A Biological Explanation for Human Sexual Orientation

I will say this: at the very least, if no one reads this entry, I've thoroughly enjoyed reading scientific papers and doing research for the first time in 6 months. I turned off my music to write this. That means a lot! Anyway, given it HAS been a long time since I've written anything like this, I hope my writing is coherent enough to make sense.

This is a lecture presented by Dr Cynthia Chappell at a PFLAG meeting in 2005, reviewing the best research done on the biological causes of homosexuality. This is not Dr Chappell's area of research - she's a Professor at the University of Texas Health Science Center's School of Public Health, her primary research interest (at the moment) is related to human parasitic diseases. After her son came out to her a few years ago she decided to do some homework into the research done on the causes of sexuality, in the hopes of a) having a greater understanding of homosexuality, and b) being able to translate this research into language more palatable to a general audience, so that this information could be disseminated. As she mentions at the end of the lecture, a study done in 2002 in Sweden, which involved 992 adult Swedes, found that there had been a significant change from previous studies in tolerance of and attitude toward homosexuals. The three main reasons for this were found to be:

1. Anti-discrimination legislation
2. Increased visibility of homosexuals (due to aforementioned legislation - more and more gay people felt safe revealing themselves, and so more people realised they had gay family members and friends); and
3. the belief that homosexuality has a biological cause and is a normal variant of human sexuality.

Since this research shows very clear evidence of sexuality as being established prenatally (before birth, and therefore not "chosen" later in life), the more people who know about it the better. I will add this - this doesn't try to explain why ALL homosexuals are the way they are, and certainly does not prove that sexuality is 100% genetic. It simply shows that there is very good evidence that sexuality is at least some part genetic.


Anyway, I found it to be a very interesting lecture. It goes for about 45 minutes, followed by about another 30 minutes of interesting questions. The information is easy to digest, and the results are fascinating. If you have time to watch it I recommend it. However, if you don't have time I've summarised some of the results I've found fascinating below.



Summary of some results:
Neurohormonal model
The current theory regarding the assignment of sexuality is the neurohormonal model. During gestation (between 8th and 16th weeks) fetal hormones act on fetal tissues (between 8th and 16th weeks of gestation) to "assign" gender (sex is established at conception, however sexual differentiation occurs at in gestation), giving rise to male and female characteristics, and thus influence characteristics such as neuroanatomy, brain function and body type. The neurohormonal model suggests that these androgens (hormones) also "assign" sexuality. Testosterone is either present during gestation or absent (or existing in very small amounts). The presence of testosterone during gestation (whether a male or female fetus) leads to a preference for females (thus a straight male or lesbian respectively). The absence of testosterone leads to a preference for males (producing a gay male in the case of a male fetus and a straight female in the case of a female fetus). Note that these hormones are produced by the genes of the fetus, and have nothing to do with the mother (other than the fact that half of the fetus's genes come from the mother).

In other words, sexuality is influenced by hormones present during gestation, not after birth. By the time the child is born sexual orientation is set and it can't be changed by hormone injections.

• Level of testosterone is same in gay and straight men.

• Testosterone increases libido, but does not affect sexual orientation.

Thus hormone levels AFTER birth do not change sexual orientation - it's the hormones present during gestation that decide a person's sexuality.

However, its very difficult to study this, as there is no way to measure the hormone levels in a fetus without destroying it, and so they've had to study this in other ways.
A number of studies to back this theory up are discussed in the lecture. Tests on traits known to be affected by hormone levels in the womb (I think, I'm very tired as I'm writing this) such as neuroanatomy, anthropometric measurements etc have been performed on straight and gay males and females, and the majority of results are consistent with the model (eg. the size of certain areas of gay male brains is very similar to that of straight females, gay males showing similar finger ratio measurements to straight females etc).

Some other results:
Childhood Gender Nonconformity - the excessive behaviour of the opposite gender in children. So for boys that means displaying behavioural traits commonly associated with girls while avoiding (or not displaying) those traits commonly associated with boys. Research has shown that around 75% of children who exhibit CGN will turn out gay or bisexual. This obviously occurs well before that person can "choose" their sexual orientation.

Family Studies

• 9-22% of brothers of gay men are also gay (compared to the 4-5% of men in society who are exclusively gay (not bisexual))

• 5-25% of sisters of lesbians are also lesbian (compared to the 2-4% of women in the general population who are exclusively lesbians)

Some evidence to suggest that opposite sex siblings of homosexuals have increased chance of being homosexual, but not as significant as for same sex siblings. This suggests that while male and female homosexuality may run in families, it doesn't mean a family with more gay brothers will have more lesbian sisters.

Twin Studies

• Identical twins (have same genes and share amniotic sac in womb) have a 52% chance of sharing the same sexual orientation.

• Fraternal twins (have different genes and each have their own amniotic sac; no different from normal brother/sister pair) have a 22% chance of sharing the same sexual orientation.

Fairly strong evidence suggesting a genetic influence on sexuality (brothers sharing the same genes have a much higher chance of sharing sexuality than brothers with different genes). Of course if it were all in the genes, the rate for identical twins would be 100%. If it were all due to the environment the children were raised in, the rate for identical twins would be the same as that for fraternal twins.

Fraternal Birth order effect
Gay men have been found to have significantly more older brothers than straight men (reported in 14 different studies).

• It's estimated around 15% of gay males in society could attribute their sexuality to the fraternal birth order effect.

• Each older brother increases a man's chance of being gay by 33% (but you would need a LOT of older brothers for the chances of you being gay to increase above 10%).

• Effects remain when parental age is controlled (ie ruling out the possibility that this high rate could be affected by the age of the mother - older eggs, genes more susceptible to mutation etc?)

• Gay males with older brothers have also been found to have lower birth weights than straight males with older brothers - suggesting fraternal birth order is associated with prenatal events, as obviously birth weight is affected by these events alone.

• None of these associations have been found with lesbians.

The effect is independent of the family situation in which the boy is raised - an increase in the number of older brothers by adopted brothers and step brothers has no effect on this, and even when the younger brother is raised by a different family (eg adopted) the effect is still seen. So the effect is not due to being RAISED WITH older brothers, it's simply BECAUSE the boy's mother has given birth to boys before him.

One hypothesis put forward to explain these results is that an immune response in the mother's body, provoked by male fetuses, becomes stronger with each male pregnancy. The different chemicals present in the womb due to this response may lead to different brain development patterns in later male children. The antigen thought to produce this response, the HY antigen, is found on the surface of the cells of male mammals, but not females, explaining why this fraternal birth order effect is not observed with lesbians.

Interestingly, one recent study (Blanchard et al. 2006) shows that the fraternal birth order effect appears to be limited to right-handed males.


Genetic Studies
Linkage studies: a method to find chromosomal regions that "match" within populations expressing the same trait. Values > 50% indicate linkage. Higher values indicate more similarity.
3 independent studies show 66-67% linkage between gay brothers (significantly higher than that shown between straight brothers); 58% between lesbian sisters.

Ultimately it's highly doubtful there's a single gay gene - so many genes contribute to sexual development, so it's impossible to identify a single gay gene.


The trouble with these studies is that there is no single objective measurement of human sexuality - ie you can't take a blood test to determine whether or not someone is gay or straight, you can only go by what they tell you. In some cultures only the receptive partner is considered gay etc, so your results are highly dependent on the thinking of your subjects.

By no means does this research show that sexuality is entirely biological - it simply shows that biology plays a very large role in a human's sexual orientation, and goes some way towards settling the nature versus nurture argument.

A very interesting article published on this can be found here. For those more adept at reading scientific papers have a look for Mutanski, Chivers and Bailey, "A Critical Review of Recent Biological Research on Human Sexual Orientation", published in Annual Review of Sex Research 13:89-140, 2002.