THE BIOLOGY OF SEX AND GENDER CHAPTER 7

THE BIOLOGY OF SEX AND GENDER CHAPTER 7 Sex as a form of motivation The biological determinants of sex Gender-related behavioral and cognitive differences Sexual anomalies Sexual orientation

Sex as a Form of Motivation • Sex is like hunger and thirst in many ways. – It involves arousal and satiation. – It involves hormonal control. – It is controlled by specific areas of the brain. • Sex also differs in important ways from hunger and thirst. – It does not fit the pattern of a homeostatic tissue need. – Hence, individuals do not require sex for survival. – On the other hand, species require sex and reproduction to survive. ◊

Sex as a Form of Motivation • In the 1960 s William Masters and Virginia Johnson conducted groundbreaking research on the human sexual response. • They identified four phases of sexual response: – The excitement phase is a period of arousal and preparation for intercourse. – During theplateau phase, the increase in sexual arousal levels off. – Duringorgasm, rhythmic contractions occur in the vagina and those in the penis are accompanied by ejaculation. – Resolutionfollows as arousal decreases and the body returns to its previous state.

Phases of the Sexual Response Cycle Figure 7. 1

Sex as a Form of Motivation • Males have a refractory phase, during which they are unable to become aroused or have another orgasm for minutes, hours, or even days. • Females do not have a refractory period, and can have additional orgasms during the resolution phase. • The Coolidge effect refers to a rapid return to sexual arousal when a new partner is introduced. ◊

Sex as a Form of Motivation The Role of Hormones – Castration, removal of the gonads (testes or ovaries) removes the major source of sex hormones. • Castration results in a loss of sexual motivation in both sexes. • Castration with drugs is sometimes elected by male prisoners to control aggression and sexual predation. Androgensare the class of hormones responsible for a number of male characteristics; testosteroneis the major hormone. Estrogenis a class of hormones responsible for a number of female characteristics and functions.

Sex as a Form of Motivation – Women initiate sex more often during the middle of the menstrual cycle, when both estrogen and testosterone peak. – Testosterone is related to sexual activity in both males and females. However, the cause may be the reverse of expected; testosterone levels go up on evenings after they have sex, and go down when they don’t. On the other hand, giving testosterone to women increases their sexual arousal and activity; estrogen doesn’t. Figure 6. 3: Testosterone and sexual behavior

Sex as a Form of Motivation • Brain areas important in males and females: – Medial preoptic area of the hyp 0 thalamus (MPOA) • More important in performance than sexual motivation – Medial amygdala(in the temporal lobe) • Involved in sexual behavior, aggression, and emotions • Brain area important for females: – Ventromedial hypothalamus • Activity increases during copulation. • Important for receptivity to male advances. ◊

Sex as a Form of Motivation • Brain areas important for males: – Paraventricular nucleus • important for performance – Sexually dimorphic nucleus • Located in the MPOA • 5 x larger in male rats • Male sexual activity is related to its size • Size is dependent on prenatal exposure to testosterone. Fig 7. 4: SDN in male and female rats

Sex as a Form of Motivation Several neurotransmitters play a role in sexual behavior. – Dopamine (DA) • DA activity in the MPOA is involved in sexual motivation in both sexes and is critical for sexual performance in males. • Drugs that increase DA increase sexual activity in humans. • DA activity parallels behavior during the Coolidge effect. • Increasing levels of DA produce erection in males, then ejaculation, and subsequent loss of erection and refractoriness.

Sex as a Form of Motivation – Serotonin • Ejaculation is accompanied by increases in serotonin in the lateral hypothalamus, also contributing to the refractory period. • Drugs that increase serotonin impair sexual ability in men and women. ◊

Sex as a Form of Motivation The neuropeptidesoxytocin and vasopressin are important for bonding. Oxytocin is more effective in female voles, and vasopressin is more effective in males. Prairie voles are monogamous, unlike meadow and mountain voles, in part due to a particular type of vasopressin receptor. In women, oxytocin facilitates lactation and infant bonding. Oxytocin contributes to social recognition in both mice and men. Men with a particular vasopressin receptor gene were more likely to be unmarried or to have had a recent marital crisis.

Sex as a Form of Motivation Odor plays a role in recognition and attraction. Humans have a few hundred receptors and distinguish 10, 000 odors. “T-shirt studies” indicate that people can distinguish family members from others based on genetically-determined odor. – Women prefer the odors of men who differ from them in the MHC (major histocompatiblity complex, genes involved in immune functioning). • Preference appears to influence mate choices. • Couples similar in MHC are less fertile. • Dissimilar matings are associated with greater sexual satisfaction.

Sex as a Form of Motivation Pheromones are airborne chemicals released by an animal that have physiological or behavioral effects on another animal of the same species. Most pheromones are detected by the. VNO(vomeronasal organ), a cluster of receptors located in the nasal cavity. This information goes to the MPOA and the amygdala. – Whether the VNO is functional in humans is uncertain. • The VNO is greatly diminished in size. • Human VNO genes may be nonfunctioning. • However, some pheromones are detected by olfactory receptors.

Sex as a Form of Motivation There are several reports of pheromonal influence on humans: Menstrual synchrony in women housed in dorms (controversial). Increased sexual activity when wearing perfume or cologne containing presumed pheromones. Higher ratings by men of the odor of sweat from women at mid-cycle (ovulating and most likely to conceive) Amygdala activation when smelling underarm pads worn by first-time skydivers. ◊

Olfactory and Vomeronasal Systems Figure 7. 6

The Biological Determination of Sex • A glossary of sex terms: – Sexis the term for the biological characteristics that divide humans and other animals into the categories of male and female. – Genderrefers to the behavioral characteristics associated with being male or female. – Gender role is the set of behaviors society considers appropriate for people of a given biological sex. – Gender identity is the person’s subjective feeling of being male or female. ◊

The Biological Determination of Sex • Sexual Differentiation of the Fetus – For the first month, XX and XY fetuses are identical. – Later, the primitivegonads(the primary reproductive organs) in the XX individual develop intoovaries, where the ova (eggs) develop. – The. Müllerian ducts develop into the uterus, fallopian tubes, and the inner vagina. – The Wolffian ducts are absorbed. – The undifferentiated external genitals become a clitoris, the outer segment of the vagina, and the labia. ◊

The Biological Determination of Sex If the fetus receives a Y chromosome from the father, the SRY gene on that chromosome causes the primitivegonads to develop into testes. • The testes begin secreting two types of hormones: Müllerian inhibiting hormone, which defeminizes the fetus by causing the Müllerian ducts to degenerate; testosterone, which masculinizes the internal organs. • The. Wolffian ducts develop into the seminal vesicles and the vas deferens. • Dihydrotestosteronemasculinizes the external genitals (the penis and the scrotum).

Development of Male & Female Internal Organs Figure 7. 8

Differentiation of Male & Female Genitals Figure 7. 9

The Biological Determination of Sex – Organizing effects mostly occur prenatally and shortly after birth. • They affect structure and are lifelong in nature. – Activating effects can occur at any time in life. • They may come and go with hormonal fluctuations or be long lasting, but they are reversible. – Hormonal surges trigger both organizing and activating effects at puberty. • Organizing effects: maturation of genitalia; stature increases. ◊

The Biological Determination of Sex • Activating effects at puberty: growth of breasts in females and egg release during the menstrual cycle; production of sperm, muscle development, and hair growth in males; and a dramatic increase in sexual interest! ◊

The Biological Determination of Sex • Sexual Differentiation of the Brain – In males, testosterone masculinizes certain brain structures. • However, the final step of masculinization is carried out by estradiol, the principle estrogen hormone. • Once inside neurons, testosterone is converted to estradiol by a chemical process calledaromatization. – Estrogens also defeminize the male brain. • Without estrogen receptors, male rats show both male-typical and female-typical behavior. – Estrogens feminize the female brain. • Without estradiol, for example, female rats show reduced sexual interest and receptivity.

Gender-Related Behavioral and Cognitive Differences Eleanor Maccoby and Carol Jacklin (1974) concluded from a review of 2, 000 studies that four male-female differences were firmly supported: 1. 2. 3. 4. Girls have greater verbal ability than boys. Boys excel in visual-spatial ability. Boys excel in mathematical ability. Boys are more aggressive than girls. However, there is much overlap between males and females and the differences are rather specific. For example, male spatial superiority is mostly in mental rotation.

Gender-Related Behavioral and Cognitive Differences The Role of Hormones in Male-Female Differences Testosterone is related to spatial ability and aggression. • Males low in testosterone during development are impaired in spatial ability. • Testosterone can improve spatial functioning in older men. • Female transsexuals taking testosterone show spatial improvement, but a loss of verbal fluency. ◊

Gender-Related Behavioral and Cognitive Differences • Male aggression has been attributed to higher testosterone levels. However, higher testosterone levels may be a result rather than a cause. For example, testosterone level increases after winning in sports and decreases after losing. – Findings related to estrogen: • Male transsexuals taking estrogen increase verbal fluency but decrease performance on spatial tasks. ◊

Gender-Related Behavioral and Cognitive Differences Brain Differences in Male-Female Performance – Verbal tasks • In a recent f. MRI study, men used primarily the left hemisphere (which was expected), • but women activated both temporal areas equally. – Spatial tasks • Males have more cortical surface in the parietal areas, and spatial performance is correlated with this measure. • Females rely more on frontal areas in performing spatial rotations.

Gender-Related Behavioral and Cognitive Differences Different patterns of activation occur in males and females during learning, pain, and stress. Males and females also differ in pain and stress resistance and response to pain medications. – Disorders: • Males are more susceptible to autism, Tourette’s syndrome, and attention-deficit hyperactivity disorder. • Females are more likely to suffer from depression. ◊

Sexual Anomalies • Male Pseudohermaphrodites – Pseudohermaphroditeshave ambiguous internal and external organs; their gonads are consistent with their chromosomes. – If dihydrotestosterone is deficient, external genitals are incompletely masculinized at birth. Masculinization is completed when testosterone surges at puberty. Causes: • deficiency in the enzyme 17α-hydroxysteroid; • deficiency in the enzyme 5α-reductase. ◊

Sexual Anomalies – Androgen Insensitivity Syndrome • Due to a genetic lack of androgen receptors in XY individuals. • Most female internal organs are suppressed, but testes do not descend. • External genitalia are more or less female in form. • Estrogen from testes & adrenals feminize the body during puberty. Fig 7. 12: Eden Atwood

Sexual Anomalies • Female Pseudohermaphrodites – CAH (Congenital Adrenal Hyperplasia) results from a genetic defect that causes the adrenal glands to produce large amounts of androgen during the prenatal period. • External genitalia are ambiguous, with an enlarged clitoris and fused labia. • If parents choose to raise the child as female, surgery can reduce the size of the clitoris and eliminate labial fusion. • If masculinization is pronounced, the choice may be to raise the child as male, and reconstructive surgery can be performed.

Masculinized Genitals of a Female Infant Fig 7. 13

Sexual Anomalies Some argue that there at least 5 sexes, with “in-between” individuals classified as intersex. The recent case of South African track medalist Caster Semenya emphasizes the complexity of sexual classification and the need for clearer understanding. ◊

Sexual Anomalies The Sexual Reassignment Controversy – Case of “ablatio penis”: a natural experiment • Bruce’s penis was destroyed during circumcision. • On physicians’ advice, Bruce received surgical and hormonal treatment and was reared as Brenda. – The “neutral at birth” argument predicted a successful gender reassignment due to the importance of “nurture” in development. • However, Brenda did not adjust well to a female role. • At 14, Brenda chose to live as David and requested testosterone injections and reconstructive surgery. ◊

Sexual Anomalies – There have been only two other reported cases of ablatio penis. • One chose reassignment as a male at age 14. • The other accepted a female identity, but was a tomboy in childhood, chose a typically masculine occupation, and was bisexual. ◊

Sexual Anomalies – The alternative to “neutral at birth” is “sexuality at birth. ” • In this view, sexuality is determined by prenatal hormones. • Reassignment is based on genital appearance, which reflects prenatal hormone influence; in these cases, reassignment is typically accepted. • In cases of ablatio penis, reassignment is contrary to prenatal hormonal influence, which explains the results in those cases. ◊

Sexual Anomalies – Opinions remain conflicted about sexual assignment. • The Intersex Society of North America and several experts advocate delaying assignment until the child indicates a clear gender preference. • Others prefer not to see the child subjected to the social difficulties that result from ambiguous appearance. ◊

Sexual Orientation • Homosexual men are often referred to as gay, and homosexual women are often called lesbian. • The term for those who are not exclusively homosexual or heterosexual is bisexual. • About 9% of men and 4% of women have engaged in same-sex activity since puberty; about 2. 8% of men and 1. 4% of women describe themselves as homosexual or bisexual. • Because same-sex experiences are fairly common, the term homosexual is usually reserved for regular activity or continuing preference. • About 1% of the population claims to be asexual, with no interest in sex.

Sexual Orientation • There is little support for the social influence hypothesis, which emphasizes home environment or early seduction as causes of homosexuality. • The Biological Hypothesis – Seventy percent of homosexuals remember feeling “different” as early as 4 or 5 years of age. – Homosexuals show a high rate of gender nonconformity during childhood: • engaging in activities usually preferred by the other sex; • preferring other-sex companions. ◊

Sexual Orientation Genetic Influence Homosexuality is 2 -7 times higher among the siblings of homosexuals than in the population. Concordance increases with genetic relationship: Non-Twins Fraternals Identicals Males 9. 2 22 52 Females 14 16 48 A few genes have been identified, but not confirmed. Two genes may increase attraction to men, both in women and in men. ◊

Sexual Orientation – Genetic influence on homosexuality may be epigenetic: • In women, one of each pair of X chromosomes is turned off. • This occurs in the same chromosome throughout the body in 4% of women with no gay sons, but in 13% of women with a homosexual son and in 23% with two or more gay sons. ◊

Sexual Orientation Hormonal Influence on Orientation Male homosexuals do not differ from heterosexuals in testosterone level. – Any hormonal influence on homosexuality likely occurs prenatally. • Early hormonal manipulation does produce same-sex preference later in several species. • Homosexuality is observed in several species, which refutes the criticism that animal studies are irrelevant. ◊

Sexual Orientation – One hypothesis is that males with older brothers are exposed to excess androgens in the womb. • Exaggeration in gays of male-typical characteristics, such as lefthandedness, lends some support for overandrogenization. • However, CAH males, who are exposed to excess androgens, do not have a high rate of homosexuality. – An older brother “immunization” effect also has no support. ◊

Sexual Orientation • The Brain’s Role in Homosexuality – The INAH 3(third interstitial nucleus of the anterior hypothalamus) was female-sized (smaller) in gay men. Figure 7. 17: INAH 3 in a heterosexual’s brain (left) and a homosexual’s (right)

Sexual Orientation – The suprachiasmatic nucleus (SCN) was larger in gay men than in heterosexual men and contained almost twice as many vasopressinsecreting cells. – The anterior commissure was larger in gay men and heterosexual women than in heterosexual men. – Gay men’s verbal and spatial performance is more similar to women’s than to men’s. – The brain response of gays and lesbians to presumed pheromones is more like that of the other sex. – Transsexual males had a female-sized (smaller) central bed nucleus of the striaterminalis (BSTc), regardless of preference.

Sexual Orientation There is little evidence that the brains of lesbians have been masculinized prenatally. For example, they perform like heterosexual females on verbal and spatial tests. However, CAH females are exposed to excess androgens prenatally and are more likely to have a lesbian or bisexual orientation. – Lesbians are like males in two characteristics associated with prenatal androgen exposure: • the index-to-ring finger ratio; • click-evoked otoacoustic emissions. ◊

Sexual Orientation Social impact differs depending on whether homosexuality is viewed as a choice or as inborn. About 75% of homosexuals believe that homosexuality has a biological basis. Some gays and lesbians fear homosexuality will be viewed as a “disease” to be prevented or corrected. However, many gay and lesbian rights activists believe that a “biological causation” approach may result in greater public acceptance. – Evidence favors the idea that viewing homosexuality as biologically based leads to more positive attitudes. ◊