Decisions have larger pay-offs when uncertainty can be reduced and the outcomes of alternative courses of action can be better predicted. Evolutionary analyses indicate that one of the key variables governing social interactions in species like humans should be formidability, the relative ability to inflict costs. These studies explore the hypothesis that the human cognitive architecture is well engineered to detect formidability in others visually. The results of studies 1–4 are summarized in . All predictions were supported. When asked to rate the strength of men from photographs of full persons or the body alone, people were able to do so accurately, even though the stimuli were small and static, and hence substantially degraded compared with real visual exposure to others in normally experienced social environments. Tellingly, when asked about strength, the subjects supplied estimates that disproportionately track upper-body strength, the strength component most relevant to the ability to win fights, over all other measures. Even more striking, when asked to rate each man's ability to win fights, their perceptions of the men's fighting ability were almost perfectly correlated with their perceptions of the men's strength. Not only was average perceived strength strongly correlated with actual strength, but performance was robust when analysed at the individual level as well, suggesting a capacity that reliably develops across individuals. Taken together, these results imply that the cognitive abilities underlying strength perception and representation were specifically shaped by selection for accurate formidability assessment.
Humans are also good at assessing strength based on the face alone. Even though no part of the men's bodies was available for inspection in these photos, the subjects were able to successfully perceive strength. Indeed, in our data, upper-body strength independently predicted facial ratings of strength, while leg strength did not. This means that the cues the strength detection system is using to judge a man's strength from his face are ones that disproportionately weight the component of strength that is most relevant to fighting ability. This supports the hypothesis that social face processing includes mechanisms designed for formidability detection.
It is often assumed that fighting ability, and judgements of fighting ability, are primarily a function of body size. Our findings indicate otherwise. Men's upper-body lifting strength robustly predicted their perceived strength and fighting ability, even when controlling for their body size and age; when pitted against each other, measured strength always predicted ratings of men's strength and fighting ability better than height, weight and age did when the body was visible, and it was usually the strongest predictor even when raters could see only the face. This means that people are tracking cues of upper-body strength, such as muscularity, that are independent of body size. All else equal, taller, leaner men were seen as stronger when their bodies were visible, but these effects were smaller and less consistent than those for upper-body strength. Height does nevertheless predict strength judgements, and it also predicts reach, which is likely to be an independent contributor to formidability. Along with sex, height is also easy to perceive at a distance, so sex and height might provide the earliest and fastest formidability assessment, to be revised on closer encounter.
Male and female distributions in upper-body strength overlap by less than 10 per cent, with over 99 per cent of women below the male mean (Lassek & Gaulin in preparation
). This renders sex itself a powerful cue in formidability detection, and underscores why in human sociality males tend to monopolize the use of force. For this reason, individual differences in female formidability might not be as urgent to discriminate. Although both men and women can judge female strength from the body and face, as expected, they perform substantially less well than they do for men. Because women's upper bodies were (unlike men's) clothed, the decrement in assessing female strength from the body alone could be an artefact; sexual dimorphism in fat deposition may also obscure critical cues. However, these problems are absent from the studies exploring strength judgements from the face. Based on the face alone, the accuracy of people's strength judgements was, on average, twice as high for male than female faces, suggesting that superior assessment of male formidability is a genuine characteristic of the system.
Although not designed to test questions of ontogeny, these studies supply some limited insights. If formidability assessment were derived from a developmental history of actual conflicts, one might expect men to be better judges of male strength than women are, given that males engage in far more rough-and-tumble play with each other during development (Boulton & Smith 1992
). Yet men and women both were accurate judges of men's strength and fighting ability. Analogously, many anthropologists might expect that humans would learn to exploit culturally specific cues through exposure. However, our 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.
These preferences have been hypothesized to result from mate-selection mechanisms that are designed to detect cues of circulating testosterone and thus a genetically high-quality immune system (Fink & Penton-Voak 2002
; Penton-Voak & Chen 2004
). However, an alternative interpretation would be that the features in the face that are perceived as masculine or dominant are cues of physical strength and hence formidability, characteristics that are inherently desirable for women to have in a mate. Formidability in males should be an important part of mate selection for women, with substantial direct benefits (e.g. Ellis 1992
; Sell 2006
; Fink et al. 2007
; Frederick & Haselton 2007
). If so, this could explain why women were as accurate as men at rating men's strength.
The formidability preference hypothesis and the testosterone preference hypothesis overlap substantially. In humans, as in similar primate species, sexual dimorphism in strength plausibly reflects an evolutionary history of male–male competition. Accordingly, a substantial subset of the long-term developmental effects that testosterone has on the body can be theoretically understood as sexually selected design for aggression. This is consistent with the fact that, in humans, male muscularity is directly related to testosterone levels and develops during puberty as testosterone levels rise (Griggs et al. 1989
). Moreover, testosterone and aggression have been linked empirically (Archer 1998
; Mazur & Booth 1998
), as have been aggression and strength (Sell 2006
; Archer & Thanzami 2007
; Gallup et al 2007
). In consequence, cumulative long-term testosterone levels (and their observable effects) will be associated with strength. It is known that testosterone affects facial morphology, specifically thickening the brow ridge, squaring the jaw and increasing the face's width relative to its length (Thornhill & Gangestad 1999
; Verdonck et al. 1999
; Schaefer et al. 2005
; Carre & McCormick 2008
); indeed, the brow ridge and jaw are the areas of the face used to distinguish male from female skulls (Buikstra & Ubelaker 1994
). These effects of testosterone on the face should covary with the effects testosterone has on the body, including increased physical strength (Bhasin et al
). Hence, it is unclear whether the adaptive benefits of preferring masculinity are strength, immune competence or both—or, indeed, whether masculinity perception is strength perception rather than testosterone detection.
In sum, both theoretical analyses and evidence from other species suggest that selection strongly favours the evolution of cognitive specializations engineered to accurately assess fighting ability. Using what can be considered a gold standard for measuring strength—lifting strength as measured on standardized weight-lifting machines—these studies provide the first direct evidence that both men and women can accurately assess adult men's physical strength, that these assessments perfectly track assessments of men's fighting ability, and that the cues employed are not solely by-products of size but instead track visual correlates of upper body strength, such as muscularity. The overall pattern of results supports the hypothesis that the human cognitive architecture contains specializations for formidability assessment.