A new study published in Evolution and Human Behavior shows that being physically strong may be linked to a greater number of sexual partners and an increased chance of long-term relationships. Researchers found that men with stronger upper bodies were more likely to be in committed relationships, while both men and women with higher upper-body strength tended to have more sexual partners over their lifetimes. The results add a surprising twist to long-held ideas about the role of physical strength in human mating behavior.
The research team set out to test ideas about how physical strength might have helped our ancestors succeed in attracting and keeping mates. Evolutionary theories have long proposed that men evolved to be stronger because physical strength provided advantages in male competition and mate guarding. However, most previous studies focused on men, and few had examined whether similar relationships exist for women. The investigators wanted to explore whether upper-body strength could also be related to mating behavior in women and to understand how these physical attributes might have influenced relationship formation in the distant past.
“We were investigating the relationship between sex differences in physical strength and sex differences in depression, and we realized that our dataset could also address the longstanding hypothesis that sex differences in strength evolved due to sexual selection: men with higher physical strength were better able to physically compete other men for access to mates, thereby leading to greater number of sexual partners,” explained senior author Edward H. Hagen, a professor of anthropology at Washington State University.
To investigate these questions, the researchers analyzed data gathered from a nationally representative sample of more than 4,300 adults in the United States. This information was collected as part of a long-running national health survey carried out by the Centers for Disease Control and Prevention. Participants in the survey, ranging in age from 18 to 60 years, provided information about their physical health, including a measure of upper-body strength. In this study, the team used hand grip strength as an easily obtained proxy for overall upper-body power. Each person’s grip strength was measured by taking three readings from each hand with a device that quantifies how hard they could squeeze.
In addition to the strength measurements, participants completed a computer-based questionnaire that asked about various aspects of their sexual behavior. They reported the number of sexual partners they had had over the course of their lives as well as in the past year. They also provided the age at which they first had sex and described their current relationship status by indicating whether they were married or living with a partner. The researchers also incorporated a wide range of other information such as age, body size, self-reported health, mood, education level, race, and even aspects of physical activity. By including all of these factors in their analyses, the team hoped to separate the effect of upper-body strength from other factors that might also influence sexual and relationship outcomes.
The results showed a clear connection between upper-body strength and aspects of mating success. For men, higher strength was associated with a greater likelihood of being in a long-term relationship. In other words, men who were stronger compared to other men were more often found in committed partnerships. The data also revealed that, on average, stronger men reported having had more sexual partners over their lives than their less strong peers.
“We found that higher upper body strength was indeed associated with more sexual partners, but also that it was associated with a greater likelihood of being in a long term relationship,” Hagen told PsyPost. “This means that higher upper body strength might have evolved, at least in part, because stronger men were better to provide benefits to women and their families, such as more food and protection, rather than simply because they were better able to physically compete with other men.”
“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,” said lead author Caroline Smith, a recent PhD graduate.
One of the more surprising outcomes of the study was that women with higher upper-body strength reported more lifetime sexual partners than their less strong counterparts. “This is contrary to the sexual selection hypothesis based on within-sex competition,” Hagen noted. “But strength was still more strongly associated with having a long term relationship for men than it was for women, consistent with evolutionary theories of sexual selection based on female choice.”
The researchers speculate that there may be several reasons behind this pattern. For example, it is possible that stronger women might be more selective about the men they choose as long-term partners, which in turn could be related to a greater willingness to explore a variety of mating opportunities. Alternatively, stronger women might be more likely to pair with men who are also physically strong, and if those men tend to have more partners, this might influence the overall number of partners reported by these women. Another possibility is that women who invest in maintaining a higher level of physical strength may also lead lifestyles that promote active social and sexual behaviors.
Further analysis showed that strength did not consistently predict the number of sexual partners people reported in the past year or the age at which they first had sex. It appears that the relationship between strength and mating behavior may be more related to overall patterns across one’s lifetime rather than to very recent sexual activity or the timing of sexual initiation.
The researchers controlled for factors like body mass index, education, race, health status, hormone levels, and activity level, among others. But the study still had some limitations to consider. One issue is that in modern industrialized societies, the number of sexual partners reported by an individual does not necessarily reflect reproductive success. With the widespread use of contraception, the number of partners a person has is not directly linked to the number of children they may eventually have.
Another limitation was that the data included a small number of individuals reporting extremely high numbers of lifetime sexual partners. The team decided to exclude those outliers from the main analyses to prevent them from skewing the results. Additionally, the researchers noted that the study could not account for some biological factors that might influence both strength and sexual behavior, such as hormone levels during early development or conditions that affect hormone balance later in life.
Looking ahead, one important goal is to design studies that can determine whether increased strength actually leads to more success in establishing long-term relationships. It is one thing to find a correlation between upper-body strength and relationship status, but another to show that being stronger causes this success.
“Our study could only show correlation, not causation,” Hagen said. “It would be important to design studies that could test if higher strength caused an increase in establishing long-term relationships. It would also be important to determine if the patterns we found were also found across cultures, especially in those cultures where men’s physical strength was important to putting food on the table and providing other benefits to their families. Finally, we don’t have a good explanation for why women with higher upper body strength also had more lifetime sexual partners, so that would be another issue to investigate in future studies.”
The study, “Strength, mating success, and immune and nutritional costs in a population sample of US women and men: A registered report,” was authored by Caroline B. Smith and Edward H. Hagen.
