Men who have sex change operations may be acting on a deep-seated urge "programmed" into the very fabric of the brain, according to a study of brain tissue from transsexuals who left their bodies to science. Dick Swaab and his colleagues at the Netherlands Institute for Brain Research in Amsterdam say they have identified a brain structure that varies in size between "normal" men and men driven to change sex.

Swaab and his colleagues believe their finding could explain why some men have such strong feelings of being born the wrong sex. All other attempts to find a biological basis for transsexuality have drawn a blank. Previous studies have reported brain differences between men and women and between gay men and heterosexual men. But this research is the first attempt to explain transsexuality in terms of brain structure. The findings will reopen the debate about the potential pitfalls - social as well as scientific - of seeking simple biological explanations for things as complex as sexual identity.

The researchers examined slices of brain tissue from 42 people, including six who were born male but changed sex later in life with the help of plastic surgery and "feminising" hormones such as oestrogen. Only one major difference came to light between the brains of normal males and those who had changed sex. A tiny cluster of cells known as the "bed nucleus of the stria terminalis", or BST, was half the size in transsexual men as it was in normal men. The same cluster is usually smaller in women than in men. In effect, the transsexuals had a female-sized BST.

Swaab and his colleagues, whose results appear in the current issue of Nature, believe that the size of the cluster may influence what sex people think they are. But they stress that it has no bearing on sexual orientation. Some transsexuals in the study had slept with men, others with women. Yet all had "feminine" BSTs. Gay men in the study had "masculine" BSTs.

The researchers do not know whether women who change sex have masculine BSTs, or whether feminine BSTs cause transsexuality in men. The small size of the brain structure could be caused by other biological events linked to transsexuality.

Nor is it clear how or when BSTs become feminised in the brains of transsexuals. One possibility is that it happens after the decision to change sex has been made, as a result of hormone treatment. Swaab's group says this is unlikely because some transsexuals had stopped taking oestrogen and their BSTs had not returned to their former size. They suggest instead that fluctuations in the hormones that affect fetuses in the uterus are more likely to affect the size of the BST.

Author - David Concar

Original posting date - 4 November 1995

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