Biol.psych.chapter11

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Biol.psych.chapter11

  1. 1. Chapter 11: Reproductive Behaviors
  2. 2. Sex and Hormones <ul><li>Sexual reproduction between two individuals increases variation in the gene pool. </li></ul><ul><li>Variation in the gene pool of a species enables quick evolutionary adaptations to change in the environment. </li></ul><ul><li>Also corrects genetic errors and disadvantageous mutations. </li></ul>
  3. 3. Sex and Hormones <ul><li>Widespread communication throughout the body is accomplished through the release of hormones. </li></ul><ul><li>Two kinds of hormones include: </li></ul><ul><ul><li>Steroid hormones </li></ul></ul><ul><ul><li>Sex hormones </li></ul></ul>
  4. 4. Sex and Hormones <ul><li>Steroid hormones are derived from cholesterol, contain four carbon rings and exert their effects in three ways. </li></ul><ul><ul><li>Binding to membrane receptors like neurotransmitters. </li></ul></ul><ul><ul><li>Entering cells and activate certain kinds of proteins in the cytoplasm. </li></ul></ul><ul><ul><li>Binding to chromosomes where they activate or inactivate certain genes. </li></ul></ul>
  5. 7. Sex and Hormones <ul><li>The sex hormones are a special kind of steroids, released mostly by the gonads and to a lesser degree by the adrenal glands. </li></ul><ul><ul><li>affect the brain, genital and other organs </li></ul></ul><ul><li>Two types of sex hormones include: </li></ul><ul><ul><li>Androgens </li></ul></ul><ul><ul><li>Estrogens </li></ul></ul><ul><li>Both sexes have these categories in varying quantities . </li></ul>
  6. 8. Sex and Hormones <ul><li>Androgens are a groups of sex hormones that include testosterone and others. </li></ul><ul><li>Generally referred to as “male hormones” because men have higher levels than women. </li></ul>
  7. 9. Sex and Hormones <ul><li>Estrogens include estradiol and others and are referred to as “female hormones” because women have higher levels. </li></ul><ul><li>Progesterone is a type of hormone that prepares the uterus for the implantation of a fertilized ovum and promotes the maintenance of pregnancy. </li></ul>
  8. 10. Sex and Hormones <ul><li>“ Sex limited genes” are those activated by androgens or estrogens and control most of the differences between male and female. </li></ul><ul><ul><li>Example: estrogens activate the gene for breast growth; androgens activate the gene for the growth of facial hair in men. </li></ul></ul><ul><li>Sex hormones increase or decrease the rate of apoptosis in various regions of the brain. </li></ul><ul><ul><li>Certain areas are slightly larger in males or females </li></ul></ul>
  9. 11. Sex and Hormones <ul><li>Sex hormones can have the following effects: </li></ul><ul><li>Organizing effects- occur mostly at sensitive stages of development. </li></ul><ul><ul><li>-Determine whether the brain and body will develop male or female characteristics </li></ul></ul><ul><li>Activating effects- occur at any time of life and temporarily activate a particular response. </li></ul>
  10. 12. Sex and Hormones <ul><li>The distinction between the activating and organizing effects of hormone is not absolute. </li></ul><ul><ul><li>Example: hormones early in life can exert temporary effects; during puberty hormones can also induce long-lasting structural changes </li></ul></ul>
  11. 13. Sex and Hormones <ul><li>Obvious differences can exist between the reproductive organs and the gonads of males and female. </li></ul><ul><li>Sexual differentiation begins with the chromosomes. </li></ul><ul><li>Female mammal has two X chromosomes and a male has an X and a Y. </li></ul><ul><li>During an early stage of prenatal development, both male and female have a set of Mullerian ducts and a set of Wolffian ducts as well as primitive gonads. </li></ul>
  12. 14. Sex and Hormones <ul><li>Wolffian ducts are the precursors to other male reproductive organs. </li></ul><ul><ul><li>Develop into the vas deferens and seminal vesicles. </li></ul></ul><ul><li>Mullerian ducts are precursors to the female’s oviducts, uterus, and upper vagina. </li></ul>
  13. 15. Sex and Hormones <ul><li>The male Y chromosome includes the SRY gene which causes the primitive gonads to develop into testes , the sperm-producing organ. </li></ul><ul><li>The developed testes produce the hormone testosterone . </li></ul><ul><li>Testosterone induces the development of the penis and scrotum. </li></ul><ul><li>Females are not exposed to high testosterone levels and their gonads develop into ovaries , the egg-producing organs. </li></ul>
  14. 16. Sex and Hormones <ul><li>Sensitive periods are early periods when hormones have long-lasting effects. </li></ul><ul><li>Sexual differentiation depends mostly on the level of testosterone during a sensitive period. </li></ul><ul><li>The human sensitive period for genital formation is about the third and fourth month of pregnancy. </li></ul>
  15. 18. Sex and Hormones <ul><li>Female rats exposed to testosterone shortly before or after birth are partly masculinized in anatomy and behavior. </li></ul><ul><ul><li>Clitoris grows larger than normal </li></ul></ul><ul><ul><li>At maturity, pituitary and ovaries produce steady levels of hormones instead of cycles </li></ul></ul><ul><ul><li>Parts of the hypothalamus appear more male </li></ul></ul><ul><ul><li>Sexual behavior becomes masculinized </li></ul></ul>
  16. 19. Sex and Hormones <ul><li>Extra estradiol does not determine whether the individual looks female or male. </li></ul><ul><li>Estradiol and other estrogens do modify various aspects of the development of the brain and the internal sexual organs. </li></ul><ul><li>The absence of sex hormones generally leads to female-looking external genitalia </li></ul><ul><li>If a male rat lacks androgen receptors or is castrated, it develops female-like anatomy and behavior. </li></ul>
  17. 20. Sex and Hormones <ul><li>Sex hormones early in life bind to receptors in specific areas of the hypothalamus, amygdala, and other brain areas and produce anatomical and physiological differences. </li></ul><ul><li>The sexually dimorphic nucleus is an area in the anterior hypothalamus that is larger in the male and contributes to control of male sexual behavior. </li></ul><ul><li>Parts of the female hypothalamus generate a cyclical pattern of hormone release; the hypothalamus of a male cannot. </li></ul>
  18. 21. Sex and Hormones <ul><li>During early development in rodents, testosterone is converted within certain brain cells to estradiol, which masculinizes development. </li></ul><ul><li>Alpha-fetoprotein is found in the blood during early sensitive periods and binds to estrogen and prevents it from entering developing cells. </li></ul><ul><li>Testosterone does not bind to alpha-fetoprotein and freely enters the cell. </li></ul>
  19. 22. Sex and Hormones <ul><li>In adulthood, sex hormones exert activating effects to temporarily modify behavior. </li></ul><ul><li>Behavior can also influence hormone secretion. </li></ul><ul><li>Hormones do not cause behavior but rather alter the activity in various brain areas to change the way the brain responds to certain stimuli. </li></ul><ul><li>Hormones also change sensitivity in the penis, vagina and cervix. </li></ul>
  20. 23. Sex and Hormones <ul><li>Sex hormones facilitate sexual behavior. </li></ul><ul><li>Arousal also depends on previous sexual experience. </li></ul><ul><li>Sex hormones activate sexual behavior by enhancing sensations. </li></ul><ul><li>Estrogens increase the sensitivity of the pudendal nerve , which transmits tactile stimulation from the pubic area to the brain. </li></ul>
  21. 24. Sex and Hormones <ul><li>Sex hormones also increase responses of certain areas of the hypothalamus. </li></ul><ul><ul><li>the ventromedial nucleus, the medial preoptic area (MPOA), and the anterior hypothalamus. </li></ul></ul><ul><li>Stimulation of an area known as the sexually dimorphic nucleus (SDN) fpound in the anterior hypothalamus is larger in males and increases sexual behavior in males of many species. </li></ul>
  22. 25. Sex and Hormones <ul><li>Testosterone and estradiol trigger the release of dopamine by the MPOA and other areas. </li></ul><ul><li>Dopamine stimulation of D 1 and D 5 receptors is associated with sexual arousal. </li></ul><ul><ul><li>Facilitates erection of the penis and sexually receptive postures in females </li></ul></ul><ul><li>Higher concentrations of dopamine stimulate D2 receptors and lead to orgasm. </li></ul><ul><li>Serotonin activity decreases sexual activity by blocking dopamine release. </li></ul>
  23. 26. Sex and Hormones <ul><li>Humans are less dependent on current sex hormones than other species but changes can increase or decrease sexual arousal. </li></ul><ul><li>For males, levels of testosterone correlate positively with sexual arousal and the drive to seek out partners. </li></ul>
  24. 27. Sex and Hormones <ul><li>Decreases in testosterone levels generally decrease male sexual activity and interest. </li></ul><ul><ul><li>Example: castration </li></ul></ul><ul><li>Impotence is the inability to maintain an erection. </li></ul><ul><ul><li>usually caused by impaired blood circulation, not low testosterone. </li></ul></ul><ul><li>Erection partially depends on testosterone increasing the release of nitric oxide. </li></ul><ul><ul><li>facilitates the hypothalamic neurons and increases blood flow to the penis. </li></ul></ul>
  25. 28. Sex and Hormones <ul><li>Most sex offenders have normal testosterone levels. </li></ul><ul><li>Hormone levels do not completely explain the behaviors. </li></ul><ul><li>Testosterone reduction has been tried as a means of controlling sex offenders as it reduces sexual activities. </li></ul><ul><li>Results favorable for those who continue taking medication, but dropout rate is high. </li></ul>
  26. 29. Sex and Hormones <ul><li>In women, the hypothalamus and pituitary interact with the ovaries to produce the menstrual cycle. </li></ul><ul><li>The menstrual cycle is the periodic variation in hormones and fertility over the course of about 28 days. </li></ul>
  27. 30. Sex and Hormones <ul><li>After the end of a menstrual period: </li></ul><ul><ul><li>the anterior pituitary releases follicle-stimulating hormone (FSH) </li></ul></ul><ul><ul><li>FSH promotes the growth of a follicle in the ovary. </li></ul></ul><ul><ul><li>The follicle nurtures the ovum and produces estrogen. </li></ul></ul><ul><li>Towards the middle of the menstrual cycle, the follicle builds up receptors to FSH. </li></ul><ul><li>As a result, the follicle produces increasing amounts of estradiol , a type of estrogen. </li></ul>
  28. 32. Sex and Hormones <ul><li>Increased estradiol causes the anterior pituitary to increase release of FSH and luetinizing hormone (LH). </li></ul><ul><li>FSH an LH cause the follicle to release an ovum. </li></ul><ul><li>The remnants of the follicle release the hormone progesterone. </li></ul><ul><ul><li>prepares the uterus for implantation of a fertilized ovum </li></ul></ul><ul><ul><li>inhibits the further release of LH </li></ul></ul>
  29. 34. Sex and Hormones <ul><li>If the ovum is not fertilized, the lining of the uterus is cast off and menstruation occurs. </li></ul><ul><li>If the ovum is fertilized, the levels of estradiol and progesterone increase gradually throughout pregnancy. </li></ul>
  30. 35. Sex and Hormones <ul><li>Birth control pills prevent pregnancy by interfering with the usual feedback cycle between the ovaries and pituitary. </li></ul><ul><li>The “combination-pill” contains both estrogen and progesterone and prevents the surge of FSH and LH that would release an ovum. </li></ul><ul><ul><li>also thickens the mucus of the cervix making it harder for the sperm to reach the egg. </li></ul></ul>
  31. 36. Sex and Hormones <ul><li>The periovulatory period is the time of maximum fertility and high estrogen levels when ovulation occurs. </li></ul><ul><li>Studies suggest that women become more sexually responsive during this time when estrogen levels are high. </li></ul><ul><ul><li>Show increased attention to sex-related stimuli. </li></ul></ul><ul><ul><li>show increased mate preference towards men who act and look more masculine. </li></ul></ul>
  32. 37. Sex and Hormones <ul><li>Oxytocin is a pituitary hormone also important for reproductive behavior. </li></ul><ul><li>Stimulates contraction of the uterus during delivery of a baby </li></ul><ul><li>Stimulates the mammary gland to release milk. </li></ul><ul><li>Released during orgasm and triggers a state of complete relaxation. </li></ul><ul><li>Facilitates formation of pair bonds between mating partners and mother and infant </li></ul>
  33. 38. Sex and Hormones <ul><li>Hormones released around the time of giving birth facilitate maternal behavior in females. </li></ul><ul><li>Late in pregnancy, the female secretes large amounts of estradiol, prolactin, and oxytocin. </li></ul><ul><li>Prolactin is necessary for milk production. </li></ul><ul><li>Oxytocin is associated with maternal behavior and social attachment. </li></ul>
  34. 39. Sex and Hormones <ul><li>Females also change patterns of hormone receptors. </li></ul><ul><li>Late in pregnancy, the brain increases its sensitivity to estradiol in areas responsible for maternal behavior, but not for sexual behavior. </li></ul><ul><li>The hormonal changes increase the attention of the mother to the young after birth. </li></ul><ul><li>Hormones also increase activity in the medial preoptic area and the anterior hypothalamus. </li></ul>
  35. 41. Sex and Hormones <ul><li>Vasopressin is a hormone synthesized by the hypothalamus and secreted by the posterior pituitary gland. </li></ul><ul><ul><li>associated with establishing long-term bonds in some species. </li></ul></ul><ul><li>Mothers are also stimulated by the odors of their babies. </li></ul><ul><ul><li>Infant rats release chemicals that stimulate the mother’s vomeronasal organ. </li></ul></ul>
  36. 43. Sex and Hormones <ul><li>Mammals have two mechanisms for stimulating maternal behavior: </li></ul><ul><ul><li>Hormones in the early phase compensate for the lack of familiarity with the young. </li></ul></ul><ul><ul><li>Later experience maintains the maternal behavior as hormones decline. </li></ul></ul><ul><li>Although hormonal changes are necessary to nurse a baby, they are not necessary to elicit care for a baby by humans. </li></ul>
  37. 44. Variations in Sexual Behavior <ul><li>A wide degree of variation exists between people in terms of frequency of sexual behavior, preferred types of sexual activity, and sexual orientation. </li></ul><ul><li>Evolutionary explanations are controversial in explaining mating behavior. </li></ul>
  38. 45. Variations in Sexual Behavior <ul><li>Gender differences in sexual behavior include the following: </li></ul><ul><ul><li>Men are more likely to seek multiple sex partners, especially for short-term encounters. </li></ul></ul><ul><ul><li>Women are more likely to be concerned about a mates earning potential: men are more likely to be concerned about a mate’s youth. </li></ul></ul><ul><ul><li>Men usually show greater jealousy at indications of sexual infidelity. </li></ul></ul>
  39. 46. Variations in Sexual Behavior <ul><li>Buss (2000) argues that gender differences reflect past evolutionary pressures. </li></ul><ul><li>Men are interested in brief sexual relationships with multiple partners because such a strategy increases the likelihood of his genes being passed along to the next generation. </li></ul>
  40. 47. Variations in Sexual Behavior <ul><li>Research has also suggested that women can gain from having multiple sexual partners as well. </li></ul><ul><li>Multiple mates increases resources available to her child and herself. </li></ul><ul><li>No direct evidence suggests that specific genes influence whether people prefer single or multiple mates. </li></ul>
  41. 48. Variations in Sexual Behavior <ul><li>Both men and women prefer a mate that is healthy, intelligent, honest, and physically attractive. </li></ul><ul><li>In almost all cultures, women prefer mates who are likely to be good providers. </li></ul><ul><li>Evolutionary explanations suggest that choosing a father who is likely to be a good provider aids the women while she is pregnant or caring for a small child. </li></ul>
  42. 49. Variations in Sexual Behavior <ul><li>Men tend to prefer a young partner. </li></ul><ul><li>Evolutionary explanations suggest that this preference exists because younger women are more likely to be fertile than older women. </li></ul><ul><li>Men remain fertile well into old age so preference for a young mate for women is not as pronounced. </li></ul>
  43. 50. Variations in Sexual Behavior <ul><li>Evolutionary explanations of gender differences in jealousy suggest that men need to be sure that the children he supports are his own. </li></ul><ul><ul><li>Unfaithful wives threaten this certainty. </li></ul></ul><ul><li>Women are always sure that the child is their own. </li></ul><ul><li>Although cultures vary in attitudes towards infidelity, no culture exists where infidelity is more acceptable for women. </li></ul>
  44. 51. Variations in Sexual Behavior <ul><li>Although mating habits of people can be explained in terms of increasing the probability of passing on one’s genes, we can not assume a genetic basis. </li></ul><ul><ul><li>Behaviors and preferences may be a product of learning. </li></ul></ul><ul><li>More data, especially about the effects of particular genes, is needed to draw conclusions. </li></ul>
  45. 52. Variations in Sexual Behavior <ul><li>Gender identity refers to how we identify sexually and what we call ourselves. </li></ul><ul><li>Biological differences are generally referred to as “sex differences”. </li></ul><ul><li>Differences that result from people’s thoughts about themselves as male or female are referred to as “gender differences”. </li></ul>
  46. 53. Variations in Sexual Behavior <ul><li>Most people have a gender identity that matches their external appearance. </li></ul><ul><li>Some people have a gender identity that is opposite their biological sex. </li></ul><ul><li>Psychologists and researchers once believed that gender identity was learned and more a product of rearing and experience. </li></ul><ul><li>Current evidence strongly suggests that biological factors, especially prenatal hormones, play a large role in gender identity. </li></ul>
  47. 54. Variations in Sexual Behavior <ul><li>Intersexes are people that have anatomies intermediate between male or female. </li></ul><ul><li>Poorly developed or ambiguous genitalia can be caused by: </li></ul><ul><li>XY males have a mutation of the SRY gene </li></ul><ul><li>Born with an XX chromosome pattern but an SRY gene translocates from the father’s Y chromosome onto another chromosome. </li></ul><ul><li>An atypical hormone pattern occurs or a mutation of testosterone receptors occurs before birth. </li></ul>
  48. 56. Variations in Sexual Behavior <ul><li>Congenital adrenal hyperplasia (CAH) is an overdevelopment of the adrenal glands from birth </li></ul><ul><li>Most common cause of the intersex condition. </li></ul><ul><li>Caused by a genetic defect in which cortisol production leads to overstimulation of the adrenal gland. </li></ul><ul><li>Overstimulation of the adrenal gland leads to extra testosterone production. </li></ul><ul><ul><li>The female fetus becomes partly masculinized. </li></ul></ul>
  49. 57. Variations in Sexual Behavior <ul><li>Hermaphrodites are individuals whose genitals do not match the usual development for their genetic sex. </li></ul><ul><li>An estimated 1 out of 100 children is born with some degree of genital ambiguity. </li></ul><ul><li>1 in 2000 has enough genital ambiguity to make the sex uncertain. </li></ul>
  50. 58. Variations in Sexual Behavior <ul><li>The brains of genetic females with CAH are exposed to higher than normal levels of testosterone during prenatal and early postnatal life. </li></ul><ul><li>Research indicates that CAH girls show a greater preference for boy-typical toys than do other girls. </li></ul><ul><li>During adolescence and early adulthood, they also show partly masculinized interests. </li></ul><ul><li>Sexual interest and activity also differs for CAH girls as well. </li></ul>
  51. 60. Variations in Sexual Behavior <ul><li>Androgen insensitivity or testicular feminization is a condition in which individuals with an XY chromosome pattern have the genital appearance of a female. </li></ul><ul><li>Production of androgens remains normal but they lack the androgen receptor that enables it to activate genes in a cell’s nucleus. </li></ul><ul><li>Condition occurs in various degrees from a smaller than average penis to genitals that develop a female appearance. </li></ul>
  52. 61. Variations in Sexual Behavior <ul><li>Physicians have traditionally recommended that intersex people be reared as girls. </li></ul><ul><ul><li>surgery was often conducted to make them look more feminine. </li></ul></ul><ul><ul><li>Assumed that children consistently raised as female would accept that identity. </li></ul></ul><ul><li>Many intersex people protest against such surgery and suggest that an informed consent would have been preferred. </li></ul>
  53. 62. Variations in Sexual Behavior <ul><li>Some genetic males fail to produce an enzyme that converts testosterone to dihydrotestosterone. </li></ul><ul><li>Most look female at birth but a penis develops during adolescence and puberty. </li></ul><ul><li>Most then accept a male gender identity. </li></ul><ul><ul><li>Brain is exposed to testosterone during early development. </li></ul></ul>
  54. 63. Variations in Sexual Behavior <ul><li>Many genetic males born without a penis or who had the penis accidentally removed and who were raised as a girl ask to be reassigned as males. </li></ul><ul><li>Many who remain female feel discontent or conflict with being female. </li></ul><ul><li>Such cases indicate that although hormones do not determine gender identity, they do play an important role. </li></ul>
  55. 64. Variations in Sexual Behavior <ul><li>Research has identified several predisposing factors in genetics and prenatal environment for male homosexuality. </li></ul><ul><li>Genetic and biological predisposition appears weaker for women. </li></ul><ul><li>Gender-typed behaviors in childhood are a stronger predictor of sexuality for men than compared to women. </li></ul><ul><li>Higher percentage of women acknowledge physical attraction to both sexes and switch between sexual orientations. </li></ul>
  56. 65. Variations in Sexual Behavior <ul><li>Subtle average behavioral and anatomical differences between heterosexual and homosexual include: </li></ul><ul><ul><li>Length of bones in the arms, legs and hands </li></ul></ul><ul><ul><li>Percentage of hair whorl direction </li></ul></ul><ul><ul><li>Use of landmarks versus distances to give directions </li></ul></ul><ul><li>Implies sexual orientation is an integral part of the individual. </li></ul><ul><li>But shifting of behavior to the opposite sex does not equally apply to both sexes. </li></ul>
  57. 66. Variations in Sexual Behavior <ul><li>Studies of twins suggest sexual orientation is influenced by genetic factors. </li></ul><ul><li>Probability of homosexuality is highest in monozygotic twins and lower in dizygotic twins, and even lower in siblings and adopted brothers or sisters. </li></ul><ul><li>No single particular gene has been identified. </li></ul><ul><li>Each of several genes may have an influence. </li></ul>
  58. 68. Variations in Sexual Behavior <ul><li>Studies also suggest a higher incidence of homosexuality among the maternal relatives of homosexual men. </li></ul><ul><li>These results suggest a gene on the X chromosome that a man receives from his mother may play a role. </li></ul><ul><li>Other studies have not replicated this result and thus the findings are inconclusive. </li></ul>
  59. 69. Variations in Sexual Behavior <ul><li>Sexual orientation may be influenced by testosterone levels during sensitive periods of brain development. </li></ul><ul><li>Studies of male animals deprived of testosterone early in life show sexual interest in other males as adults. </li></ul><ul><li>Studies of female animals exposed to testosterone during early development show an increased likelihood of mounting behavior. </li></ul>
  60. 70. Variations in Sexual Behavior <ul><li>Evolutionary explanations for homosexuality suggest: </li></ul><ul><li>Genes are maintained by kin selection </li></ul><ul><li>Genes that produce homosexuality increase probability of female relatives reproducing </li></ul><ul><li>Several genes produce advantages for reproduction, but certain combinations lead to homosexuality. </li></ul><ul><li>Homosexuality relates to activation or inactivation of genes. </li></ul>
  61. 71. Variations in Sexual Behavior <ul><li>Sexual orientation is not related to adult hormone levels. </li></ul><ul><li>May be influenced by exposure to testosterone during sensitive periods. </li></ul>
  62. 72. Variations in Sexual Behavior <ul><li>The probability of homosexual orientation is also higher among men with older brothers. </li></ul><ul><li>Number of previous sisters has no effect nor do these effects apply to females. </li></ul><ul><li>Results suggest that a mother’s immune system may react against a protein in a son and attacks subsequent sons to alter development. </li></ul>
  63. 73. Variations in Sexual Behavior <ul><li>Prenatal exposure to stress and alcohol may also play a role. </li></ul><ul><li>Stress releases endorphins </li></ul><ul><ul><li>antagonizes the effects of testosterone on the hypothalamus. </li></ul></ul><ul><li>Stress elevates adrenal hormone corticosterone </li></ul><ul><ul><li>decreases testosterone release. </li></ul></ul><ul><li>Both result in changes in the structure of the nervous system. </li></ul><ul><ul><li>makes the male anatomy closer to female </li></ul></ul>
  64. 74. Variations in Sexual Behavior <ul><li>Laboratory research has also shown that prenatal stress can alter sexual development. </li></ul><ul><li>Male subjects subjected to either prenatal stress or alcohol developed male sexual behavior in addition to female sexual behaviors. </li></ul><ul><li>Male subjects exposed to both stress and alcohol during prenatal development had decreased sexual behavior. </li></ul>
  65. 75. Variations in Sexual Behavior <ul><li>Research also suggests that certain brain structures differ in size between heterosexual and homosexual men and women. </li></ul><ul><li>On average, the homosexual brain is shifted towards the opposite sex in some (but not all) ways. </li></ul><ul><li>Studies emphasize the role of testosterone at certain times of development as certain areas of the brain have altered sensitivities to testosterone. </li></ul>
  66. 76. Variations in Sexual Behavior <ul><li>On average, differences in brain anatomy exist between heterosexual and homosexuals. </li></ul><ul><li>Homosexual men tend to have: </li></ul><ul><ul><li>Larger anterior commissure and suprachiasmatic nucleus </li></ul></ul><ul><ul><li>Smaller neurons in the third interstitial nucleus of the anterior hypothalamus (INAH-3) </li></ul></ul><ul><li>Difficult to decipher why differences exist. </li></ul>

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