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updated 4/16/2013 5:40:47 PM ET 2013-04-16T21:40:47

The human male possesses the Italian designer faucet of penises. They’re pretty big, the biggest of any primate’s relative to body size. And they’re showy, too, right out there, front and center on our upright bodies (i.e., they don’t retract), as if they were meant to be seen as part of the décor. Why?

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A study released today in the journal Proceedings of the National Academy of Sciences (PNAS) offers an explanation: Women are attracted to penises, and the bigger the better.

“Penis size does affect attractiveness,” lead author Brian Mautz, a University of Ottawa post-doctoral researcher said in an NBCNews.com interview.

Past research has seemed to indicate that women, as a group, are drawn to larger male members. But those results have been disputed as sexist, or scientifically flawed, or both.

So Mautz and his team, working at the Australian National University, designed an experiment in hopes of settling the controversy. They created 49 unique, computer-generated, nude, life-sized male figures. Each figure varied in three traits: height, shoulder-hip ratio and flaccid penis size.

The researchers then displayed all the figures to 105 Australian women with an average age of 26. The women, who were not told which traits varied, were asked to rate the attractiveness of the figures as sexual partners on a scale of 1-7. The women were alone in the room and their responses were anonymous.

As past studies have shown, women prefer tall men with broad shoulders and narrow hips, like an Olympic swimmer. But when Mautz controlled for those variables, it turned out that penis size (overall length and girth) was about as important as stature.

“As you increase penis size, the amount of attractiveness scores gets bigger” in a linear fashion, he explained, until 7.6 centimeters, or 3 inches. After three inches, attractiveness still increased, but in smaller increments.

Not only were the ratings higher, but the women also spent more time gazing at the generously endowed figures, a sign they preferred looking at them as opposed to figures with smaller penises.

Women with a greater body mass index held stronger preferences for big penises. And size was most critical in tall men, perhaps, Mautz speculated, because “a taller guy must have a disproportionately larger penis to sort of make it clear” he’s endowed.

Some have argued that penis size fretting is driven by a body-obsessed culture and porn saturation. But according to Stuart Brody, a researcher at the University of the West of Scotland who’s conducted studies on orgasm, penis size and relationship satisfaction, “some erotica might reflect fads, but there is a potent evolutionary motivation” at work, too.

That’s what interested Mautz, who studies mate-choice, or why we choose one individual over another. Women make mate choices based partly on evolutionarily constructed fitness preferences and may be using penis size as a clue, Brody said. “The results of the PNAS study (and our own penis size studies) are consistent with a mate-choice perspective.”

But a clue to what? Women may be looking for orgasms, which, in turn, Mautz suggested, may serve a pair-bonding function. In the recent book, The Chemistry Between Us: Love, Sex and the Science of Attraction (which I co-authored), Emory University neuroscientist Larry Young argues that the big human penis evolved into a tool meant to stimulate both the vagina and cervix as a way trigger the release of oxytocin in a woman’s brain, activating bonding circuits. Such bonds provide a survival advantage to offspring.

Or as Mautz puts it in his paper, “Our results support the hypothesis that female mate choice could have driven the evolution of larger penises in humans.”

Of course, this is the 21st century. Most men wear pants – or at the very least, kilts. Mautz was quick to soothe men by saying that his study did not include other proven mate choice factors like money, intelligence, hair or whether a guy drives a 1997 Chevy Astro.

Brian Alexander (www.BrianRAlexander.com) is co-author, with Larry Young, of "The Chemistry Between Us: Love, Sex and the Science of Attraction," (www.TheChemistryBetweenUs.com).

© 2013 NBCNews.com  Reprints

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