Over the past two years, more photographs of bare-naked celebrity anatomy have been leaked to the public eye than over the previous two centuries: Scarlett Johansson snapping a blurry self-portrait while sprawled on her bed, Vanessa Hudgens posing for a cellphone in a bracelet and a smile, Congressman Wiener touting a Blackberry and a mirror in the House Members Gym, Jessica Alba, Christina Aguilera, Miley Cyrus, Ron Artest, Charlize Theron, Chris Brown, Bret Favre, Rihanna, Pete Wentz, Ke$ha, and dozens more.

This flood of celebrity skin has prompted folks to wonder, 'Why are so many famous people exhibitionists?' The source of all this au naturel flaunting lies not in the culture of fame, but in the design of our sexual brains. In fact, research has unveiled two distinct explanations: Female exhibitionism appears to be primarily cortical, while male exhibitionism is mainly subcortical.

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“The desire of the man is for the woman,” Madame de Stael famously penned, “The desire of the woman is for the desire of the man.” Being the center of sexual attention is a fundamental female turn-on dramatized in women's fantasies, female-authored erotica, and in the cross-cultural gush of sultry self-portraits.

Studies have found that more than half of women’s sexual fantasies reflect the desire to be sexually irresistible. In one academic survey, 47 percent of women reported the fantasy of seeing themselves as a striptease dancer, harem girl, or other performer. Fifty percent fantasized about delighting many men.

“Being desired is very arousing to women,” observes clinical psychologist Marta Meana, president of the Society for Sex Therapy and Research. “An increasing body of data is indicating that the way women feel about themselves may be very important to their experience of sexual desire and subjective arousal, possibly even outweighing the impact of their partners’ view of them.”

The source of all this au naturel flaunting lies not in the culture of fame, but in the design of our sexual brains.The desire to be desired drives young women’s willingness to enter wet T-shirt contests and flash what their mama gave them at Mardi Gras. Whereas male exhibitionism is considered a psychiatric disorder and sometimes a crime, female exhibitionism is rarely considered a social problem. Just the opposite: It’s exploited commercially. Multi-millionaire Joe Francis built his Girls Gone Wild empire by taping college girls stripping down for his no-budget camera crew. How does he persuade young women to disrobe? He offers them a T-Shirt and a chance to be ogled by millions of men.

"Look I’m human, & just like every girl in this world, I admire my body so i take pics," wrote singer Teyana Taylor after her graphic self-portraits were leaked. International data supports Taylor's contention that the female exhibitionist urge is universal. In Brazil, Japan, Ghana, and the USA, well-trafficked websites offer galleries of tens of thousands of racy amateur self-portraits surreptitiously downloaded from women’s private MySpace or Facebook accounts or maliciously provided by ex-boyfriends. It's not just celebrities who share intimate imagery.

Though men are so eager to gaze upon women's candid photos they're willing to risk jail time by hacking cellphones, pictures of men's private parts usually come to public attention when a recipient is offended; German Olympian Ariane Friedrich, for example, outed a man on Facebook for sending her a photograph of his manhood. These pickle shots tend to elicit protests and consternation. Men do not question why Scarlett Johansson or Jessica Alba might want to sext bare skin to a guy. But women everywhere ask, 'What are men thinking when they send us photos of their junk?' The answer is that men may not be thinking at all; they may be compelled by an unconscious, evolutionary urge inherited from our primate ancestors.

Male monkeys and apes routinely display their penises to females to indicate sexual interest. Primatologist Frans de Waal writes in Peacemaking Among Primates:

Since bonobos can sheath their penis, nothing is visible most of the time. When the organ does appear, however, it is not only impressive in size, but its bright pink color makes it stand out against the dark fur. Males invite others by presenting with legs wide apart and back arched, often flicking the penis up and down – a powerful signal.

Men do not share women's desire to be desired. Instead, they emulate their bonobo brethren: The internet is saturated with penis self-portraits from every nation on Earth. At any given moment, one in four cameras on the webcam network ChatRoulette are aimed at a penis. On the adult networking site Fantasti.cc, 36 percent of men use an image of a penis as their avatar; only 5 percent of women use a vagina. On Reddit's heterosexual Gone Wild forum in 2010, where users were free to post uncensored pictures of themselves, 35 percent of images self-posted by men consisted of penises.

Though hordes of men pay to peruse amateur photography depicting the anatomy of ladies, not a single website collects cash from ladies interested in surveying amateur photography of phalluses.Anyone who has seen a koteka, the elaborate two-foot-long penis cap worn by men in Papua New Guinea, can easily believe that men have inherited our hominid cousins' exhibitionist urge regarding the penis. In fact, male exhibitionism has long been understood by clinical psychologists as a non-dangerous compulsion: Men who flash their organ to strangers rarely seek contact afterward, instead describing a powerful sense of relief from the display alone. Of course, the yawn is also a powerful biological compulsion, but as we learned in grade school it's always preferable to cover your mouth.

Though hordes of men pay to peruse amateur photography depicting the anatomy of ladies, not a single website collects cash from ladies interested in surveying amateur photography of phalluses. It is this marked gender difference in interest that reveals the dichotomous evolutionary pressures shaping male and female exhibitionism: Women feel the conscious desire to catch the universally attentive male eye, but since women's erotic attention is rarely ensnared by a penis, the male exhibitionist urge is comparatively vestigial.

There are profitable penis sites, however. They boast an engaged clientele who view male sexting as neither troubling nor distasteful and reveal the universality of male sexual circuitry. Who appreciates leaked shots of The Game's well-endowed Hosea Chanchez with the same enthusiasm heterosexual guys show for leaked shots of Mad Men's well-endowed Christina Hendricks? Gay men.

Editor: Caitlin Roper