Most people think of testosterone as a "male" hormone. That is, a hormone that is responsible for much of what we think of as characteristically male such as assertiveness, impulsivity, competitiveness and, of course, sexuality. There is substantial evidence that this is largely correct. Most of this evidence comes from studies on other species, particularly rodents. Many people imagine that humans, with their greater brains, are liberated from the control exerted in other species such as rodents by hormones like testosterone. But are they right? Have we really shrugged off our biological heritage?
When it comes to the sexuality of women, most people would suppose that it is the traditional ovarian hormones, estrogen, and progesterone, that might play a part, if indeed, women’s sexuality is in any way subject to control by hormones. It’s certainly the case in animals such as rats or cats. Is there any vestige of a similar process in women? But here there is a surprise as well.
Let’s start with males. Rats are born very immature, looking like fetuses. The testes of a new-born male rat secrete testosterone. If the testes are removed, then he grows up to behave more like a female. Conversely, if one gives a little newborn female some testosterone, then she will behave more like a male. Furthermore, she fails to show normal reproductive cycles. Similar results have been obtained in other species, including monkeys—though this happens well before birth, as it does in species that produce relatively mature young, such as guinea pigs. So testosterone has a major role in determining sexuality in these species. What about humans?
The little human male’s testes start to secrete testosterone surprisingly early: around 10 weeks of pregnancy. What does it do? There is a rare mutation that prevents the male’s body from responding to testosterone. These individuals, despite having a set of ‘male’ chromosomes (XY), are born looking like females, and grow up to think of themselves as females. Often they are not diagnosed until the age of expected puberty which, of course, does not happen. So testosterone early in life has a major influence on gender development and identity in humans as well. There is recent evidence that this may come about as one result of testosterone acting on genes in the brain. What about sexual preference? Here the evidence is much less convincing, though there have been suggestions that lack of testosterone at some stage may predispose males towards being gay.
This scenario suggests that females develop as such because of the absence of testosterone early in their lives: that is, being female is the default condition. There is some recent evidence that this might not be the whole story, but we await more information.
Now to the adults. Castrate an adult male cat (or the males of many other species) and he will gradually lose the ability and motivation for sex. Amazingly, the same is true for human males, though what happens depends crucially on when castration takes place. Castration before puberty prevents much or all subsequent sexuality: such people were well known in antiquity, and called eunuchs. But castration after puberty has much less effect, and although sexuality declines slowly, some (a variable amount) may remain. Something important happens in the brain at puberty: we don’t know what this is. It is said that some of the supposed (post-pubertal) eunuchs guarding the harems of olden times took advantage of this without their masters knowing!
Testosterone or its action is often reduced for medical reasons in older men. The results are the same: a progressive loss of sexual ability which is individually varied; we can’t yet predict how men will differ. Testosterone declines slowly with age, again something that varies between men. Does this matter? There is a wide supposition that it does, and testosterone prescriptions for men over 40 have tripled over the past decade. Whether they have the intended results on sexuality, but also on generalized vigor, is still debated. Testosterone goes on playing its ancient role even in humans, though its actions are moderated and controlled by laws, customs, prohibitions of all kinds generated by the complex human brain. So when you next see a sports car, with blaring radio, being driven past you rather too fast, or you’re in a bar and a fight between two men breaks out, and you mutter "too much testosterone," you have a good point; though it’s not too much testosterone, but too little control.
But the real surprise comes in women. Blood levels of testosterone in women are about one fifth that of men. Nevertheless, they are important. Studies on women in whom testosterone has diminished show that it has marked actions on sexuality: particularly the drive for sex, sometimes called libido. Treating such women with small amounts of testosterone restores their sexual drive. Women who complain of a low sexual drive are often treated successfully with small amounts of testosterone. The females of other species, such as rodents, don’t show this: their females respond to the ovarian hormones rather than testosterone though, interestingly, female monkeys are similar to humans: females’ sexuality is stimulated by testosterone, as in women. Why has this change happened? We have no idea, but one result is that it separates sexuality from fertility, in that ovarian hormones vary during the menstrual cycle, whereas testosterone either doesn’t or changes only a little (this may be why some women have increased sexual drive at some stage of their menstrual cycle). This can have important social consequences; fertility in women is disguised in a way not seen in other species.
Testosterone, an ancient and powerful hormone, goes on playing its role in humans as in other animals. Because this action is often not obvious, we have such complicated and varied social and behavioral structures, we should not dismiss its influence on our lives. Many of the rules developed by different societies and religions are designed to limit or control the action of testosterone. Patterns of marriage, limits on sexual aggression, personal and work-related competitiveness, disputes over territory and war itself, are all matters in which testosterone contributes a powerful, but unseen, influence. If you want to be convinced, take a look at a few advertisements.
This fascinating story is told in more detail in Testosterone: The Molecule Behind Sex, Power, and the Will to Win by Joe Herbert.