View the most complete collation of statistics on identical twins, one of whom is transsexual.


Is transsexuality biologically determined?

NE Whitehead

The entire article of which the following is an extract appeared in Triple Helix (UK), Autumn 2000, p6-8


Only about 5% of cross-dressers, or transvestites, have any desire to be the opposite sex, but those who do are often convinced they are a woman in a man’s body (Docter & Prince, 1997). Fewer women believe they are men trapped in a woman’s body. Both are usually called transsexuals and are perhaps one in thirty thousand of the population. At their request 10,000 sex-change operations have been performed to date (Israel & Tarver,1997), creating people physically of one sex but chromosomally of the other.

Today most transsexuals believe that their longing for sex-change is innate, biologically-derived and unchangeable - a stance known as “determinism”.

Biologists discarded behavioural determinism decades ago; politicians and activists are recalcitrant hangers-on. Even sociobiologists such as E.O.Wilson deny determinism: “Scientists never speak of genes causing behaviour except as a kind of laboratory shorthand and they never mean it literally”. Richard Dawkins is downright maverick in claiming : "We are survival machines - robot vehicles blindly programmed to preserve the selfish molecules known as genes".

So how do you decide whether you were inescapably born that way? Try using twin studies. Identical twins have (virtually always) identical genes. This means they are identical physically, but do they have identical behaviour? If one twin is homosexual, and if homosexuality is genetically determined all co-twins would always be homosexual. But it is now known that the percentage of homosexuality among co-twins is 11%. No scientist I know of disagrees with the following conclusion derived from twin studies: your genes do not make you homosexual. Can the same be said for transsexuality?

There are far fewer studies of transsexuality in identical twins because the condition is far rarer. However of four studied monozygotic (identical) male twin pairs, of which one was transsexual, the other twin was transsexual in only one case, so researchers concluded that genetic factors were most unlikely to be important (Buhrich, Bailey & Martin, 1991). If genes compelled transsexuality all the co-twins would have been identical. Genes may produce a tendency but not a tyranny.

The argument should stop there, but doesn't. Biological determinism is frequently insinuated in language used about homosexuality or transsexuality, mainly by the media, in ways that have become practically dishonest. Typically, a scientific study is quoted which suggests a correlation or link between transsexuality and hormones or brain structure etc. (The actual strength of the link is almost never mentioned.) This link is taken to show that transsexuality has “a biological basis”, or “is biological”, or “is genetic”, or “is due to hormones”. The implication then taken from this equivocal use of language is that the condition is biologically determined, whereas in all cases the evidence shows nothing of the kind and most of those with the physiological condition described do not become transsexual. Those conditions therefore are no more than minor influences. If they were major influences we would already understand unequivocally the origin of transsexuality in all cases. We don’t. More of these studies come out every year, get misreported, and the gap between what scientists really think and what the activist believes, becomes larger every year.

Thus there are studies on enzymic or hormonal abnormalities, physical dexterity, auditory phenomena, psychological profiles (Dörner,Poppe, Stahl, Kölzsch & Uebelhack, 1991;Bosinski, Peter, Bonatz,Arndt, Heidenreich, Siffell & Wille, 1997) The outstanding feature of these studies is poor reproducibility, no obvious single cause and only minor links.

Usually transsexuals (and homosexuals) argue that studies have shown their brain microstructure is more feminine (Gorman, 1995; Zhou, Hofman, Gooren & Swaab, 1995). Such studies are notoriously poorly replicable, and provide a very shaky foundation for such a view. The most unequivocal evidence is that brain microstructures are produced by long-continued behaviour, rather than long-established brain structures causing the behaviour. The brain changes physically in response to our behaviour - London taxi drivers have an enlarged part of the brain dealing with navigation, violinists a larger area dealing with movement of the fingers of the left hand. There is no evidence that people are born with brain microstructures unalterable ever after - but there is strong experimental evidence that experience changes that microstructure. Transsexual brain differences would be more likely the result of transsexual behaviour than its cause.

Many transsexuals (and homosexuals) showed childhood gender non-conformity (Zucker& Bradley, 1995). Boys were sissies, girls tomboys. But only a small minority of sissy boys become homosexual and a much smaller proportion become transsexual. Similarly for girls. Early sexual experience may be very important for some boys, but only a minority who have experienced it becoome homosexual as adults. Distant fathers may be a critical factor in later homosexuality for some boys, but a vast majority of boys with distant fathers do not become homosexual and even fewer become transsexual. There is no one cause, many paths, and many unique experiences.

But there is no evidence for the political case that transsexuals were born that way.





Bosinski, H. A., Peter, M., Bonatz, G.,Arndt, R., Heidenreich, M., Siffell, N. G., & Wille, R. A higher rate of hyperandrogenic disorders in female to male transsexuals Psychoneuroendocrinology, 1997, 22, 361-380.

Buhrich, N., Bailey, J. M., & Martin,N. G. Sexual orientation, sexual identity, and sex-dimorphic behaviors in male twins Behavior Genetics , 1991, 21, 75-96.

Cole, C. M., O'Boyle, M., Emory, L. E.,& Meyer, W. J. Comorbidity of gender dysphoria and other major psychiatric diagnoses Archives of Sexual Behavior, 1997, 26, 13-26.

Docter, R. F., & Prince, V. Transvestism: a survey of 1032 cross-dressers Archives of Sexual Behavior, 1997, 26, 589-605.

Dörner, G., Poppe, I. Stahl, F. Kötzi, J and Übelhack,R. Gene- and environment-dependent neuroendocrine etiogenesis of homosexuality and transsexualism Experimental and Clinical Endocrinology, 1991, 98,141-150.

Gorman, C. Trapped in the body of a man? Time, 1995, Nov 13, 66-67.

Israel, G. E., & Tarver, D. E. Transgender Care. Philadelphia: Temple University Press, 1997.

Rekers, G. A. Gender Identity Disorder. in Rekers, G. A., Oakes, J. S., &Gay, J. (Eds.) The Journal of Human Sexuality. Carrollton, Texas: Lewis and Stanley, 1996, 11-20.

Whitehead, N. E., & Whitehead, B. K. My Genes Made Me Do It! Layfayette, Louisiana: Huntington House, 1999.

Zhou, J. N., Hofman, M. A., Gooren, L. J.G., & Swaab, D. F. A sex difference in the human brain and its relation to transsexuality Nature, 1995,378, 68-70.

Zucker, K. J., & Bradley, S. J. Gender Identity Disorder and Psychosexual Problems in Children and Adolescents. New York: Guilford Press, 1995.