to-M transexuals (trans men)
• M-to-F transexuals (trans women)
 Transvestism
 Autogynephilia

Transgendered People Cross Society’s Deepest Divide
• Sex-reassignment is a multistage process
• Some transexuals do not desire surgery
• Some medical centers treat pre-pubertal children affected by gender dysphoria
• Transgenders and transexuals struggle for awareness and acceptance

Figure 4.19 Chastity Bono transitioned to Chaz Bono in 2009

Figure 4.20 The vulva after sex-reassignment surgery

Figure 4.21 Transformation of the clitoris into a small penis

Box 4.5 Controversies: How Should We Treat Gender-Dysphoric Children?

Figure 4.22 Kate Bornstein