Male Adaptations to Assess Fighting Ability
KeywordsPhysical Strength Body Strength Fighting Ability Lower Body Strength Voice Sample
Adaptations for assessing fighting ability are perceptual mechanisms designed by natural selection to predict the victor of future bouts of aggression.
Nonhuman animals are known to have evolved strategies for the assessment of fighting ability. In short, they “size up” their opponents before and during aggressive contests so as to make more prudent decisions about the fight. Evolutionary psychologists have now gathered evidence that humans also have this competence as a result of several evolved adaptations that extract cues from the body, face, and voice primarily of adult men. This entry reviews the logic of animal assessment and the data showing that humans have evolved adaptations for estimating fighting ability. It also explores some of the new data identifying which cues are used to estimate fighting ability.
The Evolutionary Function of Fighting Ability Assessment
First year psychology students are often told that there are three kinds of neurons: motor neurons to contract muscles and generate behavior, sensory neurons to respond to environmental stimuli, and interneurons to store information and connect the other two. This basic observation reflects a crucially important point about evolutionary biology: that the function of the nervous system in animals is to make muscle movement (i.e., behavior) contingent on environmental cues. The interneurons, which make up the bulk of the human brain, function to compute and store information that is then used to process sensory data and coordinate muscle movement (along with a contingent of other adaptations, e.g., the endocrine system). Analyzed from the viewpoint of evolutionary biology, the brain is literally a computer that runs behavioral programs that are continent on calculations made about information coming from perceptual systems.
Perceptual systems, then, will be designed to perceive only a narrow slice of reality, particularly the cues that enabled behavioral systems to be most efficient at overcoming adaptive problems. For an organism of a social species, these adaptive problems would include many aspects of their fellow conspecifics. For example, an individual’s access to food, territory, water, mates, and information will often depend on the behavior of others. Therefore, we can expect that natural selection would have designed social species with many perceptual adaptations that function to recognize, categorize, evaluate, and store information about other animals. For example, animals are generally designed to recognize their own species, distinguish their biological kin from nonrelatives, evaluate the quality of potential mates, and estimate a conspecific’s fighting ability (Alcock 2005; Arnott and Elwood 2009).
Why Do Nonhuman Animals Assess Fighting Ability?
It is difficult to know why animals would assess fighting ability if one does not know why animals fight. Fortunately, evolutionary biologists understand the basics of animal aggression quite well: aggression is a tool designed by natural selection that enables one organism to disrupt the functional integrity of another organism in ways that enable the aggressor to solve an adaptive problem. For example, a lion requires large numbers of calories to survive. The meat of a gazelle contains these calories, but gazelles have evolved to flee from lions. Lions then evolved neck-biting behaviors that disconnect the gazelle’s respiratory and circulatory system which causes loss of function and renders the prey inert so that the lion can consume it.
Excepting predator–prey relationships, conflicts between conspecifics rarely require that one’s opponent be killed. Conflicts between individuals of the same species are often over territory, food sources, and mates. Such fights can be “won” if an organism disorders the functional machinery that constitutes the body of the other (i.e., injures them). Thus, aggression is frequently used during these conflicts (Alcock 2005). Typically, the larger more powerful animal will win these conflicts, a pattern found in dozens of species. While physical size and strength contribute to fighting ability, in species where size is less relevant to the likely outcome of a fight, different traits will determine the winner, e.g., weapon size, physical condition, and tactical position (Arnott and Elwood 2009). Assessing these traits is therefore of crucial importance. Any animal that can assess, in advance, whether it is likely to win a conflict can make more prudent decisions about whether to instigate, escalate, or retreat during a fight. Therefore, one would predict that natural selection has designed adaptations that perceive cues of fighting ability in others and in oneself and use those estimates to modulate between different behavioral strategies before, during, and after a conflict.
Evidence shows exactly that (Arnott and Elwood 2009; Huntingford and Turner 1987). Many species estimate fighting ability from perceptual cues (e.g., vocalizations, visual inspections) but also from aggression itself. Given the importance of assessing fighting ability, and the selective damage of an erroneous estimate (i.e., retreating from a fight one could have won and commencing in a fight that one loses), it should not be surprising that natural selection has shaped complex mechanisms for these estimates. For example, domestic hens use a kind of Bayesian reasoning when watching two conspecifics fight, so that when one hen defeats another, the hen watching the fight will credit the victor with greater fighting ability if the opponent they defeated was estimated to be a good fighter (Hogue et al. 1996).
If humans have had a history of aggression, then we should expect nature to have equipped them with adaptations that assess fighting ability as well.
Are Humans Likely to Have Fighting Ability Assessment Mechanisms?
The ancestors of modern humans were highly aggressive (Pinker 2011). In past environments (in which the human mind evolved; see Tooby and Cosmides 1990), being able to accurately estimate the fighting ability of another human would have provided a powerful reproductive advantage. Therefore, one would predict on first principles that humans are designed to accurately assess fighting ability in conspecifics (Sell et al. 2009).
Crucially, human aggression is usually deployed against and by males. In this way, humans conform to a widespread mammalian pattern in which the sex that invests less in offspring competes more aggressively for access to mates and the resources used to keep and acquire them (Daly and Wilson 1983). One result is that the male human morph has been designed for combat at the expense of longevity. For example, compared to women, men have larger bodies, higher basal metabolic rates, larger hearts, better heat dissipation, more hemoglobin, more muscle, less fat, denser bones, and much more upper body strength (see Sell et al. 2012). These differences – and others – demonstrate that men have been engaged in aggressive competition for many generations. Finally, while most aggression is deployed between males, research shows that female mate choice is also influenced by factors related to fighting ability (see Collins 2000; Fink et al. 2007; Hughes et al. 2004). This would contribute to the sex differences in frequency of aggression, but also mean that women would also use mechanisms for assessing male fighting ability. Taken together, this chain of reasoning leads to three predictions: (i) that humans have likely evolved fighting ability assessment mechanisms, (ii) that those mechanisms should be fine-tuned to estimate male fighting ability, and (iii) that both men and women should have these mechanisms.
Adaptations for Assessing Fighting Ability
The primary function of assessing fighting ability is to predict the likely outcomes of fights in a way that minimizes the cost of fighting to the perceiver. Due to the nature of prediction, there is a trade-off between the accuracy and cost of the assessment. For example, one can estimate fighting ability very well by fighting someone to the death, but that comes at a great price. On the other side of the trade-off are perceptual estimates of fighting ability that are energetically cheap and do not risk injury but provide – presumably – less accurate assessments.
Because of their cheap nature, assessments in the animal kingdom frequently start with long-distance perceptual scans for cues that predict fighting ability. It is difficult to gain experimental control over realistic visual images and not possible to ask most animals whether a given opponent looks formidable, but evolutionary biologists have used controlled experiments to test whether animals use visual assessments of fighting ability. Results have shown across a number of animals species that visual assessment provides animals with information about fighting ability (see Arnott and Elwood 2009).
If there exist visual cues of fighting ability in humans, particularly human males, one would expect natural selection to have designed assessment mechanisms for the perception of those cues. Evidence supports this.
Visual Inspection of the Body
Humans have high functioning visual systems, and so it could be predicted that natural selection would tailor this system in particular to respond to any perceptible cues of fighting ability in the bodies of other humans – especially males. Such a prediction would require, however, that there actually be visually accessible cues of fighting ability.
How Could One See Fighting Ability in the Body?
A first principled analysis of human combat suggests that an individual’s fighting ability is dependent in large measure on his ability to exert force against another individual. Humans have no naturally produced poisons, toxins, inks, pincers, horns, or other ostentatious weaponry and fight instead with grips, punches, chokes, bites, and artificially constructed weapons. Such methods rely primarily on upper body strength for the application of force; indeed, no prehistoric weapon has ever been found that relied primarily on lower body strength for the production of force. While core and lower body strength is far from irrelevant, the ability to deliver a strike, choke, or hold will depend heavily on upper body strength. Therefore, one large component of human fighting ability will be upper body strength (see Sell et al. 2009). This analysis is supported by the fact that upper body strength is a highly sexually dimorphic trait in humans, with the more aggressive sex (males) having approximately 75% more of it (Lassek and Gaulin 2009).
Can Humans Visually Assess Fighting Ability?
According to this logic, males with physically stronger upper bodies should be readily perceived as such and be inferred to be better fighters. A team of evolutionary psychologists at UCSB conducted the first test to measure just how accurate assessment of physical strength is from visual inspections of the body (Sell et al. 2009). Sell and colleagues measured the upper body strength of 59 male subjects on four weight-lifting machines and took full body, shirtless, photographs of the subjects standing next to an experimenter (for size comparisons). Raters were then shown the photographs and asked to rate the subjects on how physically strong they appeared. Other raters were asked to rate the photographs on how “tough [he] would be in a physical fight.” Results showed that a static two-dimensional photograph of a man contained enough information that raters could predict about half of the variance in weight-lifting strength. Given that there was undoubtedly error in the lifting strength measure, that the photographs were static, and that the sample was truncated to primarily of young college-aged men, this 50% is certainly an underestimate of how much variance in physical strength can be assessed visually. Importantly, ratings of physical strength were identical to ratings of “toughness” (r = .96). This supports the view that upper body strength is a crucial component of fighting ability; in the minds of the raters, they were the same variable.
There is also evidence of specialized design in the assessments that supports the hypothesis that humans possess evolved fighting ability assessment mechanisms. For example, regression analyses showed that ratings of “toughness” and “physical strength” were tracking actual weight-lifting strength more so than height or weight. While taller subjects were credited as being tougher (std. Beta = .29), this effect was much smaller than actual strength (std. Beta = .49). Body weight, once its correlations with lifting strength and height were statically controlled, was no longer related to ratings of fighting ability at all (indeed it was negative in some analyses). In other words, raters were not just picking larger males along easily perceptible dimensions (e.g., size, height) but were extracting cues of actual upper body strength, crediting males additionally if they were tall, and penalizing them slightly for being overweight. Additional analyses revealed that subjects were responding to cues of upper body strength over lower body strength. In other words, their ratings were zeroing in on the particular kind of strength that is used in ancestral human aggression.
What Are the Cues of Fighting Ability in the Body that Are Being Estimated?
The question of what actual cues were being perceived has remained largely unanswered. Morphometric measurements of the subjects were taken, and analyses showed that biceps circumference was the single best predictor of lifting strength, but no bodily measurement or combination of measures predicted upper body strength as well as ratings of strength did. It can be concluded, however, that height is being assessed (and taller men are being credited as better fighters), but the ability to accurately assess upper body strength visually is largely independent of height assessments. More research, preferably experimental research, will need to be done to isolate the visual predictors of fighting ability in men.
Is the Visual Assessment of Fighting Ability More Accurate on Male Targets?
Sell et al. (2009) replicated the assessment of upper body strength on a second sample of college students that included both males and females. This necessitated less naturalistic stimuli, e.g., female subjects wore standardized white T-shirts rather than being photographed shirtless, and upper body strength was assessed using proxy measurements validated on the gym sample, specifically, a portable chest compression measure, self-report, and flexed biceps circumference. Unfortunately, the fact that most of female subjects’ upper bodies were obscured made it impossible to meaningfully compare the accuracy of assessing upper body strength across the sexes. Raters could still assess female upper body strength from photographs, accounting for approximately 25% of the variance in the composite strength measure. This was less accurate than ratings of male upper body strengths, but little can be made of this difference.
Visual Inspection of the Face
Sell and colleagues (2009) also speculated that humans might assess fighting ability via visual inspection of the face. This was based on three lines of reasoning: (i) there is a precedent in that the human brain is known to possesses specialized perceptual mechanisms for processing many kinds of information from the face, (ii) cues of the upper body in ancestral environment may have been obscured by clothing, vegetation, carried objects, tactical cover, and so on, selecting for visual assessment of the face, (iii) decisions that make use of estimates of fighting ability would sometimes need to have been made rapidly, and (iv) the face may contain additional cues of upper body strength not present in the upper body itself (see Sell et al. 2009 for references). Indeed, Fink et al. (2007) had already shown evidence that a man’s handgrip strength (a measure of upper body strength) was correlated with ratings (by females) of “dominance,” “masculinity,” and “attractiveness” once one controlled for age and body weight.
How Could One See Fighting Ability in the Face?
There are a number of reasons that the skeletomuscular structure of the face would be relevant to combat. Perhaps of most relevance is that the face is a frequent target of strikes and that strikes to the face can render an individual unconscious. There is paleontological evidence that the structure of the human face (which is markedly different than that of chimpanzees) has been tinkered by natural selection to absorb blunt force trauma more efficiently (Carrier and Morgan 2015). The possibility that differences in human faces track bite strength is also being investigated (Sell et al. 2014). These lines of inquiry suggest that faces are – to some extent – relevant to combat and therefore that different faces could be more or less proficient at aggression. Another possibility is that the face contains cues of fighting ability but that these cues are not themselves causally related to fighting ability. For example, the “masculinity” and “dominance” perceptions of a face often track testosterone levels in the body (Penton-Voak and Chen 2004).
Can Humans Visually Assess Fighting Ability From the Face?
Accuracy of fighting ability assessment from face (From Sell et al. 2009)
Gym sample (males)
UCSB students (males)
UCSB students (females)
Correlation with upper body strength r(p)
Results showed that both male and female raters could assess physical strength from a visual inspection of a static two-dimensional photograph of a man with enough accuracy to predict about a quarter of the variance in upper body strength. Again, given the static nature of the photographs, the imperfect measures of fighting ability, and the limits of the sample, this likely underestimates the accuracy of assessment. Furthermore, accuracy was not diminished at all when assessing males from unfamiliar cultures (which also had no variance in ethnicity). Assessments of strength from female faces were less accurate when rated by either sex and (unlike full body ratings) were not compromised by sex differences in clothing. This suggests that both males and females are all well calibrated for assessing male strength.
Like assessments of the full body, assessments from the face were not overly dependent on estimates of height or weight. Ratings of physical strength from the face also tracked upper body strength over lower body strength; an impressive feat given is that neither the upper nor lower body was visible. This is consistent with two possibilities: either cues in the face that are being assessed are by-products of development or the cues arise because the developmental systems that give rise to upper body strength also coordinate functional components of fighting ability in the face (e.g., blunt force resistance).
One limitation of the Sell et al. studies is that they rely only on upper body strength as a measure of fighting ability. While upper body strength is undoubtedly a large component of fighting ability, there are many additional sources of variance in the ability to fight. Ideally, researchers would use actual fights to measure fighting ability, but this presents a sizeable ethical problem. Fortunately, mixed martial arts (MMA) – in which individuals fight with relatively few restrictions – has become a popular sport with enough data for analysis. Several researchers have looked at mixed martial arts contests and tested the ability of subjects to predict winners based on photographs of the contestants’ faces. While these fights are undoubtedly better measures of fighting ability, they unfortunately severely truncate the sample to only the best hand-to-hand fighters in the world and further truncate within samples by weight class.
With those limitations in mind, there have nevertheless been several published studies showing links between features of the face and combat performance in MMA.
Anthony Little and colleagues (2015) produced arguably the most direct test of whether fighting ability can be estimated from the face by showing the faces of MMA fighters to subjects who then attempted to pick the winner of the fight. Results showed that they could do this with better than chance accuracy. These faces were also rated for strength by different subjects, and the results indicated that MMA fighters with stronger-looking faces than their opponents were more likely to win.
One of the first studies to look at MMA fighters was done by Třebický and colleagues (2013) who looked at samples of MMA fighters and correlated their recorded fights against ratings of aggressiveness and fighting ability from photographs of their face. Results were mixed because weight was confounded with ratings of fighting ability (another limitation of MMA studies is that the fighters shed body fat in order to qualify in as low a weight class as possible, leading to an artificially high correlation between body weight and muscle mass). Perceived fighting ability was only predictive of fighting records among heavyweight fighters who had no weight restrictions.
Another way to mitigate reliance on upper body strength as a measure of fighting ability is to use peer ratings. Doll and colleagues had fraternities give peer ratings of each other’s fighting ability. They then had facial photographs taken and rated by strangers on “dominance.” Results showed that these dominance ratings tracked peer ratings of fighting ability (Doll et al. 2014). And while dominance and fighting ability are not the same construct, ratings of dominance and ratings of fighting ability are very highly correlated (Toscano et al. 2014).
What Are the Cues of Fighting Ability in the Face?
Compared to research on body assessments, there are many research studies that hint at the cues that indicate fighting ability in the face.
Třebický and colleagues (2013), in their study of MMA fighters, found that aggressive-looking faces had larger chins, more prominent eyebrows, and a larger nose. Zilioli and colleagues (2015) analyzed a sample of several hundred MMA fighters and showed that having a face that was wider but shorter (i.e., facial width-to-height ratio, fWHR) was predictive of having more wins, a greater percentage of wins, and lasting for more matches in the competitive Ultimate Fighters Championship circuit. Artificial faces made to have wider faces were also rated as physically tougher than narrow faces. Toscano et al. (2014) used natural and artificial faces rated on strength and dominance. They found that low brows, large chins, wide noses, and narrow mouths were rated as stronger. Windhager and colleagues (2011) showed that physical strength (i.e., handgrip, shoulder width) was predictive of rounder faces, wider eyebrows, and a more prominent jaw, though the sample size was rather small for such analyses (n = 26). Windhager and colleagues also found that perceived dominance was the result of a wide lower face, smaller lips, shorter nose, and smaller eyes.
Research on the design of the anger face illustrated seven features of the face that are used when assessing physical strength. Sell et al. (2014) proposed that the anger face itself is a coordinated modification of cues in the face that indicate fighting ability. They had computer-generated faces rated for physical strength and showed that each of the seven major components of the anger face – independently – increased perceived strength. The features included: a lower browridge, higher cheekbones, wider nose, higher mouth, thinner lips, protruding lips, and larger chin and chin bun. While this demonstrates that these features do influence ratings of fighting ability, they did not demonstrate that these particular features are indeed present in better fighters.
Is the Visual Assessment of Fighting Ability More Accurate on Male Targets?
Sell et al. (2009) found that both males and females were more accurate at estimating male strength from photographs of the face. Most studies of fighting ability assessment, however, do not look at female faces, so this effect should be replicated before strong conclusions are drawn. Female MMA fighters are becoming more common, but so far no researchers have published work on whether female fighters’ records are predictable from facial features.
Cues of Fighting Ability in the Voice
Unlike the structure of the face, which may play some causal role in fighting (be it through biting or blunt force trauma resistance), it is unlikely that the acoustic properties of the human voice played a direct causal role in resisting or inflicting damage on others. Nonetheless, there are several reasons to believe that the voice may contain indicators of fighting ability (see Sell et al. 2010). Firstly, there are numerous animal precedents showing that vocal qualities not only indicate size and fighting ability but that animals evolved to respond to those qualities (see Huntingford and Turner 1987; Sell 2012). Secondly, research demonstrates that women’s mate choices are highly tuned to characteristics of the male voice, which indicates that there are cues of some desirable traits in the voice (Collins 2000; Hughes et al. 2004; Puts 2005). Finally, testosterone modifies the voice of males in ways that generate large sex differences in vocal characteristics (Dabbs and Mallinger 1999). Given that testosterone plausibly affects fighting ability (Bhasin et al. 1996), it is plausible that fighting ability itself can be predicted from cues in the voice.
Can Humans Assess Fighting Ability From the Voice?
There is good evidence that some physical characteristics associated with fighting ability can be assessed from the voice. Acoustic research in perceptual psychology has a long history – over 30 years – of testing subjects’ ability to estimate body size from the voice. The results depend partly on the nature of the voice samples and often have samples too small to detect the typical effect sizes reported elsewhere, but overall the research demonstrates the ability to assess height and weight from the voice (see Sell 2012 for review).
More evolutionary-minded researchers looked at aspects of the phenotype more closely linked to fighting ability rather than just height and weight. For example, Hughes et al. (2004) showed ratings of attractiveness predicted shoulder-to-hip ratio (a measure believed to be predictive of upper body strength).
Accuracy of fighting ability assessment from the voice (From Sell et al. 2010)
Correlation with upper body strength r(p)
Again, accuracy was not diminished at all when assessing males from unfamiliar cultures. Assessments of strength from female voices were generally less accurate when compared to the males of the same culture. As with photos of the face, additional analyses showed that both males and females were well calibrated for assessing male strength and that the assessments were not highly dependent on estimates of height or weight. Furthermore, because photographs of the subjects were available as well, regression analyses were able to be run that demonstrated that assessments from the voice were not redundant with assessments from photographs. In other words, having access to a man’s voice as well as visual information about his body enabled a more accurate rating of fighting ability than either did alone.
Other research has failed to find a link between various covariates of fighting ability and ratings of dominance or strength. Doll et al. (2014), for example, found no relationship between dominance ratings of male voices and their acquaintance-rated fighting ability. And numerous studies failed to show that subjects could assess height or weight reliably (see Sell 2012).
Regardless, the weight of the evidence is that various body characteristics associated with fighting ability are linked to different acoustic cues that are produced in the normal speaking voice and spontaneously extracted by the human auditory system (see Sell 2012 for review).
What Are the Cues of Fighting Ability in the Voice?
Researchers have suggested numerous acoustic traits that could covary with fighting ability, body size, or the assessments thereof. While some progress has been made, no one has produced an acoustic analysis that can be shown to mimic the ratings of human responses. Regardless, there has been data that demonstrates some recurrent relationships between acoustic variables and various body measurements. The most common are fundamental frequency and formant dispersion.
F0 – Fundamental frequency. Fundamental frequency is the result of vocal fold vibrations during speech and is not physiologically necessarily related to body size or strength. The vocal folds do, however, respond to testosterone and are highly sexually dimorphic due to a rapid thickening in adult males. The result is that adult men have much lower F0s (i.e., lower “pitched” voices). F0 clearly distinguishes men from women, and adolescent men from adult men (Hodges-Simeon et al. 2014), but generally does not highly distinguish strong from weak men or large from small men. Nonetheless, experimental manipulation of F0 does show that listeners respond to it and (mis)attribute greater size and strength to men with lower pitched voices. These results were found in Sell et al. (2010) for all tested cultures. F0 did not predict physical strength but did predict ratings of strength (see also Rendall et al. 2007). In her definitive meta-analysis on vocal correlates of body size, Pisanski and colleagues (2014) found that F0 explained less than 2% of the variance in height and weight within adult within-sex samples.
Df – Formant dispersion. Formant dispersion was first proposed by Fitch (1997) who defined it as the averaged difference between adjacent formant frequencies and is – in theory and in practice – linked to the length of the vocal tract and therefore likely covaries with height and perhaps other physical traits. It is an acoustic variable known to predict body size in a number of nonhuman species (see Pisanski et al. 2014). Sell et al. (2010) found in a single sample of recorded vowel sounds that Df did not predict physical strength and could not account for the accuracy of strength assessment. It did predict height marginally. This is consistent with the larger literature that shows various measures of variance in formant position showing weak negative relationships with height (Pisanski et al. 2014).
Pf – Formant position. Formant position was first introduced by David Puts and colleagues (2011) as a new way of measuring formants which results in a more sexually dimorphic measure similar to formant dispersion (see Puts et al. 2011 for details). Puts and colleagues showed that Pf predicts upper body strength in both a US sample and a sample of Hadza men from Tanzania (correlations were −.26 and −.23, respectively, though given the smaller sample size, the Hadza correlation was not significant).
While the details of the acoustic variables are still under study, there is good evidence that speaking voices contain cues of fighting ability and that people extract and use them to estimate it.
Self-Assessment of Formidability
Without an accurate estimate of one’s own fighting ability, knowledge of another’s would be of little functional use. Thus, it is expected that animals and humans have mechanisms for assessing, tracking, and updating their own fighting ability. Direct evidence comes from experiments such as Richard Alexander’s work with fighting crickets (1961). When crickets were allowed to physically dominate “dummy” crickets (i.e., artificial animals created by biologists to resemble crickets), the victors become more likely to challenge live competitors in the future, demonstrating that victory is tracked in the brain of the winning animal as an indicator of the likelihood of future success. Edmonds and Briffa (2016) recently showed that some animals not only track their own fighting ability, but different aspects of their fighting ability. When hermit crabs had the effectiveness of their shell knocking strikes reduced (by buffering their shell), they compensated by switching to an uncompromised and lesser used tactic – rocking their opponent’s shell. This switch suggests that the crabs are estimating the effectiveness of at least two kinds of strikes and updating this estimate continuously.
The phenomena of “badges” – in which animals signal their fighting condition via coloring or markings – also demonstrates that animals maintain an internal representation of their own fighting ability (Alcock 2005).
The most direct evidence that humans store representations of their own formidability is data showing very high correlations between objective measurements of physical strength and subjects’ own self-assessments (Sell et al. 2009). Similar research shows that individual’s self-ratings of fighting ability correspond closely to those of their peers, i.e., men and their peers agree on who are the best fighters (Doll et al. 2014). The mechanisms which humans use to do these computations, and their computational structure, remain to be mapped.
Future Research: Aggression as Assessment
From an evolutionary point of view, perceptual assessment is low cost but low accuracy. There are many factors that determine an individual’s fighting ability that will not be vulnerable to a visual or auditory analysis. For example, the following factors are sexually dimorphic and arguably causally related to fighting ability: reaction time, basal metabolic rate, mental rotation and speed of spatial visualization, throwing accuracy, history of combat practice, heat dissipation rates, hemoglobin levels in the blood, heart size, and lung capacity (see Sell et al. 2012). These traits could still be estimated if the developmental programs that gave rise to them were, for example, coordinated by pubertal testosterone levels which left perceptible cues in the voice and face. Regardless, these are imperfect estimates of imperfect indicators.
More accurate estimates of fighting ability can be gained through actual fighting. Evolution has designed mechanisms in many nonhuman animals for engaging in ritualized aggression or “conflicts of assessment” (Huntingford and Turner 1987; Alcock 2005) that function to enable two animals to assess fighting ability during a given conflict. For example, the cichlid fish studied will progress through a series of aggressive stages starting with perceptual assessment (e.g., parallel swimming), to coordinated displays of body size and strength (e.g., “tail-beating” in which the fish take turns shaking their bodies at each other to make waves), and finally actual contact aggression (e.g., mouth-locked wrestling) (Enquist et al. 1990). By following the rules of this “ritual,” both animals are able to assess fighting ability through forms of aggression that are less likely to result in injury for both contestants.
Such rituals are found among humans as well (Pinker 2011). A more detailed and thorough treatment of ritualized aggression in humans is needed and will likely be able to explain core concepts in human aggression such as honor, “fair fights,” sports, and sportsmanship.
In conclusion, given the long history of human aggression – particularly between males – there is a strong theoretical reason to believe that natural selection would have favored mechanisms for estimating male fighting ability. The computational structures of these mechanisms are now being mapped. Evidence shows that that both men and women can estimate aspects of fighting ability from cues in the body, face, and voice. While the exact nature of the cues is still being explored, there is evidence that cues of fighting ability in the face include aspects of the brow, the width-to-height ratio of the face, nose and mouth size, and jaw size. Cues of fighting ability in the voice likely include aspects of formant position that result from the length of the vocal tract and possibly fundamental frequency. Future research will likely show that a great deal of human interpersonal aggression is also designed for assessment.
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