Furthermore, we know that the concept of "gender identity" itself is not simple: It has many dimensions, each of which can vary. There is overlap between the classical genders, as there is in physical measures such as height or strength. This means that, even if we find such apparent gender differences in the brain, they might be difficult to interpret. Furthermore, such differences may be related to factors other than gender identity (for example, sexual preference).
So what has been found? A considerable number of gender differences in the brain have been described. Reassuringly, many of them lie in those parts of the brain that we know are concerned with sexuality (though also with other functions). For example, an area of the hypothalamus, part of the brain well known to be concerned with sexuality (but also with eating, drinking, aggression and other means of survival) is larger in males than females, and smaller in male-to-female transgender brains.
What does this tell us? In fact, not very much, though it’s intriguing. First, because the same finding has been reported in gay men compared to straight ones (so is it concerned with sexual preference?) and secondly, the size of a brain area tells us very little about what it does. Why should a larger INAH3 (the relevant area of the hypothalamus) alter any aspect of sexuality? The brain is not a muscle: What matters is the way it functions (that is, its connections and electrochemical activity), not its size.
There have been reports of chemical differences in male and female brains, but no one knows what these mean, or how (or whether) they relate to particular gender differences in brain function (and these are disputed).
