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Overview:
The debate over the causes of sexual orientation is extremely controversial. One the one hand there are those who claim that homosexual behaviour is an innate characteristic like race or gender whilst others argue that "there is evidence to suggest that homosexual behaviour represents a socio-cultural phenomenon of learned behaviours and attitudes which are shaped by a society that increasingly tolerates, encourages and even glorifies sexual experimentation and adventurism - so much so, that even young children are victims in the form of sexual abuse" (Tahir).

Tahir's article) is recommended supplementary reading. One way of approaching this controversy is to see it as an example of the nature-nurture debate, but of course the implications and emotions aroused are a long way from discussion of the development of birdsong or pecking in newborn chicks. I hope you enjoy this topic - it affects us all - but there are no clear cut answers available.

Lecture outline

Points to ponder
A woman with AGS was banned from competing in the Olympic Games as a woman. Do you agree with this decision?
Do you think a person with male (XY) chromosomes but suffering from Testicular Feminising Syndrome (see Carlson) should be allowed to enter womens' events?
How would you decide whether a person should be allowed to compete in mens' or womens' events?
What tests would you employ? Would you base your testing on a person's sex or sexuality?
This material from Integrative Topics in Psychology: Gender Differences may help you appreciate just how complicated this issue has become.

Popular belief & base rate of homosexuality


.. 43% of Americans believe that 'young homesexuals became that way because of older homosexuals'

 
In an interview with Science in 1992 Bailey stated:
'No one has ever found a postnatal social environmental influence for homosexual orientation - and they have looked plenty'

Changes in the psychiatric status of homosexuality


Genetic Studies of homosexuality

Point to ponder: 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?


Sexual differentiation

We covered this material previously. It is reproduced here for your convenience.
In early interuterine development males 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 these diagrams 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.

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.


Sexual Anomalies:

Frequency


Variables that effect gender:

The eight Variables of gender:

  1. Chromosomal gender
  2. Gonadal gender
  3. Prenatal hormone gender
  4. Internal accessory organs
  5. External genital appearence
  6. Pubertal hormone gender
  7. Assigned gender
  8. Gender identity

Two theories of gender identity

development_nature.gif (37675 bytes)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:
  • How do you think theories of gender development might have influenced the way parents raise their children?




Aromatization

Steroid hormones such as testosterone and estrogen pass through the cell membrane and bind with intracellular steroid receptors. This complex then passes into the nucleus where it activates mRNA transcription which leads to the production of proteins.The proteins then leave the nucleus and pass into the cell cytoplasm.

In some cells of the body the effects of testosterone are carried out by estrogen. Testosterone is aromatized to estrogen, which activates estrogen receptors in the nucleus. In females alphafetoprotein captures estrogen so that it cannot cross the cell membrane and androgenise the female brain.

The following account of the aromatization hypotheis 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:

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."


Aromatization


Location of SDN in rats

Use arrow / slider to rotate brain Sagittal view of rat brain Coronal section of rat brain

Common abbreviations:


Effect of testosterone on SDN

Photomicrographs of sections through the preoptic area of the rat brain. SDN-POA = sexually dimorphic nucleus of the preoptic area
Effect of testosterone on SDN
Neonatal hormone exposure influences volume of sexually dimorphic nucleus. Females androgenised with testosterone proprionate (TP) on day 4 have dose-related significantly larger volume than control (oil-treated ) females; volume is reduced in neonatally castrated males as compared with males castrated on day 21. Redrawn from Gorski, In Kreiger & Hughes (Eds), Neuroendocrinology
Effect of testosterone on SDN volume
Anatomic dimorphism in preoptic area of hypothalamus of intact and castrated male rats and female rats treated with testosterone or oil. Data from Raisman & Field.
Effect of testosterone on SDN spines


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.

INAH nuclei in human hypothalamus img024.jpg (20493 bytes) 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.

Researchers Brain region
INAH1 INAH2 INAH3 INAH4
Swaab & Fliers, 1985 Larger in men Not studied Not studied Not studied
Allen et al, 1989 No sex difference Larger in men than in some women Larger in men No sex difference
LeVay, 1991 No sex difference No sex difference Larger in heterosexual men than in women or homosexual men No sex difference

LH response & sexual orientation

LH response and 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, noninstitutionalized 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).


Bem's developmental theory of sexual orientation

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

bem_figure.gif (6993 bytes) 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

"...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:

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


References and online resources:

Recommended reading that expands on points covered in lecture:


Supplementary material

Study on the sexual behaviour of American men
Abstract:This study--known as the 1991 National Survey of Men--was based on what its authors describe as a "nationally representative" sample of 3,321 U.S. males ages 20 to 39. The survey was conducted through interviews done in 1991. It is one of just a few large-scale studies on the topic. The study's most controversial aspect was its finding that only 2 percent of the men it surveyed reported having had sex with other males during the past 10 years, and that just 1 percent indicated that over this time their sexual partner(s) had been exclusively male. Many reacting to the study pointed out that this finding places the survey at odds with other estimates. For example, one study has placed the percentage of men who engage in sexual behavior exclusively with other men as high as 10 percent.

Controversy over Biological Theories of Sexual Orientation

  • The full text of Tahir's article HOMOSEXUALITY - An Analysis of Biological Theories of Causation is worth reading. He provides detailed critisisms of genetic, hormonal and neuroanatomical studies which claim that homosexual behaviour is an innate characteristic like race or gender.
  • Newspaper report of Swaab's work on brain anatomy in transsexuals.
    Abstract:Scientists in Holland have found preliminary evidence that male transsexuals -- men who identify sexually with women -- have a strikingly different brain structure from ``ordinary'' men, at least in one key area about one-eighth of an inch wide. A team of researchers from Amsterdam reports that it conducted post-mortem exams on the brains of six male-to-female transsexuals. Specifically, the researchers studied one particular part of the hypothalamus, called the central division of the bed nucleus of the stria terminalis (BSTc). This area, which is thought to influence sexual behavior, is on average 44 percent larger in men than in women. Yet all six subjects had BSTc regions that were the size of women's, Dick Swaab of the Netherlands Institute for Brain Research and colleagues found.
  • Here is an article that provides a summary of Tahir's critique of Swaab's work on tranasexuals and LeVay's work on homosexuals
    Abstract:Critics of Swaab's transsexual study state that the study was performed on a small number of specimens and the results may have been skewed by other factors. For one, only six brains of transsexuals were autopsied. Secondly, all the transsexuals had been exposed to large quantities of estrogen, the feminizing hormone. Hormones can alter the state of the brain and scientists questioned whether the discovered results could have been the result of estrogen use within the samples. Further tests to confirm the results will have to be made before these findings can be seriously analyzed.
    A 1991 published study by Simon LeVay that claimed that an area of the hypothalamus known as INAH3 is smaller in homosexual men and heterosexual women. The report was the result of the study of the brains from 41 cadavers and stated that there was a similarity in the size of the hypothalamus in the gay men and heterosexual women, both smaller than that of the heterosexual men. Tahir again disputes these findings by stating that the study had too many unknown factors to make them valid. For one he states that the sexual histories of the subjects were not absolutely known. Secondly he states that the 19 homosexual subjects had died of AIDS and studies have shown that brain tissue has been documented to deteriorate in patients with AIDS and may have affected the study's results. Lastly, the hypothalamus was larger in 3 of 19 of the homosexual men than in the mean size of the heterosexual men. Dr. Paul Cameron states, "According to [LeVay's] theory, 3 of the 'heterosexuals' should have been homosexual, and 3 of the homosexuals should have been heterosexual. When you completely misclassify 6 of 35, you don't have much of a theory." Both agree the study cannot be considered valid.
  • The Complex Interaction Of Genes And Environment: A Model For Homosexuality by Jeffrey Satinover, MD
    Source: Collected Papers from the NARTH Annual Conference, Saturday, 29 July 1995.
    There is essentially no dimension of behavior which is not both environmentally and genetically influenced. Genes and environment interact in extraordinarily complex ways with each other, as well as among themselves to produce a final result; the environmental influences are multi-factorial and affect each other; in human behavior, the pertinent genes are also multiple. Furthermore, there are very few circumstances where free will plays little or no role in what we do. These facts are true of homosexuality as of all other dimensions of human behavior. Indeed, there are no features of human behavior which are not influenced in some fashion by our genetic makeup and no features of human behavior which are not influenced in some fashion by our environment. more ....

Frequently Aasked Questions (FAQ): Hormone Therapy for Transsexuals

  • FAQ: Hormone Therapy for Female to Male Transsexuals
    Abstract This document contains a list of frequently asked questions and their answers regarding hormone therapy (secondary sexual reassignment) for female-to-male transsexuals. More generally, this document contains information about gonadal hormones and anti-hormones, so it can be a helpful reference for the treatment of androgen and estrogen-sensitive conditions--for example, certain cancers of the reproductive organs and breasts.
  • FAQ: Hormone Therapy for Male to Female Transsexuals
    Abstract This document contains a list of frequently asked questions and their answers regarding hormone therapy (secondary sexual reassignment) for male-to-female transsexuals. More generally, this document contains information about gonadal hormones and anti-hormones, so it can be a helpful reference for the treatment of androgen and estrogen-sensitive conditions--for example, certain cancers of the reproductive organs and breasts.

The Endocrine Society Factsheets

  • Congenital adrenal hyperplasia
    What is CAH?
    CAH is a disease that affects the manufacture of the "stress" hormone, cortisol. Cortisol is produced by the adrenal gland, a small organ near the kidney. CAH occurs in two forms: severe or "classical" and mild or "nonclassical." Girls with classical CAH are born with masculine-appearing external genitals but with female internal sex organs. Boys with classical CAH look normal at birth, so their diagnosis of CAH is sometimes missed. People with CAH are likely to have trouble retaining salt, a condition that can be life-threatening. Everyone with CAH also has unusually high levels of the male sex hormone testosterone in their blood. Among the difficulties it causes are infertility, excessive hair growth, acne, and early growth of sexual hair. In addition, adrenal crises can occur and result in death.
  • Turner's syndrome
    What is Turner's syndrome?
    Turner's syndrome is a relatively common disease in females, affecting many body systems. In this genetic disorder, the absence of all or part of one sex chromosome is associated with short stature and failure to mature sexually. Other problems can include learning difficulties, heart and kidney abnormalities, infertility, and thyroid dysfunctions.

American Psychological Association publication


E-mail from Cheryl Chase, Executive Director, Intersex Society of North America

Dear Dr. Kenyon,
I happened upon your "Gender Differences" integrative topic page while searching for popular media coverage of the following new report.
Bradley, Susan J., Gillian D. Oliver, Avinoam B. Chernick, and Kenneth J. Zucker. 1998. Experiment of Nurture: Ablatio Penis at 2 Months, Sex Reassignment at 7 Months, and a Psychosexual Follow-up in Young Adulthood. Pediatrics (Electronic Pages) 102 (1):E9. http://www.pediatrics.org/cgi/content/full/102/1/e9
You have provided your students with a wide variety of challenging and interesting material. But, for understanding the experience of intersexuality (as opposed to how it has been used to support various "scientific" agendas, also an important topic of study), I recommend that you send them where intersexuals speak for themselves.
Please do visit our web site, www.isna.org. From there you can reach the web sites of a variety of intersex support groups in worldwide.
Regards,
Cheryl Chase
Intersex Society of North America
http://www.isna.org


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