Just like penguins and primates, people trade sex for resources

Female penguins mate with males who bring them pebbles to build egg nests. Hummingbirds mate to gain access to the most productive flowers guarded by larger males.

New research shows that even affluent college students who don't need resources will still attempt to trade sexual currency for provisions, says Daniel Kruger, research scientist at the School of Public Health.

The exchange of resources for sex — referred to by scientists as nuptial gifts — has occurred throughout history in many species, including humans, Kruger says. The male of the species offers protection and resources to the female, and offspring in exchange for reproductive rights. For example, an arranged marriage can be considered a contract to trade resources.

The recent findings suggest, however, that such behaviors are hard-wired, and persist no matter how much people obtain wealth, resources or security.

"It's remarkable to find these patterns in the students in the study," Kruger says. "We have seen many examples where people do this out of necessity, but we still see these tendencies in people who are already well provided for."

In addition, there are predictable, sexual differences in the types of exchanges attempted. Men are more likely to attempt to exchange investment for sex, females are more likely to attempt to exchange sex for investment, Kruger says.

For the study, researchers interviewed 475 U-M undergraduate students to discover if they attempted exchanges in reproductively relevant currencies outside of dating or formally committed relationships, and if they were aware of attempts others tried with them. While the study population was limited to students, these types of exchanges happen all over the world in different cultures and species, he says.

The majority of students were well aware of their own attempts to trade reproductive currency, Kruger says. If they were in committed relationships, however, they did not view the partnership as trading in reproductive currencies.

Overall the strategy of attempting to exchange investment for sex only is successful about 25 percent of the time, the paper found. Some of the attempted trades included: tickets to the U-M versus Ohio State football game, studying assistance, laundry washed, a Louis Vuitton bag and voice lessons, among other things.

Students in the study were 18-26 years old. For exchange attempts made, 27 percent of men and 14 percent of women reported attempts to trade investment for sex, 5 percent of men and 9 percent of women reported attempts to trade sex for investment. Of exchange attempts initiated by others, 14 percent of men and 20 percent of women reported that someone else attempted to trade investment for sex with them, and 8 percent of men and 5 percent of women reported that someone else attempted to trade sex for their investment.

Kruger says the findings are remarkable in that any exchanges were reported at all, considering the subjects' youth and affluence.

"The confirmation of hypothetical predictions regarding these exchanges once again demonstrates the power of an evolutionary framework for understanding human psychology and behavior," he says.

The paper "Young Adults Attempt Exchanges in Reproductively Relevant Currencies" appears this month in the Journal of Evolutionary Psychology.