Why are human penises so large? New evolutionary study finds two main reasons

“Size matters” sounds like a tabloid cliché, but for evolutionary biologists the size of the human penis is truly a puzzle.

Compared to other great apes, such as chimpanzees and gorillas, the human penis is longer and thicker than expected for a primate of our size.

If the primary role of a penis is simply to transfer sperm, why is the human penis so much larger than those of our closest relatives?

Our new study, published today in PLOS Biology, reveals a larger penis in humans serves two additional purposes: to attract mates and to threaten rivals.

Why so prominent?

Understanding why the human body looks the way it does is a popular topic in evolutionary biology. We already know that physical features like greater height and a more V-shaped torso increase a man’s sexual attractiveness.

But less is known about the effect of a larger penis. Humans walked upright long before the invention of clothing, which made the penis highly conspicuous to mates and rivals during most of our evolution.

Might this prominence have been selected for greater size?

Thirteen years ago, in a landmark study we presented women with life-sized projections of 343 videos of anatomically correct, 3D computer-generated male figures that varied in their height, shoulder-to-hip ratio (body shape), and penis size.

We found that women generally prefer taller men with broader shoulders and a larger penis.

That study made global headlines, but it only told half the story. In our new study we show that men also pay attention to penis size.

Read more: Penis size may be driven by women (oh, and it matters)

A dual function?

In many species, traits that are more strongly expressed in males, like a lion’s mane or a deer’s antlers, serve two roles: they are attractive to females, and they signal fighting ability to males. Until now, we didn’t know if the human penis size might also serve such a dual function.

In the new study we confirmed our earlier finding that women find a larger penis more attractive. We then tested whether men also consider a rival with a larger penis as more attractive to women and, for the first time, we tried to determine if men treat a larger penis as a signal of a more dangerous opponent when it comes to a fight.

To find these answers, we showed more than 800 participants the 343 figures that varied in height, body shape and penis size. The participants viewed and rated a subset of these figures either in person as life-sized projections, or online where they were viewed on their own computer, tablet or phone.

We asked women to rate the figures’ sexual attractiveness; and we asked men to assess the figures as potential rivals, rating how physically threatening or sexually competitive each figure appeared.

What we discovered

For women, a larger penis, greater height, and a V-shaped upper body all increased a man’s attractiveness. However, there was a diminishing effect: beyond a certain point, further increase in penis size or height offered smaller returns.

The real revelation, however, came from the men. Men considered a larger penis as an indicator of a rival with both greater fighting ability and as a stronger sexual competitor. Males also rated taller figures with a more V-shaped torso in the same fashion.

However, in contrast to women, men consistently ranked males with ever more exaggerated traits as stronger sexual competitors, suggesting that men tend to overestimate the attractiveness of these characteristics to women.

We were surprised by the consistency of our findings. The ratings of the different figures yielded very similar conclusions regardless of whether participants viewed life-sized projections of the figures in person, or saw them on a smaller screen online.

Instant judgement – with limitations

It’s important to remember that the human penis primarily evolved for sperm transfer. Even so, our findings show it is also a biological signal.

We now have evidence that the evolution of penis size could have been partly driven by the sexual preferences of females, and as a signal of physical ability used by males.

Note, however, that the effect of penis size on attractiveness was four to seven times higher than its effect as a signal of fighting ability. This suggests that the enlarged penis in humans evolved more in response to its effect as a sexual ornament to attract females than as a badge of status for males, although it does both.

Interestingly, our study also highlighted a psychological quirk. We measured how quickly people rated these figures. Participants were significantly quicker to rate figures with a smaller penis, shorter height, and a less V-shaped upper body. This rapid response suggests that these traits are subconsciously almost instantly rated as less sexually attractive or physically threatening.

There are, of course, limitations to what our experiment reveals. We varied male height, penis size and body shape, but in the real world characteristics such as facial features and personality are also major factors in how we rate others. It remains to be seen how these factors interact.

Additionally, while our findings were robust across both males and females of various ethnicities, we acknowledge that cultural standards of masculinity vary across the world and change over time.

Upama Aich receives funding from the Forrest Research Foundation to be based at the University of Western Australia and received a Monash University Research Reactivation Grant to conduct the study.

MIchael Jennions does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.