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The Nature and Nurture of Gender Author Paul Kenyon Overview Gender What do we mean by the terms sex and gender? The development of gender: Nature or Nurture? Are there sex differences in human behaviour? Sexual differentiation Development of male and female external genitalia Activational and organizational effects of hormones Feminization of male rat by castration in infancy Masculinization of female pups by injection of testosterone in infancy Summary of the effects of castration and testosterone treatment Aromatization Gender role behaviours Behaviours influenced by hormone exposure in infancy Do hormones affect psychosexual differentiation in humans? Studies of congenital adrenal hyperplasia: activity and aggression marriage and motherhood gender role preference behaviours sexual orientation Gender identity "Boy raised as girl discovers happiness as a man" Effect of testosterone on the brain

Sexual orientation Location of INAH nuclei in humans The debate moves on LH response & sexual orientation Popular belief & base rate of homosexuality Genetic Studies of homosexuality Bem's developmental theory of sexual orientation Evolutionary theories of homosexuality Seminar discussion topics References Supplementary Web Sites Further reading Enter your search terms Web www.flyfishingdevon.co.uk Submit search form

What do we mean by the terms sex and gender?

The development of gender: Nature or Nurture?

There are two fundamentally different explanations for how gender develops.

Many psychologists believe that gender is the result of environmental influences, particularly the way we are treated by our parents, guardians, friends and relatives. According to Dr John Money we are psychosexually neutral at birth, and our gender is a consequence of the nurture we receive as children.

A less popular view is that gender is the result of nature, particularly the effects of hormones on the developing brain.

This is not a dry academic argument. The lives of a significant number of people have been changed as a result of the application of these theories.

This web page describes some of the evidence that suggests that gender may the result of exposure to hormones in the first few weeks of life.

Are there sex differences in human behaviour?

Prenatal exposure to androgen could influence the development of gender role behaviours - behaviours that are typical of one or the other sex e.g. boys playing with construction toys; girls playing with dolls Berenbaum (1999) has shown clear differences in activities and job interests between adolescent boys and girls.

Male adolescent gender role activities (Berenbaum,1999) Female adolescent gender role activities (Berenbaum,1999) Work with engines and electronics Think about becoming a secretary or typist Football Needlepoint, embroidery, macramé Think about becoming a car mechanic Go to slumber parties Ride motorcycle Collect dolls Build models Try out for cheerleading Hunting Write in diary Yard work or take out garbage Read teen magazine Baseball Read glamour or fashion magazines Wash or wax the car Sewing or knitting Read sports magazines Read romantic novels Play sports games Make jewellery Fishing Play dress-up Basketball Play with make-up

Here is some unpublished data collected by Corrine Hutt (University of Reading, UK) who observed aggressive behaviour in pre-school children in a nursery situation. She found that overall boys were more aggressive than girls; most aggressive acts tended to involve boys fighting with other boys. The amount of aggression by boys directed towards girls and vice versa tended to be relatively low. Corrine argued that this sex difference in aggression was due to masculinization of the brain by testosterone

Development of male and female external genitalia

Perhaps the first question asked by parents and relatives after a child is born is "What sex is the child?". According to the nurture view of psychosexual differentiation this is the point at which shaping the child's gender identity begins. But the process may have started many weeks before when the baby was developing in the mother's womb. This series of diagrams shows how the development of a child's external genitalia is affected by exposure to the androgen dihydrotestosterone.

An important message from this diagram is that - regardless of genetic sex -

• a child will develop female external genitalia unless it is exposed to dihydrotestosterone
• a child will develop male external genitalia if it is exposed to dihydrotestosterone

	In early intrauterine development male and female external genitalia (visible sex organs) are identical.
	Under the influence of the androgen dihydrotestosterone, the external genitalia develop in the male direction. In the absence of androgens female external genitalia develop.
	The crucial point about this diagram is that tissues that are equivalent for the two sexes in the undifferentiated state will become different organs by the time of birth. You can follow this change by tracing the fates of the different coloured tissue. The top diagram shows the undifferentiated state. The middle diagrams shows differentiation beginning during the third and fourth months of pregnancy. The bottom diagrams show complete differentiation of the external genitalia at birth. These diagrams are based on the drawings found in most textbooks that cover the the biological bases of sexual behaviour.

Here is an animated diagram of the effects of androgen exposure on external genital development that will open in a new browser window.

Activational and organizational effects of hormones

The nature and nurture views of psychosexual development differ in the significance they attach to the importance of hormones in the development of behavioural differences between males and females.

Hormones have two fundamentally different effects on sexual behaviour:
Organizational effects refer to the effects of hormones during the early development of an animal
Activational effects refer to the effects of hormones in the adult organism

There is evidence that exposure to hormones during a critical period of development changes the way in which the organism reacts to hormones in adulthood. Notice that this does not mean that early exposure to hormones has a permanent effect on behaviour. Instead it suggests that exposure to hormones in infancy affects how the adult reacts to hormones.

In other words, early exposure to hormones organizes the way behaviour is activated by hormones in adulthood. We need to spend a few moments reviewing how the activational effects of hormones on behaviour are measured.

Hormones such as testosterone, estrogen and progesterone, activate sexual behaviour of adult male and female rats

Many studies measure female sexual behaviour in terms of 'lordosis'. Lordosis refers to a characteristic posture in which the female rodent arches her back and moves her tail to permit penetration by the male.

Sexual behaviour in male rats consists of three behaviours:

• Mount: the animal assumes the copulatory position on top of the female and grasps her flanks, but does not not insert his penis into the female's vagina
• Intromission: the male mounts the female and briefly (200-300 milliseconds) inserts his penis. . Semen is not released during intromissions or mounts.
• After 10 to 12 intromissions spaced 20-30 seconds apart, the rat ejaculates semen.

The activational effects of hormones are discussed in greater detail on a separate page of this website.

This page focusses on the organizational effects of hormones.

According to the nature theory of psychosexual differentiation, hormones organize the brain during development. In a nutshell:

The organizational effects of the presence or absence of testosterone are only apparent in adulthood when the organism is under the activational effects of testosterone or estrogen and progesterone.

Feminization of male rat by castration in infancy

Exposure to testosterone between about day 17 of gestation to day 8-10 of postnatal life organizes the brain of a male rat to determine how it will react to hormones in adulthood.

If a male rat is castrated at birth, and then given an injection of estrogen in adulthood, it exhibits lordosis, - a female sexual response - when tested with a sexually vigorous male rat.

If a male rat is castrated at birth, and then exposed to a female rat which is in estrus (heat) he will not mate with her.

In contrast, if a normal male is injected with estrogen in adulthood it does not display the lordosis response in the presence of a sexually vigorous male rat. A normal male rat will mate with a female rat in estrus.

Testosterone is thought to establish male circuits (masculinization), and inhibit the development of female brain circuits (defeminization).

Masculinization of female pups by injection of testosterone in infancy

The absence of testosterone organizes the brain of a female rat to determine how it will react to hormones in adulthood.

If a female rat is injected with testosterone during infancy, and then given an injection of testosterone in adulthood, she will show male sexual responses - mount, intromission and ejaculation behaviours - when tested with a female rat in estrus.

If a female rat is injected with testosterone during infancy, and then injected with estrogen in adulthood, she will not exhibit lordosis behaviour when she is tested with a sexually vigorous male rat.

In contrast, if a normal female rat is injected with testosterone in adulthood, she will not exhibit male sexual behaviours.

A normal female rat in estrus will exhibit lordosis when paired with a sexually vigorous male rat.

Testosterone is thought to establish male circuits (masculinization), and inhibit the development of female brain circuits (defeminization).

In the absence of testosterone brain circuits are feminized and demasculinized.

Summary of the effects of male castration and female testosterone treatment

Adult sexual behavior in the rat depends on whether the brain was organized by gonadal hormones during the first few days after birth.

Normal adult males display mounting behavior because their brains were subjected to a dose of testosterone from the gonads just after birth. The same effect can be produced in females by injecting the hormone testosterone.

Depriving male pups of testosterone by castrating them at birth results in a female brain organization. As adults these feminized males, like normal females, display very few attempts to mount, but a high frequency of lordosis when mounted.

Aromatization

The DNA in a cell's nucleus is essentially a set of plans. The plans contain instructions that allow a cell to manufacture proteins. Steroid hormones exert powerful effects on the growth and specialization of cells. When hormones enter a cell's nucleus they open up its set of plans, and set protein manufacture in motion. (See Becker et al, 2002, p32-34)

Hormones travel through the cell membrane and bind with intracellular steroid receptors. This hormone-receptor complex then passes into the nucleus where it activates mRNA transcription which leads to the production of proteins. The resulting proteins then leave the nucleus and pass into the cell cytoplasm.

This picture shows the process:

• Run the animation
• Stop the animation

When testosterone travels across the cell membrane it is converted into estrogen by the enzyme aromatase. The estrogen attaches to a receptor which takes it into the cell's nucleus where it initiates the production of proteins based on instructions in the cell's DNA.

It is the expression of this genetic information which is responsible for masculinization.

• Run the animation
• Stop the animation

Thus, in order to masculinize the brain the 'male' steroid hormone testosterone must be converted into the 'female' hormone estrogen. You should read this sentence again. It contains a paradox. It is saying that a 'female' hormone is responsible for producing a male brain.

You might wonder if estrogen - produced by the mother's ovaries during pregnancy - would cause masculinization of a female fetus. But in females alphafetoprotein captures estrogen so that it cannot cross the cell membrane and androgenise the female brain.

• Run the animation
• Stop the animation

It is possible to masculinize the brain and behaviour of female rats by injecting them with such a large dose of estrogen that it overwhelms the alphafetoprotein estrogen-binding mechanism. Consequently the 'female' hormone estrogen can also masculinize brain and behaviour. (See Becker et al, 2002, p87-89)

The following account of the aromatization hypothesis is adapted from Milgram.

"The aromatization hypothesis is an attempt to explain how brain masculinization is caused by high levels of estrogen, which affect estrogen receptors in brain cells.  According to the aromatization hypothesis, masculinization of some brain structures results from the conversion of testosterone to estrogen by an aromatase enzyme (aromatization). This hypothesis was originally proposed to account for the findings that:

• high levels of estrogen act like testosterone in leading to defeminization; and
• dihydroxytestosterone does not mimic the effect of testosterone.

Testosterone and dihydroxytestosterone differ in their response to aromatase enzyme; only testosterone is converted to the aromatic compound estrogen.  Thus, dihydroxytestosterone does not produce defeminization.

If this hypothesis is correct, why aren't the brain of young females, who secrete estrogen, masculinized?  One reason may be that ovarian secretion of estrogens are lower than testicular secretion of testosterone. Another factor that serves to prevent the aromatization of estrogen is a protein known as alpha-fetoprotein, which binds to estrogens but not testosterone and restricts the access of estrogens to estrogen receptors in the brain. This suggestion is supported by evidence of high levels of alpha-fetoprotein in young female rodents."

Behaviours influenced by hormone exposure in infancy
Testosterone exposure in infancy affects a range of behaviours not just reproductive behaviours.

A number of behaviours in rats are effected by testosterone exposure around birth. These include:

• Intromission and ejaculation behaviours
• Exploratory behaviour
• Aggression
• Play
• Taste preference
• Feeding
• Active avoidance learning
• Maze learning

Exploratory behaviour is more extensive among female rats than among male rats. This behaviour is modified when female rats are injected with testosterone shortly after birth. The bar chart shows the frequency of defecation, which is inversely proportional to exploration, during a three minute open-field test. When the females had not been injected with testosterone after birth (left and centre groups) their boli count was significantly less than males. Females that had been masculinized defecated at the same rate as males.

Do hormones affect psychosexual differentiation in humans?

We have already described how androgen masculinizes the external genitalia of a developing baby.

Sometimes female embryos are exposed to abnormally high levels of androgen before birth. A small number of genetic females are born with ambiguous external genitalia. The most common cause of female pseudohermaphroditism is congenital adrenal hyperplasia (CAH) which occurs in about 1 in 5,000 to 15,000 live births. Congenital adrenal hyperplasia (CAH) also known as the adrenogenital syndrome (AGS).

CAH is a disease that affects the manufacture of the "stress" hormone, cortisol. Cortisol is released into the blood stream from the adrenal gland, a small organ near the kidney.

The diagram shows how a deficit in cortisol release disrupts the normal negative feedback between cortisol and ACTH secretion from the pituitary gland.

In CAH patients a metabolic error causes overproduction of androgens (e.g. testosterone) in the adrenal gland. This androgen leads to partial masculinization of the external genitalia of female patients which is corrected surgically at birth and with artificial cortisol supplements.

A great deal of attention has been given to the consequences of excess androgens on the psychosexual differentiation of these children.

We will examine some early psychological studies of these girls (Ehrhardt, 1975) which suggests that exposure to androgen during development causes a partial masculinization of human female behaviour.

Ehrhardt's study of fetally androgenized genetic female children
This picture shows a baby girl born with congenital adrenal hyperplasia (CAH) . Her external genitalia have been partially masculinized (virilized) as a result of exposure to high levels of testosterone whilst in the womb.

Ehrhardt (1975) studied 17 female CAH patients: age 4.3 to 19.9 years, most of the girls were in middle childhood and early adolescence. The comparison sample (n=11) consisted of the girls' sisters who did not have CAH. All the patients were under long-term corrective treatment with replacement cortisol and had undergone surgical correction of the external genitalia, usually in infancy or early childhood. Interviews with children and their mothers, fathers and siblings were tape-recorded. Interview transcripts were rated according to coded scales to elicit information about the child's behaviour.
Activity and aggression in congenital adrenal hyperplasia patients
Girls with CAH were more often described as having high levels of energy expenditure compared to their unaffected siblings. They also tended to prefer to play with boys rather than other girls. Although they tended to start fights more frequently than their sisters, this difference was not statistically significant.

Marriage and motherhood in congenital adrenal hyperplasia patients
This diagram shows that girls with CAH were not very interested in playing with dolls, instead they tended to play with cars, trucks and blocks; toys that are generally preferred by boys.

They showed little interest in future roles as brides or mothers, but were much more concerned with their careers.

Their relatives described them as being indifferent to - or avoiding - contact with babies. For example, they did not participate in caring for infants at home or go out baby-sitting.

Gender role preference behaviours in congenital adrenal hyperplasia patients
Relatives and girls with CAH describe themselves as 'tomboys' during all of their childhood.

35% of the sample were unsure or said that they might have chosen to be a boy if they could start their lives over again. However, Ehrhardt points out that none of the girls were unsure about their gender identity. They did not feel that they were boys and being a girl did not make them unhappy. In other words - as a group - they did not exhibit gender dysphoria.

Conclusion: CAH appears to have a significant effect on gender role behaviours. Patients exhibit significantly more male-typical behaviours than unaffected siblings.

Sexual orientation in CAH/AGS patients
Prenatal exposure to androgen could influence the development of:

• sexual orientation - sex of preferred romantic partner and
• gender identity - the feeling an individual has of being a man or a woman

Money, Schwartz & Lewis (1984) asked 30 women born with CAH about their sexual orientation. Their replies are shown in this diagram together with an estimate of the base rate of female homosexuality according to Kinsey 1953. See Carlson for further details.

• 40% of CAH patients were exclusively heterosexual
• 37% were bisexual or homosexual

Zucker et al (1996) review eight studies that have explored sexual orientation in women with CAH.

Zucker et al (1996) found that most women with CAH have a female gender identity. However, significantly more women with CAH live as men than would be expected by chance.

They conclude that "excessive exposure to prenatal androgens in women with CAH shifts psychosexual differentiation to a point somewhere in between a female-typical pattern and a male typical pattern."

 Here is a summary of Diamond and Sigmundson's (1997) paper:

Dr Milton Diamond, Hawaii University

"This article is a long-term follow-up to a classic case reported in paediatric, psychiatric, and sexological literature. The penis of an XY individual was accidentally ablated and he was subsequently raised as a female. Initially this individual was described as developing into a normally functioning female. The individual, however, was later found to reject this sex of rearing, switched at puberty to living as a male, and has successfully lived as such from that time to the present. The standard in instances of extensive penile damage to infants is to recommend rearing the male as a female. Subsequent cases should, however, be managed in light of this new evidence."

Here is a newspaper account of the case:

The story of a boy who was raised as a girl seems to show that gender really is all in the genes. Despite the efforts of psychiatrists, surgeons and parents, he never felt happy as a girl and eventually reverted to being a man, got married and is now living happily. The man's life history is told as a cautionary tale by Milton Diamond, a sexologist at Hawaii University in Archives of Paediatric Adolescent Medicine. He says that it is the first long-term follow-up of a male with the normal allotment of XY chromosomes who was raised as a female. Dr Diamond says that the problems for "John" began when he was eight months old, in 1963. An accident during circumcision left him without a penis. His parents took him to Johns Hopkins University in Baltimore, Maryland, where experts said that the best thing would be to raise him as a girl. His testicles were surgically removed and an artificial vagina created, as is done in sex-change operations. John became Joan. The result, says Dr Diamond, has often been extolled as the classic demonstration of how the environment can override nature in forming gender identity. In fact, he says, it was nothing of the sort; it was a disaster. Despite being raised as a girl, Joan never felt happy. At 12, she was given oestrogen therapy to complete the conversion to a woman. She grew breasts, but was never accepted by other girls, nor felt comfortable as a woman. At 14, she rebelled, confessing to her doctor: "I suspected I was a boy since the second grade." She was eventually given a mastectomy to remove the breasts and was given male hormones. At the age of 25, now John once more, he married a woman who already had children. Dr Diamond says that the case history has implications for any child born with ambiguous sexuality. "Keep your knife away," he says. "Let the kids make a decision when they get older." Michael Bailey, a psychologist at Northwestern University, Illinois, told Science Now, a daily science news service run by Science magazine, that the case was heralded by many as the pinnacle of proof that psycho-social factors could override biological factors in determining gender. Textbooks continued to claim that Joan made a successful adjustment, in spite of contradictory evidence. Dr Diamond's report, says Dr Bailey, "suggests that, if anything, how you're reared matters little". (Text extract from The Times, March 15 1997, by Nigel Hawkes. Pictures captured from a BBC TV programme "The boy who was turned into a girl", broadcast in December 2000)

Effect of testosterone on the brain

We have seen that early exposure to testosterone affects the external genitalia and adult behaviour, but does it also change the brain? This is an important question. There is evidence that the brains of male and female rodents are structurally different, and that this sexual dimorphism is caused by exposure to androgen during a critical period of development.

An area of the hypothalamus at the base of the brain called the sexually dimorphic nucleus of the preoptic area (SDN-POA) is much larger in male rats than in females. These diagrams show the location of this sexual dimorphism. You can load an animation that explains the relationship between the saggital and coronal views of the brain in a separate browser window.

	Sagittal view of rat brain	Coronal section of rat brain
Click your mouse on parts of the brain to reveal their names. Abbreviations: SDN : sexually dimorphic nucleus POA : preoptic area SDN-POA : sexually dimorphic nucleus of the preoptic area

These pictures of sections through the preoptic area of the rat brain show that:

• the SDN-POA is larger in male than female rats
• the SDN-POA is larger in females that have been injected with testosterone (androgenized females) than normal females.

The size of the sexually dimorphic nucleus is affected by the presence or absence of testosterone during a critical period around birth (neonatal) .

• The SDN is larger in males than females
• Females androgenized with a testosterone proprionate (TP) injection on day 4 have significantly larger SDN volume than control (oil-treated ) females
• SDN volume is reduced in males castrated on day 1 compared with normal males (Gorski, 1980)

Location of INAH nuclei in humans

There are four interstitial nuclei of the anterior hypothalamus (INAH) in the human brain that may be homologues (similar in structure and evolutionary origin) of the rat's sexually dimorphic nuclei (SDN).

This diagram shows a coronal section through the human hypothalamus at the level of the optic chiasm (OC). The four cell groups (INAH1, 2, 3 and 4) studied by Le Vay, (Science, Vol. 253, page 1034, 30th August 1991) are indicated by the corresponding numbers.

Le Vay (1991) reported that: INAH3 is larger in heterosexual men than in  women assumed to be heterosexual INAH3 is larger in heterosexual men than in homosexual men INAH3 is similar in size in homosexual men and heterosexual women However it is quite difficult to interpret these results: many of the homosexual men died of AIDS - what is the effect of AIDS on INAH3 size? does a small INAH3 cause homosexuality? or does homosexuality reduce the size of INAH3? Is some unknown 'third factor' responsible for homosexuality and reduced INAH3 volume? These results have not been replicated by an independent group of scientists

Caveat! This table (redrawn from Byne, 1994) shows that there is no clear agreement between researchers in which INAH nuclei differ between the sexes.

The debate moves on

• Does the influence of genes and hormones mould the brain in such a way to affect sexual orientation?
• Might sexual activity itself alter the brain?

In the 23 October, 1997 issue of Nature, Dr Marc Breedlove of the University of California, Berkeley provided an insight into these important questions.

"Breedlove studied male rats that copulated freely with constantly receptive females, and compared the structures of certain nerve tracts in their spinal cords with those of male rats caged with unreceptive females, and which therefore did not have the opportunity for intercourse. The nerve tracts studied by Dr Breedlove are known to be influenced by sexual factors, notably the presence of the male sex hormone testosterone. The result was clear -- sexual activity has an effect on the structure of the nervous system. The relevant nerve tracts were smaller in males that were free to copulate than in those that remained celibate. "
You can read more about this important piece of research in the Nature article Lifelines: Sex on the brain by Henry Gee

LH response & sexual orientation

A neuroendocrine component, the positive estrogen feedback effect, thought to be related to sexual orientation and, indirectly, to sexual differentiation, was evaluated in healthy, non-institutionalized research volunteers.  Men and women with a lifelong heterosexual orientation and men with a lifelong homosexual orientation were administered an estrogen preparation known to enhance the concentration of luteinizing hormone in women but not in men.  The secretory pattern of luteinizing hormone in the homosexuals in response to estrogen was intermediate between that of the heterosexual men and that of the women.  The diagram shows changes in LH in response to a single injection of Premarin. From: Gladue et al, Science, 225, 1496-1499, 1984. The interpretation of these results have been challenged and more recent studies have failed  to replicate the effect (Bem, 1996).

Popular belief & base rate of homosexuality

Genetic Studies of homosexuality

• One of the most frequently cited studies of homosexuality was that of Kallmann in 1952[Kallmann F. "Comparative Twin Study on the Genetic Aspects of Male Homosexuality", J Nerv Ment Dis. 1952. 115:283-298. ]. He reported a one hundred percent concordance in identical twins for homosexuality, and only twelve percent concordance in fraternal twins (identical twins result from a fertilised egg splitting in half and each half continues to grow as an embryo. Thus, the identical twins have an identical genetic code. Fraternal twins do not have identical genetic codes as each twin arises from different fertilised eggs). Subsequent studies have failed to repeat Kallman's findings. Kallman later himself postulated that this impressive concordance was an artefact due to the fact his sample was largely drawn from mentally ill, institutionalised patients[Kallmann F. In Discussion: Ranier J, Masnikoff A, Kolb L, Carr A. "Homosexuality and Heterosexuality in Identical Twins" Psychosom Med. 1960. 22:251-259 ] .
• The largest twin study was published by Bailey and Pillard[ Bailey J, Pillard R. "A Genetic Study of Male Sexual Orientation", Arch Gen Psychiatry , 1991. 48:1089-1096. ] and received considerable media coverage.
  This study included
  • 56 pairs of identical twins,
  • 54 pairs of fraternal twins,
  • 142 non-twin brothers of twins and
  • 57 pairs of adoptive brothers.

  They found that the concordance rate of homosexuality for

  • genetically unrelated adoptive brothers was 11%
  • for non-twin biologic brothers about 9%
  • the rate for fraternal twins was 22%
  • and for identical twins it was 52%

In a recent article Bem (1996) has argued that nature sets the scene for nurture to determine sexual preferences:

Abstract: "A developmental theory of erotic/romantic attraction is presented that provides the same basic account for opposite-sex and same-sex desire in both men and women. It proposes that biological variables, such as genes, prenatal hormones, and brain neuroanatomy, do not code for sexual orientation per se but for childhood temperaments that influence a child's preferences for sex-typical or sex-atypical activities and peers. These preferences lead children to feel different from opposite- or same-sex peers--to perceive them as dissimilar, unfamiliar, and exotic. This, in turn, produces heightened nonspecific autonomic arousal that subsequently gets eroticized to that same class of dissimilar peers: Exotic becomes erotic. Specific mechanisms for effecting this transformation are proposed. The theory claims to accommodate both the empirical evidence of the biological essentialists and the cultural relativism of the social constructionists."

Evolutionary theories of homosexuality

If homosexuality is due to a person's genetic make-up, how could the gene survive? Surely it would have disappeared due to homosexuals fathering less children?

"...natural selection should favor heterosexuality as it facilitates reproduction and the propagation of genes. .... what has maintained homosexuality in a small but consistent percentage of the human population?" (Muscarella et al. 2001)

Muscarella et al. (2001) review several evolutionary theories of homosexuality:

• "The first and most widely recognized evolutionary theory of homosexuality is that of E. O. Wilson and is based on the concept of kin selection (i.e., a sociobiological explanation for the evolution of altruistic behaviors). This theory holds that during the course of human evolution homosexual individuals may have helped family members, through direct or indirect provision of resources, to reproduce more successfully than they would have otherwise. Thus, genes for homosexual behavior would have been propagated indirectly through relatives." (p 394)
• "Ross and Wells propose that homosexual behavior is an ‘exaptation’ of homosocial behavior. An exaptation is not a direct product of natural selection but a neutral variation of a behavior, which with time demonstrates some fitness enhancing quality. As a result, natural selection acts upon it. According to Ross and Wells, male homosocial behavior could have contributed to male survival through increased social support and access to resources. Homosexual behavior would have reinforced homosocial bonds and thus would have been acted upon by natural selection. would interact with a range of personal experiences, ecological conditions, and psychological processes resulting in a general sexual orientation." (p 395)
• "Kirkpatrick argues that homosexual behavior comes from individual selection for reciprocal altruism, which would have contributed to resource exchange and a reduction in inter-male aggression." (p 395)
• "Rahman and Wilson propose that variations in genotypes produced hominid males who were more feminine in behavioral traits and bisexual in sexual preferences. These characteristics contributed to same sex affiliation and females were attracted to these traits because they were associated with decreased aggression and infanticide, and increased parenting behavior. Over time, females chose increasingly feminine traits in males, which led to the evolution of alleles associated with exclusive homosexual interest. The contribution of the feminine traits to parenting and the viability offspring offset the reproductively deleterious effects in males." (p 397)

It strikes me that Rahman and Wilson's  theory predicts that

• women should prefer to mate with bisexual men, and
• most men should be bisexual

At the risk of pointing out the blindingly obvious it is worth bearing in mind that homosexuals are perfectly able to have children.

Seminar discussion topics

Read Francis (2000) "Is Gender a social construct or a biological imperative?". Paper presented at the Seventh Australian Institute of Family Studies Conference Family futures: issues in research and policy: Sydney 24 - 26 July 2000. Available online

Francis argues that the term 'gender' has been politicized as part of the 'gender agenda' of contemporary feminism. She writes about her experiences in confronting these issues 'in the real world'. What is your position on  the following issues/questions that she encountered:

• Is "gender"  an arbitrary social construct?
• Is it reasonable to substitute the term 'gender' for the word 'sex'?
• Are the distinctions man/woman, masculine/feminine social constructions?
• Would abolishing gender abolish patriarchy?
• Do human beings exist on a continuum between male and female?
• What is the relationship between the 'nature-nurture' debate and the debate over gender?
• Are differences in educational outcomes between girls and boys due to 'discrimination' and/or 'social conditioning'?
• Are "the primary factors driving human psychosexual differentiation learning and environment, not biology."?

References and recommended reading that expands on points covered in lecture:

• Bailey et al, Archives of General Psychiatry, 1991, 48, 1089-1096 - argues that homosexuality has a genetic component.
• Becker, Breedlove, Crews & McCarthy (2002). Behavioural Endocrinology. 2nd Edition, The MIT Press, Cambridge.
• Bem (1996) Exotic Becomes Erotic:A Developmental Theory of Sexual Orientation. Psychological Review, Vol. 103, No. 2, 320-335
• Berenbaum (1999). Effects of early androgens on sex-typed activities and interests in adolescents with congenital adrenal hyperplasia. Hormones and Behavior, 35, 102-110.
• Carlson (2001). Physiology of Behavior, 7th edition, Allyn & Bacon
• Debate in Scientific American, May 1994:
  • Le Vay & Hamer (1994) Is homosexuality biologically influenced? Scientific American, May, 43-49
  • Byne (1994). The biological evidence challenged. Scientific American, May, 50-55
• De Vries GJ, Boyle PA. 1998. Double duty for sex differences in the brain. Behav. Brain Res. 92: 205-213. Available online
• Diamond and Sigmundson (1997). Archives of Pediatrics & Adolescent Medicine Vol.151, No.3, pp.298-304
• Diamond (1965). A critical evaluation of the ontogeny of human sexual behavior. The Quarterly Review of Biology
  Volume 40, No. 2, June 1965. Available online
• Diamond (2000).Sex and Gender: Same or Different?. Feminism & Psychology, Volume 10 (1): 46-54, 2000. Available online
• Ellis & Ames, Psychological Bulletin, 1987, 101, 233-258 an early review of the psychosexual differentiation literature that puts forward a biological theory of sexual orientation
• Ehrhardt (1975). Prenatal Hormone Exposure and Psychosexual Differentiation. In Sachar (Ed) Topics in Psychoendocrinology, Grune & Stratton.
• Francis (2000) "Is Gender a social construct or a biological imperative?". Paper presented at the Seventh Australian Institute of Family Studies Conference Family futures: issues in research and policy: Sydney 24 - 26 July 2000. Available online
• Gladue et al, Science, 1984, 225, 1496-1499 - studied release of LH in response to estrogen in homo- and heterosexual men, results suggest that homosexual men show evidence of partial demasculinisation
• Gorski (1980). Sexual differentiation of the brain. In Krieger & Hughes (Eds). Neuroendocrinology, Sinauer, Sunderland, Mass.
• Hawkes (1997). The Times, March 15th
• Holden, Science, 1992, 255, 33 - a review of Bailey et al by scientific journalist.
• Horizon, (2000). "The boy who was turned into a girl", BBC, Transmitted in December 2000)
• Le Vay, Science, 1991, 253, 1034-1037- reports that some cell groups in the hypothalamus are smaller in homosexual than heterosexual men
• Money and Ehrhardt (1972) Man & Woman, Boy & Girl. , John Hopkins University Press, Baltimore.
• Money, Schwartz & Lewis (1984). Psychoneuroendocrinology, 9, 405-414.
• Muscarella, Fink, Grammer, Karl and Kirk-Smith (2001) Homosexual Orientation in Males: Evolutionary and Ethological Aspects. Neuroendocrinology Letters 22(6):393-400. Discusses the view that some aspects of homosexual behaviour were adaptive during human evolution.
• Nimmons (1994). Sex and the brain. Discover Archives, March 1994. Report of an interview with Simon LeVay, covers LeVay's reactions to criticisms of his work, and how his career changed after publication of the science paper in 1991.
• Wilson JD. (2001) Androgens, androgen receptors, and male gender role behavior. Horm Behav 2001 40: 358-66. Available online

Supplementary Reading