Sexual Orientation

The Role of Genes in Homosexuality: Observations from Family Studies

-Homosexual men have a higher number of homosexual relatives -In families of homosexual women: Clustering of this trait -Homosexual men have more homosexual brothers, while homosexual women have more homosexual sisters -Homosexual men tend to have more homosexual male relatives on their maternal side

The "Fertile Female" hypothesis

-So-called "gay genes" are not actually genes for homosexuality per se; rather, more for androphilia (attraction to males) -Beneficial for female relatives: Greater tendency to experience sexual attraction to males, thus increasing fitness -Could explain why gay people tend to belong to larger extended families -Some observational support as well as support from mathematical modeling -Lesbianism might be female analog (gynephilia)

"Having a homosexual orientation has no effect in today's society" True or False

- False -Having a homosexual orientation in today's society still puts one at risk for anxiety, mood disorders, and suicide because of the stress of discrimination and isolation."

Social science research on Homosexuality

-Attitudes towards non-heterosexual people are most readily influenced by interaction

Distribution of Sexual Orientation

-Bimodal for men, continuous for women -Suggests different course of development of sexual orientation between men and women; possibly different mechanisms -Would explain why some gay/straight trait differences were found only for men and not women (and vice versa)

What is bisexual?

-Bisexual men and women are people who experience a significant degree of sexual attraction to both sexes

How strong is the Fraternal Birth Order Effect?

-Blanchard & Bogaert claimed a Linear effect. Compared with having no older brothers, each older brother increases the likelihood that a man will be gay by 33% -Schwartz et al. (2009): Nonlinear effect -1st & 2nd brother have only small effects, but effect grows rapidly with 3 or more brothers -Despite how it might appear, Blanchard & Bogaert's 33% is actually a very modest effect -Probability that a man with no older brothers is gay is estimated at 2% -33% of 2%=0.66%

The Role of Genes in Homosexuality: Observations from Twin Studies

-Compare monozygotic (MZ) twins & dizygotic (DZ) twins -MZ twins: About 52% (male); about 48% (female) -DZ twins: About 22% (male); about 16% (female) -Other studies have suggested even higher rates -Suggests a large genetic contribution: Homosexuality seems significantly heritable in both sexes

Research Obstacles for Homosexuality

-Despite advances in research, progress is still very slow -Severe lack of research funding due to nonpathological nature of sexual orientation -Research into human sexual behavior still controversial: Another barrier to funding

The Role of Genes in Homosexuality: Studies on Fruit Flies

-Early 1960s: Created a mutant strain of fruit flies -Males courted males and females equally -Gene was called fru, or "fruitless" -Gene is present in both sexes -Male fru product activates genes in their brains, enabling male- typical sexual behavior, while suppressing female-typical sexual behavior -Female flies engineered to process fru in male fashion: Courted other females

Education and Homosexuality

-Evidence that biology is a large part of sexual orientation can further promote acceptance through education -Much more accepting in 2004 than back in 1993

True or False "Bisexuals are loved by all"

-False -Prejudice toward bisexual people are more widespread than anti-gay attitudes -Misconception that bisexual people may be perceived as deceiving themselves or others about their "true" sexuality -Misconception that bisexual people are "oversexed"

Prenatal Hormones effect in homosexual people: Finger-length studies

-Gay Women: Lower ratios -Another study found a difference between "masculine" and "feminine" lesbians -Gay Men: Less clear; conflicting data Results from remarkably large study: Difference found only between white gay and straight men (not other ethnicities) -Taken altogether: Support the idea that prenatal androgens might exert some influence on sexual orientation

The Fraternal Birth Order Effect (a.k.a. The Older-Brother Effect)

-Gay men have more older brothers than straight men -Hypothesis: Being a later-born male member of a family with other brothers predisposes that male to homosexuality -This apparent influence appears biological rather than social -This influence appears to not be a directly from one's older brothers, but from mother's immune system -No comparable effect observed in females

Prenatal Hormones effect in Congenital Adrenal Hyperplasia (CAH)

-Genetic mutation, affects corticosteroid hormones -Consequently: Higher-than-normal levels of androgens -Result in females: More likely to show more male-typical traits -Sexual orientation: Shifted in the homosexual direction

What are the evolutionary reasons that homosexuality exists in humans?

-Homosexuality in men could result from accumulation of feminizing genes that increase attractiveness to women -Male homosexuality is a "package" of gender-variant traits -These traits are controlled by several "feminizing" genes that make them more attractive to women like empathy, kindness, decreased aggressiveness, etc. -If a man inherits all of these genes: Homosexuality; lowered fitness -Leads to equilibrium state that maintains a constant low percentage of gay men in the population -There is evidence that gender-atypical traits do increase reproductive success in both straight men and straight women

Kin Selection and Homosexuality

-Homosexuality is an evolutionary conundrum due to its non-reproductive nature -Kin Selection (inclusive fitness) -1 offspring of your own (50%), vs. helping your brother or sister produce 2 or more offspring (≥ 50%) -Comparable studies in U.S. and U.K.: No evidence in favor of kin selection model -Gay people tend to allocate their resources more to themselves

-Bouchard Jr. Twin Study on Homosexuality

-MZ twins reared separately from birth -If such twin pairs are concordant, it would suggest that homosexuality is either influenced by genes, or shared interuterine environment (not postnatal environment) -Very difficult to find such subjects -Male pair #1: Gay, concordant -Male pair #2: Probably bisexual, concordant -Female pairs (4): All discordant -Implies female homosexuality is not heritable (but low n)

The Role of Genes in Homosexuality: X-linkage Studies

-Male homosexuality observed to be maternally loaded -Prompted research on the X-chromosome -Male sexual orientation is linked to a region of the X-chromosome, called Xq28 -Female sexual orientation does not appear to be linked to Xq28 -In subsequent studies, linkages were also found in autosomes -Chromosome 7 (7q36) -Chromosome 8 (8p12) -Chromosome 10 (10q26)

Biological Possible Cause of the The Fraternal Birth Order Effect

-Maternal-immunity model: Mothers' bodies "remember" earlier pregnancies -Male fetuses produce antigens that are specific to male fetuses -In response, mothers produce antibodies -If antigens guide psychosexual development towards male-typical direction, antibodies will counter those functions -The more male fetuses the mother has had, the stronger the immunological reaction

Neuroanatomical Differences in Homosexuals in The Third Interstitial Nucleus of the Anterior Hypothalamus (INAH3)

-May be equivalent to a region within the medial preoptic area in rats that is involved in male-typical sexual behaviors (SDN-POA) -Typically larger in straight men than straight women -Study: Found to be smaller in gay men than straight men -No difference found between its size in gay men and women in sample -1 replication attempt (2001) -Confirmed sexual dimorphism of INAH3; found size to be intermediate between the average sizes for straight men and women -Also observed in domesticated sheep -The ovine sexually dimorphic nucleus (oSDN) seems to be the equivalent of the SDN-POA in rats -In homosexual rams, oSDN was half the size compared with heterosexual rams; not different from that of ewes

Homosexuality effect on Visuospatial Abilities

-Mental Rotation Task -The most consistent and largest sex difference found -Straight men tend to score more correct answers than straight women -Gay men score less correct answers than straight men -Lesbians score more correct answers than straight women

Do men or women more exclusively describe themselves as homosexual?

-More men than women describe themselves as exclusively homosexual

Prenatal Hormones effect in Androgen Insensitivity Syndrome (AIS)

-Mutation in gene coding for the androgen receptor, poor or not functioning -Usually attracted to men -Implication: Testosterone is a key biological player in the development of sexual orientation and gender in both men and women

Socialization Possible Cause of the The Fraternal Birth Order Effect

-Parents more actively guide older brothers towards heterosexuality, but give later-born sons more free rein? -Younger sons are more rebellious, or more open to experiences? - Younger sons have greater opportunity for same-sex interactions with older brothers? -No evidence for all of the above

Neuroanatomical Differences in Homosexuals in The Suprachiasmatic Nucleus

-Region of the hypothalamus involved with sleep/wake circadian rhythms -Larger and more elongated on average in women than men -Women tend to have shorter sleep durations than men on average -In homosexual men: Found to be larger and more elongated on average than heterosexual men -Homosexual men also have shorter sleep durations on average than heterosexual men

Homosexuality in Nature

-Same-sex behaviors have been observed in nearly 1,500 species, and well-documented in about 500 species

The Fraternal Birth Order Effect and Handedness

-Seems to apply only to right-handed gay men -Generally, gay men are slightly left-shifted in handedness (gay women too) -Modest effect: "Washes" away beyond 1 older brother -But left-handed + having several older brothers ≠ higher likelihood of being gay

Socialization Theory of sexual orientation

-Sexual orientation has been attributed to socialization and early sexual experiences -Young people who attend single-sex boarding schools are no more likely to be gay in adulthood -The case of Bruce Reimer, who was raised as Brenda Reimer, and then identified as David Reimer: Heterosexual orientation never changed -Socialization theories not supported by evidence

What is sexual orientation?

-Sexual orientation refers to the trait that predisposes one to experience sexual attraction to people of the opposite sex (heterosexual), the same sex (homosexual), or to both sexes (bisexual). -This attraction may not necessarily result in sexual activity, but may exist in the form of desires, interests, infatuations, and fantasies

Homosexuality effect on Object Location Memory

-Show display of many different items, display then hidden, and subjects are asked to recall as many items as possible, as well as locations -Women tend to recall more items than men -Gay men recall more items than straight men (about as well as women) -No significant difference between lesbian and straight women

The prenatal hormonal theory of sexual orientation

-Some evidence exists for a hormonal or genetic influence on sexual orientation; early androgen exposure may predispose an individual to become attracted to females -Testosterone levels during a "critical period" before the time of birth influence an animal's preference for female or male sex partners after puberty -E.g.: Pregnant rats placed under repeated stress can give birth to male rats that show sexual preference for other male rats -Something similar might operate in humans

Subtypes within Homosexual Populations

-Some gays and lesbians are more "masculine"/"feminine" than others -May be more informative to research populations of each subtype individually

Prenatal Hormones effect in Otoacoustic Emissions (OAEs)

-Spontaneous (SOAEs) & click-evoked (COAEs) -Women generate more and louder SOAEs than men; louder COAEs -Appears to result from exposure to androgens during prenatal life -Difference also exist in monkeys & sheep -SOAE study in straight, gay, bisexual men and women -OAEs of lesbians and bisexual women: Male-shifted -Interpretation: Non-heterosexual women experience higher levels of androgens during prenatal life -However: No difference among straight, bisexual, and homosexual men

Bogaert Study

-Study on 950 gay and straight men in nonstandard families -Only older brothers who had the same biological mother as the subjects increased the likelihood that the subjects would be gay -I.e., not adoptive brothers, step-brothers, half-brothers who had only the father in common, etc.

Homosexuality in Media

-There seems to be increasing acceptance: Exposure in popular culture, greater dialogue, acceptance of marriage equality, etc.

Neuroanatomical Differences in Homosexuals in The Corpus Callosum

-Thick bundle of axons that connects the left and right cerebral hemispheres and facilitates hemispheric communication -In the fetus, the developing corpus callosum reaches adult morphology by 115 days -The isthmus of the corpus callosum (between the body and splenium) is larger in females than males -Found to be larger in homosexual men than heterosexual men

Prenatal Hormones effect in healthy people: Finger-length studies

-To answer this question: Search for anatomical/functional "markers" -Candidate: Finger-length; influenced by prenatal testosterone -Index finger tends to be slightly longer in females than males -2D:4D ratio used as an index -2D:4D ratio is lower in males than females

Homosexuality effect on Verbal Fluency

-Verbal tasks such as "List as many words as you can that begin with 'd'" -Generally, women tend to list more words than men -Gay men list more words than straight men (about the same as straight women) -Lesbians list less words than straight women (about the same as straight men)

Problems with Biological Possible Cause of the The Fraternal Birth Order Effect

-Why aren't all male fetuses with older brothers affected by H-Y antibodies accumulated in the mothers? -Perhaps H-Y antibodies only one side of equation: Genetic susceptibility of the male fetus could also come into play -Maternal-immunity model explains why only males with brothers are affected, not females/sisters -Does not explain handedness caveat -Maternal-immunity hypothesis offers best biological explanation, but lacks direct supportive evidence

Do men or women more exclusively describe themselves as bisexual?

-Women are more likely than men to label themselves as bisexual or "something else"

The Role of Genes in Homosexuality: Studies on Klinefelter Syndrome

-XXY karyotype -Tendency to have traits shifted towards female-typical patterns -Reported to have higher rates of homosexual orientation -Current studies suggest X-chromosome effect -Mice with XXY karyotype also exhibit elevated partner preferences for other male mice -Animal models may offer fertile grounds for research on sexual orientation

Until what year, was homosexuality considered a psychological disorder?

1973

What percentage of men and women in the US are homosexual?

Although the majority of individuals in the US are heterosexual, approximately 1-2% of women and 2-5% of men categorize themselves as exclusively or "mostly" attracted to the same sex

Lessons and Implication of Studies on Fruit Flies

Lessons & Implications -Understanding basis of sexual partner preference in fruit flies on a genetic level: Perhaps humans? -Courtship patterns of both males and females exist; one is usually inhibited -Examples: Klüver-Bucy syndrome, chemical castration, etc. -Possible mechanism? -In the SDN-POA of male rats, the predominant neurotransmitter is GABA -Inhibitory action of GABA on cell groups elsewhere that promote attraction to males

Is homosexuality a result of abuse/molestation/seduction by adults when they were children?

No

Is homosexuality a result of the disruption of "normal" psychosexual development?

No

Is homosexuality linked with problems in a child's relationship with parents (e.g., Domineering mother? Absent father? Possessive mother? Hostile father?)

No

Will children raised by same-sex parents grow up to be homosexual themselves?

No