Unless you’re living under a rock, you might suspect that stereotypes about men being hornier than women and wanting more sex without strings have some truth to them.
And they do. Researchers have collected a huge amount of data on sex differences in libido and preference for sexual novelty, using a variety of different methods, and from a wide range of diverse cultures.
Repeatedly, studies have shown that while “sociosexuality” (the willingness to engage in sexual activity outside of a committed relationship) varies between men in countries all over the world, on average, men have a significantly higher libido than women, even if they can’t always act on their sex drive.
The conclusion drawn from this research is thatsociosexuality results from “cultural influences superimposed on biological predispositions.”And central to those predispositions is testosterone.
My new book,Testosterone: The Story of the Hormone that Dominates and Divides Us, looks at how testosterone creates many of the differences between men and women, and there is little doubt that sex drive is one of them.
It isn’t always easy to measure libido, given the surveys can rely on self-report – and respondents may not be answering honestly.These problems can be overcome to some extent by including questions designed to ensure that answers are consistent.
And then there are other, more creative methods, like hooking people up to fake lie detectors. But the penis never lies. At least, that’s what sex researchers seem to think, and it strikes me as a pretty solid observation.
We can’t stick microdialysis probes into the brains of men and women as they get it on or ask them to switch out familiar sex partners for new ones just to see what happens.But we can let sex researchers use some of their favorite tools, and the penile plethysmograph has got to be near the top of the list.
This involves strapping a ring onto a man’s member that registers changes in its circumference as he watches nature documentaries or p*rnography, or listens to erotic recordings or classical music.
When men – mostly college undergraduates in Europe and the Americas – come into the lab for one of these experiments that include more “objective” measures of arousal like those from the plethysmograph, the results match their self-reports.
In other words, what the penis says about how turned-on a man is matches his verbal reports about how turned-on he is. As a man watches p*rnography, his sexual arousal (as indicated by changes in penile tumescence) wanes over time if the actors in the movie don’t change. But it pops right back up when new actors come on the scene.
One group of researchers included other measures of participants’ arousal in response to watching p*rnography: the amount of time it takes them to ejacul*te, the volume of that ejacul*te, and the hardiness of the sperm (the portion that were good swimmers).
All of these measures show that a hearty sexual response returns in men who have sexual access – or the virtual suggestion of sexual access – to a new mate. The converging evidence for the Coolidge effect in men, regardless of sexual orientation, is strong. If the effect is present in women, it is much weaker.
Other lines of evidence support this sex difference in the preference for novelty and in libido. For instance, men out-masturbat* women. Men are also the main consumers of p*rnography worldwide, which allows them to view naked, apparently sexually eager women (or men) without working for it.
They fantasize more about sex with strangers and multiple partners, while women tend to fantasize about current partners or people they know. Men are almost exclusively clients of prostitutes, the world’s oldest profession. They are more likely than women to use infidelity websites and to respond positively to invitations to have sex with someone they don’t know.
Given what we know about human evolution, and the evidence for competition between men for sexual access to women, the most parsimonious explanation of these sex differences is that men are adapted to have a greater sex drive than women and a stronger preference for sexual novelty.
Culture matters, but there is no evidence that it can account for the consistent patterns of sex differences in sexuality across the globe. Adaptations need mechanisms, which arise from the genes that have been preferentially selected because they confer a reproductive advantage.
Once it is granted that men’s greater sex drive and preference for novelty are an adaptation, there is little doubt that testosterone is part of the mechanism. Whatever the mechanism is, it of course has to differ between males and females. And high testosterone, a product of male sperm-producing testicl*s, clearly promotes physical and behavioral features designed to increase mating success.
There is every reason to think that the mechanisms that explain the greater male libido and preference for sexual novelty likewise involve testosterone. Animal studies aside, we know that large increases in men’s testosterone levels, going from extremely low to normal, will increase sex drive, sexual arousal, and sexual function.
And the reverse is true. Those measures all decline for men who shut down their testosterone production for medical reasons, or as part of a gender transition. A man’s testosterone responds to physiological, social, and environmental circ*mstances to incline him to behave in ways that have been, on average, reproductively beneficial. Sometimes that will involve seeking out multiple partners, but often it won’t.
Testosterone levels can also change when men become fathers. First, testosterone may drop in newly paired men who are in committed relationships, and lower testosterone in this situation is associated with being a more devoted partner. Conversely, higher testosterone is more characteristic of men who are keeping an eye out for opportunities to cheat.
When the baby comes, testosterone may drop even more. This drop, along with other hormonal changes, may serve to make the hard work of parenting feel less like a chore and more like a rewarding and enjoyable activity (the balance there is, of course, often tipped in the other direction). In addition, it may dampen one’s drive to compete with other males and seek out other mates.
As ever, there are cultural differences and outliers, but these findings show just how testosterone drives and divides the genders. It’s present in our blood in minute quantities, but its effects are untold.
This is an edited extract fromTestosterone: The Story of the Hormone that Dominates and Divides Usby Carole Hooven, alecturer and co-director of undergraduate studies at the Department of Human Evolutionary Biology at Harvard University. The book is published by Octopus on July 8. To order, visit books.telegraph.co.uk or call 0844 871 1514
