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Just like penguins and other primates, people trade sex for resources

ANN ARBOR, Mich.—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, said Daniel Kruger, research scientist at the University of Michigan 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 said. 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.

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

"It's remarkable to find these patterns in the students in the study," Kruger said. "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 were more likely to attempt to exchange sex for investment, Kruger said.

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 said.

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

Overall, the strategy of attempting to exchange investment for sex is only successful about 25 percent of the time, the paper found. Some of the attempted trades included: tickets to the

U-M versus Ohio State 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.

A sample of older individuals, especially one that is more representative of the general population, would likely report higher frequencies of experiences, Kruger said. The assumption is an older population would have more unmet needs and would be more sexually active.

In fact, Kruger said the findings were remarkable in that any exchanges were reported at all, considering the subjects' youth and affluence—in other words, they don't want for much yet they still attempt these exchanges.

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

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

The University of Michigan School of Public Health has been working to promote health and prevent disease since 1941, and is consistently ranked among the top five public health schools in the nation. Faculty and students in the school's five academic departments and dozens of collaborative centers and initiatives are forging new solutions to the complex health challenges of today, including chronic disease, health care quality and finance, emerging genetic technologies, climate change, socioeconomic inequalities and their impact on health, infectious disease and the globalization of health.