Andy Dubbin/FUSION

The human male's so-called biological drive to "spread his seed" has been well-documented in scientific literature—it's even used as an excuse for modern-day cheating. As evolutionary theory goes, men are hardwired to procreate with as many sexual partners as possible to ensure the survival of their genes and the human race. Or something like that.

Women, meanwhile, are supposedly wired to commit to one sexual partner forever and ever, thanks to the demands of childrearing and the risk of sexually transmitted diseases.

Yet many women like to play the field, too—and many women cheat. So can we write off women's promiscuity as evolutionary biology as well? Historically, when has mating with multiple sexual partners benefited women?

That's exactly what a new study, published in the Journal of Personality and Social Psychology, sought to find out. Based on the behavior of other species (such as the Seychelles warbler and Atlantic salmon) researchers from Texas Christian University hypothesized that human females may have also evolved to desire multiple partners as a means to ensure genetic variance in their offspring—especially when threatened by disease.

"Mating with a variety of partners has been shown to increase the reproductive success of females in contexts characterized by high levels of disease by ensuring that some of their offspring will possess the genes necessary to survive and thrive under these conditions," the authors write. "In the current research, we demonstrated that disease cues may similarly affect women’s mating psychology."

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To study this theory, the researchers conducted five experiments and found that "vulnerability to disease—based on developmental history and current exposure to disease threat—may be an important and largely unexamined factor that influences sexual decision-making among women."

The experiments

The researchers tested this hypothesis on modern-day women by placing them in a hypothetical environment in which they were threatened by a potentially lethal disease. For example, in the first experiment (which involved 84 heterosexual females), the researchers created three priming conditions using slideshows: one that illustrated the growing threat of illness and disease (the experimental condition), one about the economic recession (control) and one discussing academic failure (control). Each woman was shown one slideshow.

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Afterward, participants were asked how many sex partners they would like to have over the next month, six months, year, or five years, as well as questions about their vulnerability to disease (how often they get sick, how prone to infectious disease are they, and so on).

The women were told the experiment was about how information is presented and then memorized—they did not know they were being "primed."

The researchers found that women who viewed the slideshow about life-threatening diseases (aptly titled "The Growing Problem of Disease in America: A Sick Future Ahead") also wanted to sleep with more men.

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"Women in the disease threat condition, with higher vulnerability to disease
predicted a greater desire for a variety of partners," they wrote. The same did not occur for women in the economic threat condition, or academic failure condition.

Several of the experiments used this same model—priming the participants with slideshows and then asking them questions—and all found that women primed in the disease condition desired more sexual partners.

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For example, in the third experiment—which used the disease slideshow and academic failure slideshows as a control—71 heterosexual women were primed and then given the following scenario:

"Imagine that you will go on a date twice a week for 1 month. We would like to know which men you’d like to see on your dates. You can choose the same man for every date or choose a new man for each date. It’s up to you! For each day, please write the number that corresponds to the man that you would like to date."

They were then given ten "men" to choose from (i.e. text descriptions of men, with a name and facts such as "John: has a dog, volunteers at a local animal shelter"). The women who viewed the threatening disease slideshow chose to go on more dates with more men versus the control conditions.

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To ensure these results were gender specific, the researchers tested whether men would react the same way. In these experiments, they repeated the "hypothetical dating" experiment, this time using 153 men and women. Again, women primed in the threatening-diseases-are-ruining-the-world scenario, who were also vulnerable to illness, chose more dating partners. However, this was not the case with men. The various scenarios and susceptibility to disease had no effect on their mating strategy.

Overall, the researchers were convinced they had found confirmation for their hypothesis, explaining, "Across five experiments, we found evidence that the threat of illness and disease may play an important role in shaping women’s desire for sexual variety. Specifically, our experiments found that women with a history of vulnerability to illnesses respond to disease threat cues by desiring a greater number of novel sexual and dating partners."

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What's the takeaway? 

The researchers believe these experiments offer some explanation as to why women might also have a biological desire to play the field, despite the perceived risk.

"Mating with multiple men—whether in the form of serial monogamy or extra pair mating—is risky for a woman, potentially decreasing the likelihood of receiving investment from any one of her partners," the researchers note. However, they go on to argue that the long-term rewards might be greater. By ensuring more genetic variety in her offspring, a woman—especially one who is susceptible to illness and disease—can ensure higher survival rates.

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"Having genetically diverse offspring increases the likelihood that at least one will possess the immune genes necessary to survive into adulthood," write the researchers.

According to the study, the desire for genetic variance has been observed in women before. "Research finds that women mated to men of relatively low genetic quality are more likely to desire sex with extrapair mates near ovulation when conception is possible—a shift not exhibited by women with higher quality mates," the authors write. This behavior is also seen in non-human primates.

So while evolutionary biology may not trump modern life (most women aren't actively running around trying to get impregnated by multiple men), it does offer some insight into women's sexual desires.

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Another way to look at it? Contrary to stereotypes, women don't necessarily commit to one man forever because "it's biology"—it may be the opposite, since a human's most primal instinct is to survive.

Taryn Hillin is Fusion's love and sex writer, with a large focus on the science of relationships. She also loves dogs, Bourbon barrel-aged beers and popcorn — not necessarily in that order.