Exploring the link between physical strength and sexual behavior in humans

While many studies have looked at possible evolutionary links between men’s strength and sexual behavior, a Washington State University study included data on women with a surprising result. Women, as well as men, who had greater upper body strength, tended to have more lifetime sexual partners compared to their peers.

The study, published in the journal Evolution and Human Behavior, was designed to test evolutionary theories for human sexual dimorphism-namely that in early human history there was likely a reproductive advantage selecting for men’s greater upper body strength.

Another finding in this study did hint at a reason for that physical difference: men with greater upper body strength were also more likely to to be in long-term relationships.

People have assumptions about men’s sexual behavior and how that’s related to evolution. Besides acquiring more sexual partners, establishing long-term relationships was likely also important for men in evolutionary history.”

Caroline Smith, Study Lead Author and Postdoctoral Fellow, Washington State Univeristy

For this study, Smith and her advising professor, WSU evolutionary anthropologist Ed Hagen, analyzed data on 4,316 U.S. adults from 2013-2014 from the National Health and Nutrition Examination Survey conducted by the Centers for Disease Control and Prevention. They primarily used grip strength, a common measure to approximate upper body strength, and compared it to participants’ survey responses about their sexual behavior.

The findings present a mixed picture, the authors said.

There are several hypotheses around why men have greater upper body strength. One popular theory, known as the sexual selection hypothesis, is based on competition: that like other primates, human males competed against each other for access to mates so needed to be physically formidable to pass their genes on. This theory predicts little relationship between women’s strength and their mating success.

“Men are stronger than women, on average, and men report more lifetime partners than women, but men and women are on the same regression line,” said Hagen. “Regardless of whether they’re males or females, stronger individuals have more lifetime sexual partners. That was a surprising finding and somewhat contrary to the sexual selection hypothesis.”

On the other hand, this study’s finding about long-term partners seems to support another theory based on “provisioning.” Since human babies require a lot of care and resources, particularly from women during pregnancy and lactation, men were more desirable as partners when they could provide meat through hunting, which for hundreds of thousands of years before the modern era required upper body strength. The stronger human males, who also stuck around and helped provide more food resources to those children as they grew, also would better ensure their survival.

While there are theories for men’s strength in relation to reproductive success, there are not so many for women’s strength, in part because women are not often included in these types of studies.

There was not an obvious explanation in this study’s data why women with greater upper body strength also had greater number of lifetime partners. The researchers controlled for many variables, including general health and testosterone levels, but the connection still held. They did cite a few potential theories, including that it is due to “assortative mating,” meaning physically stronger people tend to partner with each other more frequently. It could also be that women who are physically stronger require less male investment or feel like they can take more risks.

Ultimately, more studies involving women would be needed to uncover more evidence for the reasons behind this connection as well as a better understanding of human evolution in general.

“I believe it’s important to continually test our theories, especially by expanding our research questions to include women,” said Smith.