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|Sex and gender: Nature
Author Paul Kenyon
contents of this lecture should surprise and interest you. The heart of
the lecture is a diagram showing two opposing views on the development
The widely held view that gender is the result of the nurture we receive as children is contrasted with the idea that gender is influenced with biological factors in our nature. The lecture explores some evidence that supports the latter position.
A series of experiments showing that it is possible to effect the sexual behaviour of animals by manipulating hormones circulating in their bodies during early development is presented.
An important message from these studies is that the mammalian brain and external genitalia tend to develop as female unless they are exposed to androgens in infancy.
finding has important implications for human development which are
explored by presenting a study that suggests that girls that are
exposed to testosterone in utero exhibit masculine behaviour patterns
After studying the material on this page you should be able to:
What do we mean by the terms sex and gender?
is a biological term which refers to the functional
differences between males and females and their reproductive potential
Gender is a psychological term which refers to our awareness and reaction to biological sex
and psychological variables that affect gender
|Gender consists of
What do you think is the gender role, gender identity and sexual orientation of these two characters played by Jack Lemmon and Tony Curtis in the 1959 film Some Like it Hot?
The development of gender identity: Nature or Nurture?
Each of us has a gender identity - a private feeling that we are male or female. There are two fundamentally different explanations for how this develops.
The most influential theory is that our 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.
The less popular view is that gender may 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.
|Point to ponder:
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.
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 -
|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.
|The 'Guevedoces' of the Dominican Republic. "During the early 1970s, Dr. Julianne Imperato, a Cornell endocrinologist, conducted an expedition to the Dominican Republic to investigate reports of an isolated village where children appearing to be girls turned into men at puberty. In the village, these children were known as 'guevedoces' (literally, penis at 12 years)." Their underlying pathology was found to be a deficiency of the enzyme 5-alpha Reductase which converts testosterone into 5 dihydrotestosterone|
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:
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:
of male rats in infancy causes them to become:
Treatment of female rats with testosterone in infancy causes them to become:
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.
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.
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.
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.
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.
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:
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
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.
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.
These pictures of sections through the preoptic area of the rat brain show that:
The size of the sexually dimorphic nucleus is affected by the presence or absence of testosterone during a critical period around birth (neonatal) .
Dr Roger Gorski,
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. But gender role behaviours may be influenced by the media.
|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
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.
examine some early psychological
studies of these girls (Ehrhardt, 1975) which suggests that exposure to
development causes a partial masculinization of human female behaviour.
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 testesoterone whilst in the womb.
(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
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.
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.
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 babysitting.
Gender role preference behaviours in congenital adrenal hyperplasia
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.
orientation in CAH/AGS patients
Prenatal exposure to androgen could influence the development of:
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.
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.
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
|Points to ponder
"Boy raised as girl discovers happiness as a man"
According to a nature view of psychosexual differentiation, prenatal exposure to androgen could influence the development of gender identity - the feeling an individual has of being a man or a woman. In contrast the nurture position holds that we are psychosexually neutral at birth and that socialization is responsible for the development of gender identity.
In 1972 Money and Ehrhardt reported the case of a 7 month old baby boy - one of a pair of twins - born in 1963 whose penis was removed after an operation for circumcision damaged the child's penis. At 22 months old the child was surgically reassigned as a girl and brought up according to the prevailing view at the time that we are psychosexually neutral at birth. This case entered the textbooks and informed medical opinion for several decades because Money reported that the child had adapted well as a girl. But long term follow up of this case by Milton Diamond paints a very different picture of the success of this application of the nature theory of psychosexual differentiation.
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 pediatric, 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."
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.
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.
testicles were surgically removed and an artifical vagina created, as
is done in sex-change operations. John became Joan.
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.
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
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 from "The boy who was turned into a girl", Horizon, BBC, December 2000)
Online text resources :
HEFCE, the funding body for universities and colleges for the UK, has purchased a 3 year licence to IDEAL, the Academic Press online journal library. If you are a member of a UK academic institution (HEFCE funded) you now have full access rights to this online library which enables you to read the full text of articles in Academic Press journals.
The following articles in the journal Hormones and Behavior cover topics raised in the lecture in greater depth: