I need to say right now that we are talking more about the social evolution of human sexual behavior here than the physical evolution of the structure of the penis!

Sociobiologists argue that in our evolutionary history, male and female proto-humans were engaged in a mating game, a war of attrition, which led to different strategies evolving around reproduction: these roles were shaped by the biological investment each sex made in the sperm and egg, and the subsequent investment in the child. Obviously this is much greater for a female than a male, because she has to carry the developing child for nine months. This means that females would naturally look for a mate who was trustworthy and reliable, and likely to stay around for long enough to help raise the child. This was the way that a woman would ensure her reproductive success - the way she would most effectively spread her genes.

But for men, the pressure was different - a male had no especial investment in the child, except for an ejaculation. His best strategy therefore, would be to assert himself and breed as often as possible, possibly at the expense of other males lower down the social scale. His reproductive success would be ensured if he could fertilize as many females as possible. Equally, of course, the survival of the child would be most likely if a female could attract the alpha male, the strongest, fittest, or best adapted to survive (assuming these characteristics were passed on to the child, or that the man stayed around to protect the female and child). In the conditions in which we evolved as a species, therefore, there appear to have been two contrasting forces at work. And the end result is that a man wishes to use his penis to inseminate as many women as possible, so that through sheer reproductive force of numbers, he leaves as many children as possible. A woman wishes to mate selectively, with the best males, and keep them around after the birth.

Now does this bio-social theory have any relevance to the human penis? Possibly. Let's begin by looking at the unexpectedly large size of the human penis: unexpected, that is, compared to the size of the gorilla penis. Five or so inches on the man, but only one inch on the gorilla! How can that be? First, perhaps a large penis could have been an evolutionary turn-on long ago, a sexual signal to women of male dominance? A signal of strength, a kind of peacock's tail? (The peacock's tail is a sexual signal, for peahens are attracted by the largest and most colorful display. And while such displays do indeed tell the female something about the male's strength and fitness, a large penis doesn't really do that. At least, I don't think it does.....no, it's impossible! Still, we do rather admire a large penis. Oh, sorry, it's men who want a large cock, not women. I forgot.)

Second, therefore, the function of a large penis might just be that it was designed to impress other males - and perhaps put them off a sexual challenge. Could this, perhaps, explain men's obsession with penis size? Certainly dominant male monkeys may use their erections to intimidate other subordinate males - and while we might like to think that this could hardly be human behavior, well, if you have seen adolescent boys at work in the locker room, you know this might actually not be so far from the truth.

Biologists correlate penis size, testicular mass, and volume of ejaculate to frequency of copulation, body size, and other factors. The resultant theory is predicated on the fact that we have sex far more often than gorillas, and the assumption that in our past had sex much more often than we do now - thus we have larger testicles and a larger penis. But why would penis size be so correlated? It isn't, as we have already seen, correlated with body size (the gorilla is much bigger than a human). One theory is that the male with the longest penis and the largest volume of ejaculate was going to have the reproductive edge because he would shoot his load near the woman's cervix, and thus be more likely to fertilize her.

Bear in mind also that the reproductive edge would also be enhanced by a rapid ejaculation. This wasn't a time when sexual pleasure was especially important - this was about reproduction. We can assume there was a distinct possibility that a man who was copulating was at risk both from predators and from other men, who might want to pull him off a woman as he was mating, and replace his semen with their own. Men who made sex last for a long time might therefore be evolutionarily discriminated against and a rapid ejaculation might have become the norm. So - we have a (comparatively) large penis, and we ejaculate quickly.....the first trait might please women, the second tends not to. But why are there men with small penises? How come they escaped this evolutionary lottery? That's a bit of a nonsensical question, because there are plenty of conditions which have been carried through the population even if they were evolutionarily disadvantageous. (Mind you, even the idea that we ejaculate quickly is open to contention - gorillas ejaculate within a minute. Chimpanzees last all of seven seconds. And you could argue that women would wish to get sexual pleasure, so there would have been an evolutionary pressure on men to actually last longer - otherwise, how could the G spot have evolved? Generally this organ only gives a woman a vaginal orgasm during intercourse after prolonged male thrusting.)

But what about men today who cannot get a sexual partner? This is usually due to social issues (like childhood abuse) but we can assume they too have a biological urge to spread their genes. Randy Thornhill controversially claims that human males may have adapted to avoid the effects of female choosiness by using another strategy - rape. Thornhill calls this a genetic programme, a way that nature has evolved of allowing all men a chance to reproduce. Hmm.....social controls on rape would hold this in check, I suppose, but my main problem with the idea is that rape seems often to be about anger and power, and, I assume, is a conscious choice. I think that's pretty clear. And some rapists are already fathers. And most rape is violent. My other big problem is that Thornhill is an entomologist. (He studies insects.) I think we can dismiss this idea.

If this were true, then women are encouraged by generations of evolution to resist men

Other pages on the penis in the human psyche

The penis and the phallus
Penis, phallus and the male ego
A cultural history of the penis
Black men and penis size
Evolution and the penis

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