Causes of sexual orientation (was: Disorders)

Anders Sandberg (
13 Jul 1998 19:26:25 +0200

"Jim Barnebee" <> writes:

> There is no such thing as a "gay gene". Homosexuality is not
> genetic, but environmental.

Sorry, but it isn't that simple. Practically nothing in psychology is 100% genetic or environmental, usually it is a complex mixture. Often people misreport the discovery of a gene linked to a trait as if the gene was the sole cause.

> Homosexuality is caused by the
> underproduction or overproduction of a certain hormone in the mother
> during the gestation of the fetus (I believe in the third trimester).

(Based on chapter 11 in _Biological Psychology_ by James W. Kalat, which I happen to have handy. References at the end)

There is some data that levels of testosterone during the middle of the second month to the fifth month might influence sexual orientation; decreased levels increase the number of homosexual males (Ellis & Ames 1987) in animal models (Adkins-Regan 1988), and increased levels the number of homosexual females. However, these studies also caused anatomical changes, which are not observed among humans. Stress during the last part of the pregnancy might also increase the number of homosexual offspring, perhaps due to endorphines from the mother acting as anti-testosterone in the fetus' hypthalamus (Ward, Monaghan & Ward 1986).

However, the evidence among humans for a link between maternal stress and homosexuality is relatively weak: one study (Ellis, Ames, Peckham & Burke 1988) found an increased number of stressful events during the second trimester of pregnancy, but another study failed to observe it (Bailey, Willerman & Parks 1991).

The consensus seems to be that prenatal hormones play a role, but are not that strong influence.

The evidence for a genetic link is fairly clear, but does not explain all homosexuality. Bailey & Pillard 1991 and Bailey, Pillard, Neale & Agyei 1993 produced the following picture in a twin/family study of homosexuals:

                     Probability of
                     homosexual orientation

                      Male           Female

Monozygotic twin       52%            48%
Dizygotic twin         22%            16%
Sister                                14%
Adopted brother/sister 11%             6%

The percentage of homosexuals in the population in general is around 5-10%, with fewer reported females. These results suggest that there is a genetic correlation, but even in monozygotic twins it is not strong enough to account for more than 50%.

In a study of more remote relatives, it was observed that for homosexual males there was an increase in probability of homosexuality on the mother's side, suggesting an X-chromosome linkage (Hamer, Hu, Magnussoon, Hu & Pattuccio 1993).

So, to sum up, there is evidence for a genetic link, but it doesn't account for all the variation, and the evidence for hormonal effects isn't that strong. These results, together with the observed biological changes in homosexual's brains (mostly in the diencephalon) suggest that purely psychological explanations do not work to explain it.

ObTranshuman: An interesting question is what would happen if it was possible to relatively simply adjust one's brain to change sexual preferences at will. How many people would try it? My guess is that relatively few adults would do it, since their preferences had already become a part of their self-image and they would not want to change that. Among adolescents experimentation might reasonably be expected to be higher. It would be interesting to study more what traits people regard as too central to be changed and what traits they could accept changing. For example, practically everybody seems to accept a better memory or sharper perception, many people would likely want to 'polish' their personalities but not change them outright, and very few if any people want to change their basic value systems. Is this because the things we might accept changing are not as rooted in our values as the 'sacred' parts?

Adkins-Regan, E 1988 Sex hormones and sexual orientation in animals, Psychobiology 16 335-347

Bailly, Willerman & Parks 1991 A test of the maternal stress theory of human male homosexuality. Archives of sexual behavior, 20 277-293

Bailley, Pillard 1991 A genetic study of male sexual orientation, Archives of General Psychiatry 48 1089-1096

Bailley, Pillard, Neale & Agyei 1993 Heritable factors influence sexual orientation in women, Archives of General Psychiatry 50 217-223

Ellis, L & Ames, MA 1987 Neurohormonal functioning and sexual orientation: a theory of homosexuality-heterosexuality. Psychological Bulletin 101 233-258

Ellis L, Ames M, Peckham W & Burke D 1988 Sexual orientation of human offspring may be altered by severe maternal stress during pregnancy, Journal of Sex Research 25 152-157

Hamer, Hu S., Magnuson, Hu N., Pattatucci 1993 A linkage between DNA markers on the X chromosome and male sexual orientation. Science 261 321-327

Ward, OB, Monaghan EP, Ward IL 1986 Naltrexone blocks the effects of prenatal stress on sexual behavior in male rats. Pharmacology Biochemistry & Behavior 25 573-576

Anders Sandberg                                      Towards Ascension!                  
GCS/M/S/O d++ -p+ c++++ !l u+ e++ m++ s+/+ n--- h+/* f+ g+ w++ t+ r+ !y