Bigger is better—at least when it comes to a woman's penis preferences, a new study says. The findings suggest that female choice may have driven the evolution of larger penises in humans.

Past psychology studies that have asked women about their preferences for penis size have produced mixed opinions, with some saying they don't care about the size, while others say they prefer larger penises. (Also see "Why Human Penises Lost Their Spines.")

But those women may have felt pressured to say the politically correct thing: That size doesn't matter, said study leader Brian Mautz, a biologist at the University of Ottawa, Canada.

So his team set up an experiment in which 105 Australian women—averaging 26 years old—each looked at 53 life-size images of various computer-generated male silhouettes projected onto a screen.

The gray, homogenous-looking men had different heights, shoulder-to-hip ratios (or body shapes), and flaccid-penis lengths. In past studies, women have generally perceived tall men with greater shoulder-to-hip ratios as more alluring, making these factors important in how a woman judges attractiveness.

The women in the current study were then asked to rate the men on an attractiveness scale of 1-7, with 1 being least attractive and 7 being most attractive.

Although body shape was the most important factor in whether or not women rated a male shape as attractive or not, penis size also significantly influenced attraction.

However, the relationship wasn't necessarily linear—i.e., "as you get a larger and larger penis, it doesn't necessarily mean you're getting more and more attractive," Mautz noted. (Read about the barnacle, which has the longest penis relative to body size in the animal kingdom.)

For example, the rate at which women rated the men as attractive fell when flaccid penises were longer than 2.99 inches (7.6 centimeters).

What's more, height was also a factor: "Tall guys with really large penises ended up being most attractive relative to other figures," Mautz said.

It's not totally clear why women prefer bigger penises, but studies have shown that women prefer larger sizes because they can increase sexual satisfaction, Mautz said.

The Evolving Penis

Since early humans didn't wear clothes, male penises were obvious to women. So if women chose their mates based on the size of their genitalia, it's possible that these decisions influenced the evolution of bigger penises, according to the study, which was published in the journal Proceedings of the National Academy of Sciences. (Explore an interactive of the human body.)

"We can't say for sure that female choice is what drove the evolution of penis size," but "at this point in time, penis size has an effect on attractiveness," said Mautz.

But evolutionary psychologist Gordon Gallup, Jr. said in an email that there are some limitations to the team's theory that female choice played a role in penis evolution.

For one, previous studies have shown that "the flaccid human penis is an imperfect indicator of the size of the erect penis," said Gallup, of the State University of New York at Albany, who was not involved in the study.

Theoretically, women in the early days of human evolution would have seen most penises in a flaccid state.

What's more, the results show that women paid more attention to shoulder-and-hip ratios than penis size when rating an image as attractive, he said.

Gallup has done his own research on penis evolution, which he said has been shaped by a man's competition with other males to fertilize a woman's egg.

Gallup's research has found that the penis's arrow-like shape is able to scoop rival male semen out of the female's reproductive tract. (Also see "Sperm Works Best in the Winter.")

What's more, studies using models of human genitalia have shown that deeper thrusting displaces more semen from a woman's reproductive tract than shallow thrusting. Therefore, "long penises may have also evolved to deposit semen in the deepest and more remote parts of the vagina to promote more effective sperm retention," he said.

Whatever the reason for large penises, this field of study addresses a question that some will find humorous, study leader Mautz acknowledged.