Sexual Differentiation, Hormones, and Sexual Behavior

165 question-and-answer flashcards covering sexual differentiation, hormone action, intersex conditions, sexual orientation, and related neuroendocrine concepts.

What is the main benefit of sexual reproduction between two individuals?

It increases variation in the gene pool, enabling rapid evolutionary adaptation and correction of genetic errors.

What event initiates prenatal sexual differentiation in humans?

The chromosomal complement—XX for females and XY for males.

During early prenatal development, which two types of ducts are present in all embryos along with undifferentiated gonads?

Müllerian ducts and Wolffian ducts.

Into which adult structures do the Wolffian ducts develop?

The vas deferens and seminal vesicles.

Into which adult structures do the Müllerian ducts develop?

The oviducts, uterus, and upper vagina.

What is the SRY gene?

A gene on the Y chromosome that directs the undifferentiated gonads to become testes.

What two substances do developing testes secrete?

Androgens and Müllerian-inhibiting hormone (MIH).

What happens to the undifferentiated gonads in the absence of the SRY gene?

They develop into ovaries.

Relative to estrogens, which class of hormones do testes produce in larger quantities?

Androgens.

What are androgens and estrogens?

Classes of steroid hormones, androgens being more abundant in males and estrogens more abundant in females.

What structural feature do all steroid hormones share?

Four carbon rings.

List three ways steroid hormones can influence target cells.

By binding to membrane receptors, entering cells to activate cytoplasmic proteins, and binding to chromosomes to alter gene expression.

What is the most widely known androgen?

Testosterone.

What is the most common estrogen?

Estradiol.

What is progesterone’s principal reproductive function?

It prepares the uterus for implantation and helps maintain pregnancy.

Besides hormonal influences, how else do genes contribute to sex differences?

Genes on the X and Y chromosomes themselves directly create sex differences.

Approximately how many Y-chromosome genes are active in specific brain areas?

At least three.

Is there a gene on the X chromosome that is active only in the female brain?

Yes.

When do the organizing effects of sex hormones primarily occur?

During sensitive developmental periods before birth and again at puberty.

What do organizing effects determine?

Whether the brain and body develop male versus female characteristics.

What are activating effects of sex hormones?

Temporary, reversible effects that can arise at any time, such as during the menstrual cycle or pregnancy.

Is the separation between organizing and activating effects absolute?

No; the two categories can overlap.

In humans, when is the sensitive period for external genital formation?

The first trimester of pregnancy.

What is the result of exposing female rats to testosterone during the sensitive period?

They become partly masculinised in anatomy and behavior.

What happens to male rats that lack functioning androgen receptors or are castrated early?

They develop female-like anatomy and behavior.

Do estrogens alter the external anatomy of male mammals?

No, but estrogens can influence internal structures such as the prostate.

In the complete absence of sex hormones during development, what type of external genitalia form?

Female-appearing external genitalia.

What is the consequence for females that lack estradiol early in life?

They fail to develop normal female sexual behavior.

Which major brain areas are particularly sensitive to early sex-hormone exposure?

The hypothalamus, amygdala, and related regions.

What is the sexually dimorphic nucleus (SDN)?

An anterior hypothalamic area larger in males that is critical for male sexual behavior.

How does the female hypothalamus typically regulate hormone release?

In a cyclic pattern.

During prenatal development, testosterone in the brain is enzymatically converted into which hormone?

Estradiol.

What is the role of alpha-fetoprotein in early development?

It binds circulating estradiol and keeps it from entering fetal cells.

Why can testosterone affect brain development even when alpha-fetoprotein is present?

Testosterone bypasses alpha-fetoprotein, enters neurons, and is then converted to estradiol inside the cell.

Exposure to testosterone or estradiol in the medial preoptic area (MPOA) has what effect?

It increases prostaglandin production and neuronal growth that support male sexual behavior.

What effect does estradiol have on neurons in the ventromedial nucleus of the hypothalamus?

It increases dendritic branching and promotes aggressive and sexual behaviors.

How does estradiol influence the arcuate nucleus and anteroventral periventricular (AVPV) nucleus differently in males and females?

It decreases branching in males but supports sexual behaviors and cyclic hormone release in females.

In general, what types of toys do young boys tend to prefer?

Vehicles, rough-and-tumble objects, and vigorous activities.

Are toy preferences stable over time?

Yes; they show consistency and reflect both biological and social influences.

Do infant rhesus monkeys display human-like sex differences in toy choice?

Yes, although the differences are smaller than in humans.

How does elevated prenatal testosterone affect girls’ later toy preferences?

It modestly increases preference for toys usually favored by boys.

How is high prenatal exposure to phthalates related to boys’ toy choices?

It is associated with greater interest in toys typically preferred by girls.

What is oxytocin and what are two of its reproductive roles?

A pituitary hormone that triggers uterine contractions and milk let-down.

In males, how does testosterone affect sensory responses associated with arousal?

It heightens touch sensitivity in the penis.

Name three brain areas in males on which sex hormones act to facilitate sexual behavior.

The medial preoptic area (MPOA), anterior hypothalamus, and ventromedial nucleus.

How does dopamine contribute to sexual arousal?

Moderate D1/D5 receptor stimulation promotes erection and lordosis, whereas higher dopamine levels activate D2 receptors leading to orgasm.

What is serotonin’s general effect on sexual activity?

It inhibits sexual activity by suppressing dopamine release.

How does circulating testosterone relate to men’s sexual interest?

Higher testosterone correlates with increased arousal and motivation to find partners.

How does being in a committed relationship typically affect men’s testosterone levels?

Married or long-term partnered men usually have lower testosterone concentrations.

What is the most common physiological cause of male impotence?

Impaired blood circulation to the penis.

Which drug enhances penile blood flow and is used to treat erectile dysfunction?

Sildenafil (Viagra).

What gradual change occurs in adult males after castration?

A slow decline in sexual interest and behavior.

How can sexual behavior be restored in castrated male animals?

By administering testosterone or drugs that stimulate dopamine receptors.

In women, sexual interest shows a mild rise during which phase of the menstrual cycle?

The periovulatory phase when estradiol peaks.

Name two social or sexual effects facilitated by oxytocin.

Enhanced pair bonding and increased trust and calmness.

What change in oxytocin levels is observed immediately following orgasm?

A pronounced increase.

In male prairie voles, what role does vasopressin play?

It promotes bonding with the female partner after mating.

Which hormone increases humans’ attention to social cues like facial expressions?

Oxytocin.

Contrast activating and organizing effects of hormones.

Activating effects are temporary and reversible, whereas organizing effects occur during critical developmental windows and are long-lasting.

What intersex condition is most common?

Congenital adrenal hyperplasia (CAH).

What causes congenital adrenal hyperplasia?

A genetic deficiency in cortisol synthesis that leads to adrenal over-secretion of androgens.

How does CAH influence the external genitalia of genetic (XX) females?

It causes varying degrees of masculinisation.

How do girls with CAH typically differ in play behavior?

They show more interest in boy-typical toys than other girls do.

Do most individuals with CAH identify as male or female?

The majority identify as female.

What is androgen insensitivity syndrome (testicular feminization)?

A condition where XY individuals lack functional androgen receptors.

How do people with androgen insensitivity appear externally?

They have a typical female external appearance.

Why do individuals with androgen insensitivity not menstruate?

They possess internal testes and lack a uterus and ovaries.

What gender identity is reported by most people with androgen insensitivity syndrome?

Female.

Define gender identity.

A person’s internal sense of being male, female, or another gender.

What are gender roles?

Societal expectations for behaviors regarded as appropriate for men and women.

Name three broad influences on the development of gender identity.

Biological factors (such as prenatal hormones), upbringing, and cultural expectations.

Approximately what proportion of people identify as transgender or gender-diverse?

About 1 in 100.

In many transgender individuals, how does the size of INAH-3 relate to gender identity?

Its size tends to resemble that of individuals who share the person’s experienced gender.

How can hormone therapy help transgender people?

It aligns physical traits with gender identity and can modify certain brain structures and functions.

List four common sexual orientations.

Heterosexual, homosexual, bisexual, and asexual (others exist as well).

Is there evidence for a single “gay gene”?

No; multiple genetic and environmental factors contribute.

How does the number of older biological brothers affect a male’s odds of being gay?

Each older brother slightly increases the likelihood.

What is the maternal immune hypothesis?

The idea that a mother’s immune response to male fetuses produces antibodies that can influence sexual orientation in later-born sons.

Regarding brain symmetry, how do homosexual men typically compare with heterosexual men?

Homosexual men tend to have more symmetrical brains, resembling heterosexual women.

What have twin studies suggested about sexual orientation?

Monozygotic twins show a higher concordance than dizygotic twins, implying genetic influence.

How might prenatal hormones influence later sexual orientation?

Variations in prenatal androgen and estrogen exposure may shape later preferences.

What digit-ratio (2D:4D) pattern has been reported more often in lesbian women?

A more “male-typical” ratio (shorter index finger relative to ring finger).

How do brain responses to male pheromones differ by sexual orientation?

Gay men and heterosexual women show similar hypothalamic activation to male pheromones.

How does INAH-3 size in gay men typically compare to that in heterosexual men?

It is usually smaller and similar to that of heterosexual women.

Which two limbic structures commonly activate when people view sexual images?

The hypothalamus and the amygdala.

Is there a single consistent cause of human sexual orientation?

No; multiple biological and environmental factors interact.

What does current research indicate about the heritability of sexual orientation?

It has a moderate genetic component.

How is childhood gender nonconformity related to adult sexual orientation?

Children who strongly prefer activities typical of the other sex are more likely to become gay or lesbian adults.

Roughly what percentage of the population identifies as homosexual?

About 2–6 percent, depending on definitions and surveys.

What is the scientific consensus on “conversion therapy” aimed at changing sexual orientation?

It is ineffective and often psychologically harmful.

Have consistent hormone differences been found between gay and straight adults?

No; adult hormone levels are generally similar.

At what age do many individuals become aware of their sexual orientation?

Often in early childhood, typically before adolescence.

What are pheromones?

Chemical signals released by one individual that alter the behavior or physiology of others of the same species.

In humans, is the vomeronasal organ (VNO) functional?

It exists but appears to be vestigial and non-functional.

How are pheromone-like effects probably detected in humans?

Through ordinary olfactory receptors rather than a dedicated VNO.

What is the VNO’s function in many non-human mammals?

Detecting pheromones important for mating and social behavior.

Give an example of a possible human analogue to animal pheromone systems.

Sensitivity to compounds in sweat that can subtly influence mood or attraction.

Exposure to certain sweat-borne chemicals can have what subtle effects on people?

It can change mood, hormone levels, or perceived attractiveness of others.

Which endocrine and neural structures coordinate human reproductive behavior?

The hypothalamus, pituitary gland, and sex-hormone systems.

What role does the medial preoptic area (MPOA) play in male sexual behavior?

It is crucial for sexual performance and motivation.