Study reveals that the size of the human penis evolved for two purposes

For evolutionary biologists, the (huge) size of the human penis is a real enigma.

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

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

A study this Thursday in PLOS Biologyreveals that a larger penis in humans performs two additional functions: attract partners and intimidate rivals.

Why is it so prominent?

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

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 throughout most of our evolution.

Could this prominence have been selected for greater size?

Thirteen years ago, in another milestone, the same team presented women with life-size projections of 343 videos of anatomically correct, 3D computer-generated male figures that varied in height, shoulder-to-hip ratio (body shape) and penis size.

They discovered that women, in general, prefer taller men, with broader shoulders and a bigger penis.

That study made headlines around the world, but the team warns that it only tells half the story. The new research surprises and shows that men also pay attention to penis size.

A dual function?

In many species, features more strongly expressed in males, such as a lion’s mane or a deer’s antlers, serve two functions: they are attractive to females and signal fighting ability to males. Until now, it was not known whether the Human penis size could also serve a dual function of this kind.

The new study confirmed the previous finding that women find a larger penis more attractive. It then tested whether men also consider a rival with a larger penis to be more attractive to women and, for the first time, attempted to determine whether men treat a larger penis as a sign of a more dangerous opponent when it comes to a fight.

To find these answers, more than 800 participants were shown 343 figures that varied in height, body shape and penis size.

Participants observed and classified a subset of these figures, either in person as life-size projections, or online, where they were viewed on their own computer, tablet or cell phone.

The women were asked to rate the sexual attractiveness of the figures; and the men were asked to evaluate the figures as potential rivals, classifying the degree of physical threat or sexual competitiveness that each figure appeared.

And it turned out that…

For women, a larger penis has increased a man’s attraction. However, there was a diminishing returns effect: beyond a certain point, further increases in penis size or height offered smaller benefits.

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

However, in contrast to women, men consistently rated males with increasingly exaggerated features as stronger sexual competitorssuggesting that men tend to overestimate the attraction of these characteristics to women.

Classifications of the different figures produced very similar conclusions, regardless of whether participants observed full-size projections of the figures in person or viewed them on a smaller screen online.

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

There is now evidence that the evolution of penis size may have been partially driven by females’ sexual preferences and as a signal of physical ability used by males.

However, although the results were consistent across both men and women of various ethnicities, it was also noted that cultural standards of masculinity vary across the world and change over time.