Bartering sex for stuff or services

While she was studying in Brazil during college, the one thing Stephanie Gerson longed to do before leaving was spend time in the thick of the Amazon rain forest. Unfortunately, she couldn't find a tour that would take her past the forest's edge.

So, when a college-aged busboy at a resort she was visiting began flirting with her, she asked him if he thought a tourist could survive alone in the jungle.

"He laughed and told me I was nuts," says Gerson, 27, who works part-time in online marketing for a chocolate company in San Francisco.

Then he told her that he'd grown up in the jungle in a nearby indigenous community. That was all Gerson needed to hear. Although she wasn't attracted to the guy, Gerson flirted right back in the hopes that he would be her jungle tour guide. It worked. The busboy wormed his way out of work, and the two headed into the rain forest.

"It was amazing," Gerson says of her adventure in 2000. "We built our homes out of palm leaves, I saw animals I'd never seen before, he taught me the medicinal properties of all the plants, we picked fruit off the trees, we swam with and ate piranhas. And, of course, we had sex ... for almost two weeks."

Body currency system

Gerson never felt sleazy or uncomfortable with her unspoken arrangement with the busboy.

"It was a good barter both ways," she says. "I got to stay in the jungle, and he got to have sex with a cute, young American girl."

Such trades aren't so unusual. Throughout history, humans have used their bodies to get what they want -- from ancient Egyptian ruler Cleopatra, who cemented her power through liaisons with Roman rulers Julius Caesar and Mark Antony, to the man and woman who were arrested at a Fort Wright, Kentucky, motel in late June for allegedly swapping sex for gasoline. Regardless of our motivation, scientists say we're hardwired to use our bodies as a bargaining chip.

A recent study of 475 University of Michigan undergraduates ages 17 to 26 found that 27 percent of the men and 14 percent of the women who weren't in a committed relationship had offered someone favors or gifts -- help prepping for a test, laundry washing, tickets to a college football game -- in exchange for sex. On the flip side, 5 percent of the men surveyed and 9 percent of the women said they'd attempted to trade sex for such freebies.

And although they weren't hard up for resources, the students surveyed "recognized the value of this socioeconomic currency system," says Daniel Kruger, research scientist at the University of Michigan School of Public Health, who published his findings in the April issue of "Evolutionary Psychology."

"It's more about getting what you want than getting what you need," he says. "Unless you think everyone needs a $200 Louis Vuitton bag."

The handyman hookup

But unattached coeds aren't the only ones who barter with their bodies. Some professionals will attest that their skills are, well, sexy.

"Women are turned on just by the simple idea of their guy getting off his ass and doing something for them," says Rocky Fino, author of "Will Cook for Sex: A Guy's Guide to Cooking."

It works both ways, he adds.

"Give it to me first thing in the morning, and I'll play [handyman] all day," says Fino, a 39-year-old father of two and part-time construction worker.

Ben Corbett, a 39-year-old contractor from Boulder, Colorado, credits his tool belt with prompting the barrage of come-ons he fields from female clients -- most of them married -- on a regular basis.

"It starts with the flirting, and it just progresses," says Corbett, who has run a construction and remodeling business for 20 years. "They'll touch my hand, and there's all this physical contact. Or they'll run around in their pajamas."

"Once," he says, "I was painting the hallway right outside a client's bedroom, and she was lying on her bed like a girl at a slumber party with her legs up and her arms crossed and her head resting on them, asking me if I had a girlfriend.

"It's all about the fantasy of being taken by the rough-hewn construction guy," muses Corbett, who, despite the temptation, has avoided getting sexually involved with his clientele for fear of jeopardizing his business.

It's the biology, stupid

Call it crass, sexist or gender stereotyping all you want, but there are thousands of years of biological programming at work here, says Dr. Chris Fariello, director of the Institute for Sex Therapy at the Council for Relationships, a nonprofit relationship-counseling group based in Philadelphia.

Plain and simple, a partner who provides more resources -- wealth, shelter, home repairs -- is seen as more attractive and stands to reap more sexual rewards.

Or, as Fariello puts it, "I don't get anybody in my office who says, 'My husband sits on the couch all day and eats bonbons, and I want to have sex with him all the time.'"