Human penis size indicates male attractiveness and fighting ability, with a larger size more attractive to women and more likely to intimidate rivals, according to a new study.
Lead author Forrest Research Fellow Dr Upama Aich, from The University of Western Australia's Centre for Evolutionary Biology, said the human penis is unusually large for our body size compared to other primates, and evolutionary biologists had long debated why.
The study published in PLOS Biology used 343 anatomically correct computer-generated male figures that varied in height, body shape and penis size.
More than 800 men and women were shown life-sized projections in-person or participated in online surveys to test if penis size affected sexual attractiveness and was perceived as a sign of fighting ability.
"A larger penis made a man more attractive to women and perhaps more surprisingly, we also found that men use penis size to judge both a rival's attractiveness and his ability to win fights," Dr Aich said.
"Men rated rivals with a larger penis as being more physically threatening and sexually competitive."
The results highlighted that female choice and male-male competition have jointly favoured the larger penis size seen in humans. However, female attractiveness seems to be the stronger force that has driven the evolution of penis size in humans.
Co-author Emeritus Professor Michael D Jennions, from the Australian National University, said while larger penis size increased male attractiveness to women and signalled perceived fighting ability, the effect of penis size on attractiveness to women was four to seven times higher.
"This suggests the enlarged penis of humans evolved more in response to its effect as a sexual ornament to attract females than as a badge of status used by males, although it does both," Professor Jennions said.
The study also found that penis size interacted with body shape and height to affect male attractiveness. A larger penis provided a weaker benefit to smaller, more pear-shaped men.
"We measured how fast participants reacted to figures and they were significantly quicker to rate figures with a smaller penis, shorter height and a less V-shaped body as unattractive," Dr Aich said.
"This suggests that these traits are subconsciously almost instantly rated as less sexually attractive and physically threatening."
Researchers at ANU, Monash University and the University of Melbourne collected data for the study.
