Do long-term relationships help explain human sexual dimorphism in upper-body strength?

– by Caroline B. Smith & Edward H. Hagen

Humans are sexually dimorphic, meaning there tend to be differences between men and women on traits like height and weight. In our study we focused on the sexual dimorphism in muscle mass: men have more muscle mass than women. Why? The standard story is that higher male strength leads to more sexual partners, leading to sexual selection on male upper body strength. Previous research found that higher male muscle mass was positively associated with more sexual partners and lower age at first sex, supporting the sexual selection hypothesis. That study also found that higher strength comes at a cost, specifically decreased immune function and higher energy needs.

However, past research did not investigate women. The sexual selection hypothesis predicts no association between women’s strength and mating success, because partner numbers are not thought to be related to women’s fitness in the same way they are for men’s. We tested the prediction from the sexual selection hypothesis that the association of upper-body strength, proxied by grip strength, with mating success is significantly positive for males and significantly less so for females. We also tested for tradeoffs of upper-body strength with immune function and nutritional intake.

In our study, we used two large independent representative samples of US adults, collected by the National Health and Nutrition Examination Survey (NHANES) in 2011-2012 (N=2853) and 2013-2014 (N=3244). In studies of human behavior, it is important that hypotheses are clearly specified prior to statistical analysis to prevent “fishing” for statistically significant results. To do this, we first conducted an exploratory pilot study using the data from 2011-2012. We used these data to refine our models and predictions, and then tested them in data from 2013-2014 which we had not observed.

We used four proxies of mating success as our model outcome variables: Number of lifetime sexual partners, number of sexual partners in last year, age at first sex, and being currently partnered (True/False). We used grip strength as our main explanatory variable as a proxy for upper-body strength. We also controlled for five groups of possible compounds including anthropometrics (e.g. height and weight), physical activity (e.g., vigorous recreation), socioeconomic factors (e.g., education, ethnicity), hormones (i.e., testosterone), and health factors (e.g., chronic illness, depression). We tested for tradeoffs of strength with immune function (white blood cell count) and energy and protein intake.

We found mixed evidence for the sexual selection explanation overall. In models of current partnered status, we found a significant interaction of strength and sex such that stronger men, but not stronger women, were more likely to be partnered, consistent with sexual selection. But no such significant interaction emerged in models of lifetime sexual partners—stronger men and stronger women both have more lifetime sexual partners contrary to the sexual selection hypothesis. We found no consistent association between strength and either age at first sex or number of past-year sexual partners.

We found positive associations of strength with protein and energy intake for both sexes. We found no significant association of strength with immune function, perhaps because we controlled for height, which was negatively associated, suggesting a tradeoff.

The stronger association of grip strength with partnered status for men than for women is consistent with a role for sexual selection in sexually dimorphic upper body strength, perhaps via female choice for long-term relationships with more productive mates.

It is not clear why women’s strength would be positively associated with sexual partner numbers. It could be that there was selection for more formidable men to prefer more partner variety, and stronger women have a similar preference as a byproduct of selection on men. It could be that there is assortative mating on strength: if stronger men are motivated to switch partners more frequently, their (stronger) mates would also likely have more mates. It might be that stronger women require less male investment, or can take more physical risks, and so instead benefit from greater partner numbers through, e.g. genetic bet hedging, forging relationships with multiple males, ability to conduct a more extensive search for a high-quality long-term mate, or through avoiding or leaving costly long-term partnerships. It might be the case that there are some sex-specific confounds that we failed to control for, or that our models are otherwise misspecified. There also might be reverse causation, e.g., women who are interested in greater partner variety keep in better shape. The sexual selection hypothesis also might be wrong. Or these results might simply be noise.

There were some limitations to our study which are important to note. The data we analyzed for our study are cross-sectional data, meaning we cannot infer causal relationships between variables. Participants self-reported their sexual behavior, including their numbers of sexual partners, and might have either accidentally or purposefully reported their behavior inaccurately. This study used data from the US, where widespread access to contraceptives uncouples reproductive success from mating behavior for both men and women. In addition, we removed 68 participants with 100 or more lifetime sexual partners on the grounds that these might involve evolutionarily novel patterns of behavior that would not conform to predictions of the sexual selection hypothesis.

In conclusion, the sexual selection hypothesis was clearly supported for the positive association of male strength with the probability of being partnered, but not for lifetime number of sexual partners which had a positive relationship with strength for both men and women. We found no consistent association between grip strength and age at first sex or number of past-year sexual partners.

We argue that in human evolution, men likely increased their fitness, not by obtaining additional sexual partners, but mostly by investing in long-term relationships and offspring, and reaping the efficiency benefits of a sexual division of labor. Our results suggest that, for human males, high upper body strength helps obtain a long-term partner through female choice, perhaps due its advantages in provisioning resources through big game hunting and providing protection, and in male-male physical competition.

Read the full article here: Smith, C.B., & Hagen, E.H. (2025). Strength, mating success, and immune and nutritional costs in a population sample of US women and men: A registered report. Evolution & Human Behavior, 46(1), 106647.