The Biology of Sex and Gender Chapter 7

1. Sex as a form of motivation The biological determinants of sex Gender-related behavioral and cognitive differences Sexual anomalies Sexual orientation The Biology of Sex and GenderChapter 7

2. 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. ◊

3. Sex as a Form of Motivation • In the 1960s 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.

4. Phases of the Sexual Response CycleFigure 7.1

5. 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. ◊

6. 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.

7. 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

8. Sex as a Form of Motivation • Brain areas important in males and females: • Medial preoptic area of the hyp0thalamus (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. ◊

9. Sex as a Form of Motivation • Brain areas important for males: • Paraventricular nucleus • important for performance • Sexually dimorphic nucleus • Located in the MPOA • 5x 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

10. 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.

11. 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. ◊

12. 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.

13. 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.

14. 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 theVNO(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.

15. 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. ◊

16. Olfactory and Vomeronasal SystemsFigure 7.6

17. 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. ◊

18. 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. • TheMüllerian 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. ◊

19. 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. • TheWolffian ducts develop into the seminal vesicles and the vas deferens. • Dihydrotestosteronemasculinizes the external genitals (the penis and the scrotum).

20. Development of Male & Female Internal OrgansFigure 7.8

21. Differentiation of Male & Female GenitalsFigure 7.9

22. 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. ◊

23. 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! ◊

24. 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.

25. 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: • 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.

26. 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. ◊

27. 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. ◊

28. Gender-Related Behavioral and Cognitive Differences • Brain Differences in Male-Female Performance • Verbal tasks • In a recent fMRI 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.

29. 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. ◊

30. 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. ◊

31. 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

32. 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.

33. Masculinized Genitals of a Female InfantFig 7.13

34. Sexual Anomalies • Some argue that there are 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. ◊

35. 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. ◊

36. 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. ◊

37. 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. ◊

38. 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. ◊

39. 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.

40. 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. ◊

41. Sexual Orientation • Genetic Influence • Homosexuality is 2-7 times higher among the siblings of homosexuals than in the population. • Concordance increases with genetic relationship: • A few genes have been identified, but not confirmed. • Two genes may increase attraction to men, both in women and in men. • ◊

42. 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. ◊

43. 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. ◊

44. 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 left-handedness, 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. ◊

45. Sexual Orientation • The Brain’s Role in Homosexuality • The INAH3(third interstitial nucleus of the anterior hypothalamus) was female-sized (smaller) in gay men. Figure 7.17: INAH3 in a heterosexual’s brain (left) and a homosexual’s (right)

46. Sexual Orientation • The suprachiasmatic nucleus (SCN) was larger in gay men than in heterosexual men and contained almost twice as many vasopressin-secreting 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.

47. 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. ◊

48. 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. ◊