There is no simple explanation for the cause of transsexualism. Many psychological theories have been proposed and more recent research has focussed on looking at biological causes. It is likely that there are several possible elements operating at the same time. This multifactorial hypothesis includes things such as differences in how the human brain functions, neurochemical pathways, the endocrine (hormone) glands as well as many cultural and environmental factors.

There have been several studies looking at differences in the human brain. One study (Swaab et al, 1995) examined the brains of heterosexual and homosexual men and women, and six male-to-female transsexuals. They identified a small region in the brain known as the Central region of the Bed Nucleus of the Stria Terminalis (also known as the BSTc). They found this area to be significantly larger in men than in women. The BSTc of the six transsexuals was very similar to that of the women in the study, suggesting that the male-to-female transsexuals and the biological females have brains very similar to each other. Both heterosexual and homosexual men showed much larger BSTc's suggesting male brain structuring in this area.

Unfortunately the BSTc is very small and cannot be measured by conventional brain scans, such as CT or MRI. It can only be studied by removal of the brain after death! Therefore there is no scope for it to be used as a diagnostic test although the research has been vital in establishing that the brain itself is physically different.

The biological theories are certainly not new ideas. Dr Harry Benjamin, in the early 1950's, also shared a biological approach to explaining transsexualism, believing that genetic and hormonal factors also had strong influences. He also acknowledged the impact that environmental factors and early life experience play.

"There is no one cause of a gender role... Nature alone is not responsible, nor nurture alone. They work together, hand in glove" (Dr Harry Benjamin).

In the 1950's, there was a huge interest by psychologists to study gender development in young children, which has led to a much better understanding of early life experience and socio-cultural factors. In particular they wanted to look at the influence the social environment has when forming personal concepts of gender. Developmental psychologists began to identify important changes in how children think about their own gender. The researchers found that children are able to label themselves as male or female from as young as 2 or 3 years old.

The formation of a gender identity is a complex process that evolves from this very young age with the suggestion that it is fixed in early childhood and thereafter cannot change. This is why part of the assessment process for transsexualism involves understanding the individual's own early life experiences. For example, psychologists have demonstrated that children actively begin to seek out information about what it means to be a boy or a girl. From as early as 3 years old, children can be seen to show an interest in masculine or feminine toys, games and activities, behaviours and occupations, with varying degrees of enjoyment or discomfort. These are all explored in detail in the assessment process.

There are strong cultural and environmental influences on gender. Most societies have strong gender stereotypes with only 2 distinct gender roles- male and female, which correspond to biological sex. However this is not the case for all societies. For example, the Two Spirit people of some indigenous American tribes, are accepted by their community without question despite adopting a gender role opposite to their biological sex.

Sociologists therefore believe that gender identity is socially constructed and heavily influenced by the social and environmental factors of the local community. Contemporary sociology also acknowledges the importance of masculinities and feminities in terms of gender roles, suggesting diversity is not only within and across different cultures, but between individuals themselves.