Presentation on theme: "Glenn Wilson PhD, Gresham College, London"— Presentation transcript:
1Glenn Wilson PhD, Gresham College, London BORN GAY?THE PSYCHOBIOLOGY OF SEXUAL ORIENTATIONGlenn Wilson PhD, Gresham College, London
2PSYCHOANALYTIC WRITThe consensus is that (homosexuality) is caused psychically, through a disabling fear of the opposite sex. The origins of this fear lie in the homosexual’s parents. The mother, either domineering or contemptuous of the father, or feeling rejected by him, makes her son a substitute for her husband, with a close-binding, overprotective relationship. Thus she unconsciously demasculinises him. If at the same time the father is weakly submissive to his wife or aloof and unconsciously competitive with his son, he reinforces the process.Time Magazine article, 1966.Such theories have been effectively debunked. There is no satisfactory evidence that parenting affects sex orientation.
3THE STABILITY OF SEXUAL ORIENTATION This New Guinea tribe removed young boys from female company and “virilised” them by having them ingest the semen of older boys.After spending their teenage years fellating or being fellated by other boys most took comfortably to married life.Only about 5% continued to seek boys for sexual pleasure in adulthood.Apparently, sexual orientation is not easily modified by teenage experiences.
4AGAINST NATURE?Many animals engage in homosexual behaviour in the wild, sometimes for reasons of play and dominance rather than sexual preference. Bonobos use sex to form bonds and ease social tensions.The best animal model of homosexuality is sheep, where around 6% of rams will only mount other males, even when given a choice. These rams show levels of oestrogen in the amygdala similar to females.
5THE SEXUAL LIE DETECTOR It is fantasies and preferences that define sexuality, not behaviour. Erectile responses to erotica suggest that males are polarised – even those calling themselves “bisexual” respond only to male images, or female, not both.Women seem to have innate bisexual potential. Heterosexual women and lesbians respond equally to male and female erotica.
6RATES OF HOMOSEXUALITY Proportions reporting homosexual behaviour in last 5 years (Johnson et al, 2001):London: men 5.5% women 3.9%UK: men 2.1% women 2.4%Gay people seem to gravitate towards large cities.Rates of homosexuality are fairly stable across time and place. Only about 1-2% of men shift their orientation; women slightly higher (Dickson et al, 2003).The only gay in the village?
7CHILDHOOD GENDER NONCONFORMITY Cross-gender toy and play preferences are strongly predictive of adult sexual orientation. Around 75% of “sissies” and “tomboys” (usually identifiable by age 3) grow up to be gay or lesbian.This places the origin of sexual preferences well before any episodes of teenage “seduction” or “contagion”.
8MIDDLE-SEXIndividuals born with inter-sexual conditions provide clues as to the inborn origins of sexual orientation.Congenital adrenal hyperplasia (XX women masculinised by excess adrenal male hormones) show increased lesbian interests.
9THE CASE OF “BRENDA”David Reimer was born a boy but a botched circumcision led to his reassignment as a girl. Money (1975) reported that he had successfully adapted to female identity but by age 14 he had reverted to being a boy and he committed suicide at age 38.
10POSSIBLE EFFECTS ON SEXUAL ORIENTATION 1. Genes (inherited plans for brain construction).2. Epigenetics (early environmental factors that switch genes on or off).3. Prenatal hormones (from both mother & foetus).4. Other prenatal chemistry (medicines, drugs, plastics).5 Maternal stress (war, marital strife).6. Infectious agents (viruses, bacteria).7. The social environment (parental treatment, childhood and teenage experiences).
11THE BIG BROTHER EFFECTMen who have older brothers are more likely to be gay (each older brother raises chances by 33%).Effect applies only to biological brothers, not stepbrothers, hence not due to upbringing.Probably due to progressive build-up of antibodies in mother that affect masculinisation of foetal brain.There is no “big sister” effect because there is no hormonal conflict between mother and female foetus.
12BIG BROTHER EFFECT AND HANDEDNESS The fraternal birth order effect seems to apply only to right-handers (Blanchard, 2008).Given an elevated frequency of left-handedness in both gay men and lesbians this is curious.It appears that older brothers cause younger brothers to be either gay or left-handed but not often both.The reasons for this, whether genetic or environmental, are not known.
13DIRECTION OF HAIR WHORL Klar (2004) reports that 30% of gay men show an anti-clockwise hair whorl, compared with 8% of straight men. The genetic mechanism connects with that responsible for handedness (hence hemispheric brain specialisation).
14SEXUALLY DIMORPHIC NUCLEI IN THE HYPOTHALAMUS LeVay (1991) found differences between gay and straight men in certain nuclei in the hypothalamus that differentiate men and women and are concerned with sexual behaviour.These were primarily the interstitial nuclei of the anterior hypothalamus (INAH-3) and the suprachiasmic nucleus (SCN).Because many of his post-mortem subjects had died of AIDS he was obliged to show that this did not account for his results.
15WELL CONNECTED BRAINSWitelson et al (2008) have shown that that isthmus region of the corpus callosum (the conduit between the two halves of the brain) is larger in gay than straight men. This supports the idea that gay men (like women) have less structural and functional asymmetry in the brain than straight men.
16AMYGDALA CONNECTIONSUsing PET and MRI scans, Savic & Lindstrom (2008) showed that the amygdala is wired more for fight-flight reactions (action) in straight men and lesbians and more to emotional responses such as anxiety (feelings) in gay men and straight women (see widespread connections on left side). They also found more rightward asymmetry in straight men (+12cc) and lesbians (+5cc), whereas the two sides were much the same volume in gay men and straight women.
17GRAY MATTER IN THE PERIRHINAL CORTEX Using structural MRI, Ponseti et al (2007) found that lesbian women had less gray matter in the left perirhinal cortex than heterosexual womenIn this respect they were more like men. This area is involved in olfactory and visual processing but the functional meaning of the finding is not clear.No such differences were found between gay and straight men.
18TASK PERFORMANCE PROFILES Straight women and gay men excel in verbal fluency and memory. Lesbians are better than straight women at spatial tasks (Rahman, Wilson & Abrahams, 2004).
19PREPULSE INHIBITION IN LESBIAN WOMEN Startle reactions (strength of eye-blink) are reduced if the startle stimulus (sudden loud noise) is preceded by a weaker version of itself. PPI occurs less in women, but lesbian women respond more like men. Since this is an involuntary reaction it suggests hard-wiring rather than social learning.
20FINGER RATIOSMen typically have a shorter forefinger than ring finger. In women they are much the same or reversed. This seems to be a marker of exposure to prenatal testosterone.
21FINGER RATIOS AND SEX ORIENTATION Williams et al (2000) found that gay and straight men were similar in 2D:4D but lesbians were inclined toward the male pattern. Relationships were stronger on the right hand compared with the left.There was a significant tendency for men with older brothers to have a masculinised finger ratio regardless of their sexual orientation.
22BEHAVIOUR GENETICSLangstrom et al (2008) studied the origins of sexuality in 7600 twins:Men: 35% genetic, 1% shared (family) environment, 64% non-shared (unique) environment.Women: 18% genetic, 16% shared (family) environment, 66% non-shared (unique) environment.This effectively rules out parenting as a cause of male homosexuality.
23LOCI OF GAY GENESMuch interest in X chromosome, especially Xq28 (Dean Hamer)More recent work (Risch et al) implicates autosomal chromosomes (7, 8, & 10). Former two contributed equally by mother and father, latter of maternal origin only.Almost certainly, many different genes are involved.
24SKEWING OF X-INACTIVATION Women have two X chromosomes (compared to one in men) but one is inactivated early in development.Choice of which to inactivate is usually random, resulting in even mosaics (c.f., tortoiseshell cats).Bocklandt et al (2006) found extreme skewing more common in mothers of gay men. (0 gay sons 4%, 1 gay son 13%, 2+ gay sons 23%).Meaning not clear but supports involvement of X-chromosome.
25THE DARWINIAN PARADOXTheories as to why gay genes are not eliminated by natural selection (gay people having fewer children):Kin-altruism – homosexuals assist family members in raising more children. No satisfactory evidence.Balanced polymorphism – genes causing homosexuality confer a direct reproductive benefit to relatives, c.f., sickle cell anaemia. (e.g. male bonding & empathy, female competitiveness may be useful traits).Sexually antagonistic selection – genes which decrease fitness in one sex are maintained because they increase fitness in the other (some evidence that female relatives of gay men are more fecund).
26MITOCHONDRIAL DNASykes (2003) suggests that inheritance of homosexuality might be via mitochondrial DNA. This is extra-chromosomal genetic material passed down the maternal line only. It is “selfish” in that it would prefer a female-only species. Male homosexuality could be due to mDNA sabotaging sons (c.f. the beehive in which sterile males work for the queen).Currently little supportive evidence but remains an intriguing idea.
27CHANGING ATTITUDESThe last two decades have seen a dramatic increase in acceptance of homosexuality in the US. The UK has gone from homosexual behaviour being a criminal offence in 1967 to homophobia being an offence today.
28INTERNATIONAL LAWS RE HOMOSEXUALITY There is considerable variation around the world with respect to laws on homosexuality. These range from countries with laws against discrimination to those which impose the death penalty.