is a hot topic. The transgender movement has popularized some of the ethical, medical and social problems faced by those whose bodies do not seem to fit their idea of themselves.

In the simplest, and most extreme cases, their bodies – particularly their genitals – do not correspond to what they feel is their true gender. There is heated debate, not all of it very well-informed, about how to help such people, who are often very distressed.

At the center of this debate lies a question: What is it that determines whether someone thinks of themselves as ‘male’ or female’ or something else – either in between, or neither? It must be in the brain.

If we knew what determined gender identity in the brain would this help? And if we could manipulate this brain area, in a way that was safe and effective, to alter identity after birth, or assess it prenatally, would this be useful or even desirable? But first, let’s look at the information we have at present.

It all begins with a search for areas of the brain that differ between males and females. If these are found, they might indicate a neural foundation for gender identity. Now, this assumes a binary division that applies to most people. This is plausible because at the chromosomal level, nearly everyone (but not everyone) has either an XY or an pair of "sex" chromosomes. This allows us to "binarise" most people.

But just because two people have, say, XY, does not mean that these chromosomes are genetically identical. The they carry can, and do, vary: so their actions vary. This means that "binarisation" becomes inadequate.

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 (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, 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).

Scanning the brain has not helped much either. There are different types of brain scan; the simplest tells us the size of different areas. As we’ve seen, not very useful. Alterations in the brain’s blood flow in response to certain stimuli have been reported, but these often confirm what we already know – for example, that gay men respond differently to pictures of other men than do straight ones. Connections between brain areas may differ between the sexes, but we don’t know how to interpret them in a very meaningful way.

Experimental evidence has helped us understand sexual behavior, and how it is regulated, and revealed some of the factors influencing sexual preference. But we can't ask a rat whether it considers itself male or female, so there is no basic information on gender identity, though there have been attempts. All the evidence we have has to come from humans. That's an obvious problem.

But the real problem is a much deeper one. Contemporary is unable to give the answers we really need. We know a lot about the structure, arrangement, connections and variations in individual nerve cells (neurons). We also know something about which part of the brain does what (though recent progress has been less). But the brain is not a pile of individual neurons. They form assemblies, and it’s these assemblies that produce the functions we all know about: thoughts, emotions, perceptions, memories, and so on.

Now here’s the problem: We just don’t know how neuronal assemblies do these things. There isn’t even a generally accepted theory. Until we have one, we can’t even speculate how the brain might encode gender identity, along with all its other functions. So even if we find a plausible site in the brain that might be responsible for gender identity, we wouldn’t know how it did it. Therefore, we wouldn’t know why it was that a person’s idea of him/herself or their social role etc. was at variance with their body. To describe something is not to explain it, though it may be the first step.

Those who take up vociferous positions on the social position of transgender people, or the best way to help them, should recognize the limits of their knowledge, and maybe adopt a more humble and tolerant point of view. There are many uncertainties, much ignorance, and plenty of topics rich for genuine disagreements. No one has the complete answer, certainly not neuroscientists, though it continues to be a topic of intense interest.

Transgender affairs cross many boundaries, including , medicine, psychology and the law (lawyers are still debating how to define gender, or even if such a definition is necessary). Everyone has a contribution, for how we view gender identity affects us all, whoever or whatever we are. The transgender community has done us all a service by making us think hard about a topic that, only a few years ago, seemed of little interest. Will neuroscience finally yield some of the answers to the questions posed at the start of this piece? Maybe, and with it a series of new, and perhaps very difficult questions: That’s what progress means.

Let’s gaze into the future, and make some assumptions, ones that go way beyond what we actually know. The first, almost certainly wrong, is that gender identity can be assessed along one parameter; rather like the Kinsey scale for hetero/ . But an important point is that this scale is continuous, not a binary one. The second is that there is a single brain area responsible for an individual’s position on this scale. The third is that we that we have a method, perhaps a scanning system, that allows us to assess an individual’s score on this ‘gender identity scale’. How would we use such a method?

Initially, we would want to use it as a research tool. We could find out the distribution of the scores, and how they relate to other aspects of behavior, including sexual activity, but also choice and progression; but there are many other interesting correlations we might choose. We could also see how the score varies during the lifespan, particularly the earlier years, and what factors influenced this trajectory.

Perhaps the most important use might be as a diagnostic guide: to confirm or support treatment options in those with . No longer would we need to rely solely on what the individual or his/her parents told us about his/her gender identity. Instead, we would have objective evidence to guide both his/her advisors and the patient about decisions (lifestyle, , surgery, etc). And we could repeat the scan to see if people with uncertain or unstable gender identity solidified their position with time or circumstance.

So, such a method would have massive usefulness. Would it also be useful in legal or workplace contexts? Probably less, since the relevance of gender in these contexts is also becoming less.

Now let’s make a further step: suppose there was an easy and safe way to alter an individual’s gender score by acting on the relevant part of the brain: how, and when would we use it?

In cases of gender dysphoria, which can cause great distress, then reconciling a person’s gender identity with his/her genitalia (and internal genital organs) might be a highly effective solution. Clearly this would need to be under strict medical (and legal) supervision. But it would represent a ‘normalization’ process, no different, in principle, from surgical corrections of congenital hip displacements etc.

Might there be political or ethical objections? Almost certainly, but this is the case for any new medical procedure, and adequate safeguards could be devised. Would this abolish a transgender community? Not necessarily, since there will be those who prefer that status. But like all scientific and medical advances, this one would offer, along with problems about decisions and , a resolution for those who need it that is currently not available, but theoretically attainable. It will come.