Adaptations for detecting physical strength

Human adaptations for the visual assessment of strength and fighting ability from the body and face by Aaron Sell, Leda Cosmides, John Tooby, Daniel Sznycer, Christopher von Rueden, and Michael Gurven in Proceedings of the Royal Society London, (Biological Sciences), October 2008.

Like other animals, ancestral humans needed to know when to defer or persevere in conflicts and negotiations, given that others can resort to violence.  These studies report evidence that humans evolved mechanisms designed to accurately assess men’s fighting ability to assist this strategic choice.  Visual assessment of men’s fighting ability is almost perfectly correlated with perceptions of their strength.  Importantly, perceptions of strength strongly track objective measures of upper body strength—the component of strength most relevant to premodern combat. Subjects can even reliably detect upper body strength from viewing the face alone, including faces from culturally unfamiliar indigenous groups.

Selection in species with aggressive social interactions favors the evolution of cognitive mechanisms for assessing physical formidability (fighting ability or resource holding potential).  The ability to accurately assess formidability in conspecifics has been documented in a number of non-human species, but has not been demonstrated in humans.  Here, we report tests supporting the hypothesis that the human cognitive architecture includes mechanisms that assess fighting ability—mechanisms that focus on correlates of upper body strength.  Across diverse samples of targets that included US college students, Bolivian horticulturalists and Andean pastoralists, subjects in the US were able to accurately estimate the physical strength of male targets from photos of their bodies and faces.  Hierarchical linear modeling shows that subjects were extracting cues of strength that were largely independent of height, weight, and age, and that corresponded most strongly to objective measures of upper body strength—even when the face was all that was available for inspection.  Estimates of women’s strength were less accurate, but still significant.  These studies are the first empirical demonstration that for humans, judgments of strength and judgments of fighting ability not only track each other, but accurately track actual upper body strength. 

The major findings of the four studies were:

1.  Correlations between ratings of strength from photographs of male bodies and actual weight-lifting strength measured across five weight lifting machines are extremely high (r = .70).
    It is common for animal species to be equipped with mechanisms designed to assess physical formidability prior to the engagement of aggression.  This data point suggests that humans are also designed to compute strength from cues in the body, and that they can do this very well even with the comparatively narrow range of strength present in a campus gym (i.e. there were no elderly men or infirm men in the study).
2.  Correlations between ratings of strength from photographs of male faces and actual weight-lifting strength measured across five weight lifting machines are also high (r = .45).
    This data suggest an alternative to the previous literature suggesting that ratings of “masculinity” in the face are designed to signal current or pubertal levels of testosterone.  Ratings of “masculinity” or “dominance” in previous studies may simply be the output of a mechanism designed to evaluate fighting ability.
3.  Correlations between ratings of strength from photographs of female faces and actual strength measured is significant but much lower than male faces (r = .21).
    Because men have faced a longer and more intense history of selection for combat, it makes sense that mechanisms for strength assessment would work better on male faces than female faces.  Rates of physical aggression for males are often 4-5 times those for females.
4.  Men and women are equally good at assessing strength from male faces and bodies.
    Previous research has shown that women are attracted to physically stronger men.  This preference requires mechanisms that can assess physical strength in men.  Men, on the other hand, will require mechanisms for strength assessment to make informed decisions about resource conflict, aggression, ally assessment and mate rivalry.  Both sexes would need to assess fighting ability in males to predict winners in aggressive contests.
5.  Ratings of strength track actual strength better than height or weight.  In other words, raters are detecting muscularity from the body and face, not just body size.
    This data point suggests that muscularity per se is being assessed, and that one cue of physical strength that subjects are likely using is muscle definition and the distribution of weight to the upper body.  Cartoons and other media depictions of strong men support this view.
6.  Raters can estimate with equal accuracy the strength of men from cultures they are completely unfamiliar with.
    Subjects were just as good at judging strength from the faces of men of other cultures as from their own.  That is, thousands of times more experience with members of one’s local culture had no effect on the accuracy of the system.  This reinforces the belief that ratings are the output of an evolved system for detecting strength.
7.  Ratings of strength are tracking upper body strength – the kind of strength most sexually-dimorphic and most useful in aggression – more than lower body strength.  This is true for ratings of the body and ratings of the face.
8.  Ratings of fighting ability and ratings of strength are perfectly correlated with one another (r = .96).  Subjects are not distinguishing between strength and “ferocity” or any other cue of fighting ability.

UCSB Press Release click here

Andrea Estrada
(805) 893-4620
George Foulsham
(805) 893-3071
October 22, 2008

UCSB Study Finds an Evolved Ability for Judging Physical Strength and Fighting Ability from the Face and Body

(Santa Barbara, Calif.) – For our ancestors, misjudging the physical strength of a would-be opponent would have often meant suffering painful — and potentially deadly — defeat.

A study conducted by a team of scientists at the University of California, Santa Barbara has found that a mechanism exists within the human brain that enables people to determine with uncanny accuracy the fighting ability of men around them by honing in on their upper body strength. What’s more, that assessment can be made even when everything but a man’s face is obscured from view, and even when the face is culturally unfamiliar. A paper highlighting the researchers’ findings appears in the current issue of the Proceedings of the Royal Society.

“Assessing fighting ability was important for our ancestors, and the characteristic that the mind implicitly equates with fighting ability is upper body strength.  That’s the component of strength that’s most relevant to premodern combat” said Aaron Sell, a postdoctoral fellow and the paper’s lead author.  “The visual assessment of fighting ability is almost perfectly correlated with the perception of strength, and both closely track actual upper body strength.  What is a bit spooky is that upper body strength can even be read on a person’s face”.   “”

Sell conducted the study with Leda Cosmides, a professor of psychology and co-director of UCSB’s Center for Evolutionary Psychology; John Tooby, a professor of anthropology and also co-director of the Center for Evolutionary Psychology; Michael Gurven, an associate professor of anthropology; and anthropology graduate students Daniel Sznycer and Christopher von Rueden.

The study consisted of four sections, each of which asked the test subjects to assess the physical strength of individuals based on photographs of their faces, their bodies, or both. Subjects were asked to rate the physical strength or fighting ability of the people in the photographs on a scale of one to seven. When the photographs were of men whose strength had been measured precisely on weight-lifting machines, the researchers found an almost perfect correlation between perceptions of fighting ability and perceptions of strength. “When you see that kind of correlation it’s telling you you’re measuring the same underlying variable,” said Tooby.  They also found that perceptions of strength and fighting ability reflected the target’s actual strength, as measured on weight-lifting machines at the gym.  In other sections of the study, the researchers showed that this result extends far beyond the gym—both men and women are accurate judges of men’s strength, whether the men were drawn from a general campus population, a hunter-horticulturalist group in Bolivia, or a group of herder-horticulturalists living in the Argentinian Andes. 

Leg strength was measured along with upper body strength in both the US and Bolivian populations, but the results showed that perceptions of men’s strength and fighting ability reflect upper body strength, not leg strength. “That makes sense,” said Cosmides, “If, for example, you’re trying to lift something really heavy, or run a long distance, your lower body — your legs — will also be significant. But for fighting at close quarters, it’s the upper body that really matters.”

            Added Tooby: “Whether people are assessing toughness or strength, it’s upper body strength they implicitly register. And that’s the critical information our ancestors needed in deciding—or feeling—whether to surrender a disputed resource or to escalate aggressively.”

The study found that the ability to judge physical strength and fighting ability is present equally in men and women, but the researchers suggest it exists for different reasons. In men, the mechanism serves as a barometer for measuring potential threats and determining how aggressive or submissive they should be when facing a possible enemy. For women, the mechanism helps identify males who can adequately protect them and their children.  Men have a lot more experience with rough and tumble play, and direct experience with fighting, yet women are just as good at assessing these variables.  The authors also point out that both men and women are worse at assessing women’s strength, which is what one would expect since inflicting violence ancestrally was mostly the province of men.

“The next step is to isolate what it is in the face that indicates upper body strength,” said Sell. He suggests that the correlation may lie in the heavier brow ridge and thicker jaw that result from increased levels of testosterone. “Many studies have been done of the effects of testosterone on the face. There’s a good chance testosterone is involved in regulating the body for battle, and men with high testosterone — those with a heavy brow ridge and thicker jaw — developed bodies that were more prepared for combat.”

“One reason we evolved the ability to perceive physical strength in the face may be that it’s where we focus our attention when we look at someone,” said Cosmides. “Even if we are able to see someone’s body, we always look at the face. It’s so rich in social information — what a person is thinking or feeling — and adding the assessment of physical strength is a huge benefit. A person who is angry and strong offers a much greater threat than the person who is angry but weak.”