Neuro-cognitive processes involved in estimating others’ and one’s own ability to inflict harm and other fitness-related costs in physical confrontation based on available cues related to the likelihood of success in physical conflict.
When organisms are competing for the same resource there are multiple strategies that each individual might take. They might cooperate, they might try to scramble to outcompete, they might leave and seek alternative sources, or they might aggressively compete (Duntley 2005). This last choice is a decision to inflict costs on the other and is likely made where at least one of the other strategies are available. It is therefore an interesting question to analyse when and how organisms decide to aggressively compete.
While a number of factors will be at play in the wider context, such as the value of the resource and how divisible it is, for our purposes here we focus on the relative fighting abilities of the prospective combatants in a human context or as Parker (1974) describes it the decision whether to fight or not may be based upon an assessment of “relative resource holding potential.”
It is possible that assessment of the other prospective candidate may be unnecessary. Arnott and Elwood (2009) report that in a variety of species, there is evidence for a pure self-assessment where the prospective combatant pays attention to its own resources and health and will only deescalate from a fight if these resources drop below some trigger point. This strategy avoids the costly effort of evaluating others. There is some evidence for use of this reduced cost strategy in humans. Taller men (Watkins et al. 2010a) or merely individuals that feel powerful in the moment (Watkins and Jones 2012; Watkins et al. 2010b) have a reduced ability to discriminate differences in male facial dominance. This suggests that their self-assessment is sufficient to at least ignore rival characteristics although not precisely evidence that self-assessment would be enough to escalate to a fight.
What is more likely, however, is that human competitors mutually assess rivals before considering escalating to physical aggression. Various models exist to describe this but all involves initial low-cost sampling of the rival abilities before escalating (Arnott and Elwood 2009).
In many species, agonistic conflicts with conspecifics are key determinants of access to fitness contributors like resources, territory, and mating opportunities or promotion in social hierarchy (Ellis 1995). Species that engage frequently in physical confrontations with conspecifics calibrate their behavior according to their relative fighting ability. Many such species assess fighting ability from visual, auditory, or olfactory cues. Moreover, individuals remember these assessments over time, use them to determine relative fighting ability, calibrate their assessments based on previous victories or losses in fights, and engage in ritualized behavior involving displays and mutual assessments of formidability, and these contests are frequently resolved when one of the antagonists surrenders, which prevents actual costly fights (Sell et al. 2012).
Any individual that can reliably assess the chance to win a conflict in advance and that can make more sensible decisions about whether to initiate, escalate, or retreat from a potential fight is likely to obtain selective advantage. Thus, it seems likely that natural selection would have favored the evolution of cognitive and behavioral mechanisms that would facilitate assessment of fighting ability and that promote the most appropriate decisions and responses to decrease costs and increase benefits from potential confrontations.
Perception of numerous socially relevant characteristics that are important in social contexts (e.g., confrontations and negotiations) are frequently determined by physical traits like conspecifics’ morphology. In many species, direct visual inspection of others’ physical size is well documented (Arnott and Elwood 2009) as primary determinant of social dominance and formidability (see Třebický and Havlíček 2017). Individuals across various taxa can recognize the relative dominance, including fighting ability, of their conspecific upon first encounter depending on displays of body size or presence and size of armaments, ranging from paper wasps (Tibbetts and Lindsay 2008), hermit crabs (Neil 1985), monitor lizards (Frýdlová et al. 2017), numerous avian species (Fretwell 1969; Senar and Camerino 1998), domestic pigs (Rushen 1987) to non-human primates (Bergman et al. 2009; Ghazanfar and Santos 2004).
Recalibrational Theory of Anger
Fighting ability could be described as an ability to efficiently use physical strengths (among other means), against a conspecific antagonist. Greater fighting ability of the given individual should increase the costs the individual is able to inflict upon the opponent. The willingness to use ones’ fighting ability against competitors is commonly mediated via anger. Individuals are deemed to use anger to convince others to treat them better, and the more power they had to harm others, the more convincing they would have been (Price et al. 2012).
The recalibrational theory of anger (Sell 2011) is a computational evolutionary model which proposes that the function of anger is to recalibrate individuals who place insufficient weight on the welfare of the angry individual when making fight-or-flight decisions. According to this theory, (i) individuals differ in terms of their anger thresholds and selective usage of negotiation tactics, (ii) they need to estimate others’ traits relevant to the ability to inflict any costs (e.g., fighting ability) and provide any benefits (e.g., mate value or resource gathering potential), (iii) and use these estimates to calibrate their welfare trade-off ratios (Tooby et al. 2008). More weight is placed on the welfare of individuals who have higher Resource Holding Power – the abilities and willingness to inflict higher costs or appropriate benefits from others (Parker 1974; Sell 2011). Individuals with higher resource holding power easily win conflicts, attain preferential access to mates, and benefit themselves at the expense of others in other ways (Sell 2017). Individuals are expected to have evolved to calibrate their welfare trade-off ratios to maximize their own welfare. Such recalibration strategy confers important selective advantage over fighting without prior assessment.
Due to the significant role of mate selection in evolutionary theorizing, the vast majority of investigations in current behavioral sciences emphasize that intersexual selection (i.e., processes underlying mate choice) is responsible for men’s phenotype (but see Sell et al. 2017; for review Třebický et al. 2012). However, recent studies suggest that a variety of masculine traits appears to be primarily designed for intrasexual competition rather than for attraction of potential mates (Hill et al. 2013; Puts 2010; Sell et al. 2017).
Human inclinations towards antagonistic interactions have been displayed throughout our evolutionary history. Forensic evidence suggests that antagonistic interactions among our ancestors – especially among men – have been rather common and significant selection force in our ancestral environment (Manson et al. 1991; Walker 1997, 2001). Facial skeletal trauma supposedly caused by fist fighting exists in the fossil record of Australopithecus (Roper 1969) and early Homo (Wu et al. 2011) and many other samples of historic skeletal remains (Walker 1997). Historical records and ethnography literature contain numerous examples that acts of physical encounters are not exceptional in many human cultures across the globe (Archer 2009; Horns et al. 2015; Walker 2001) and ritualized combats in the form of folk wrestling and martial arts are also common and have a long history (Green 2010). As in many other vertebrate species, men show higher levels of physical aggression, engage in physical contests and homicides to members of the same sex more frequently than women, and since practically all wars were fought prevailingly by men, this potential seems to be much higher among men than women in virtually all societies worldwide (Archer 2004, 2009). These patterns suggest a relatively high level of male intrasexual physical competition in human evolutionary history. This may account for some typical features of the human male physique, especially for the upper body shape and a greater amount of muscle mass and strength compared to the female body (Lassek and Gaulin 2009). Evolutionary-informed scholars therefore argue that physical encounters have probably generated powerful selection pressures in humans as well that have contributed to the establishment of various neurocognitive and behavioral complexes, whose function is to perceive, assess, and specifically act in antagonist situations or even prior to actual encounters (Carré et al. 2013; Puts 2010; Sell et al. 2009a).
Physical competition often favors evolution of anatomical armaments (structures used as weapons or armors). Unlike some other animals, humans are not equipped with dangerous weaponries such as sharp canines, claws, or poisonous glands. Before humans started using and fabricating objects as weapons, physical combat had depended on hits executed by hands and striking with fists appears to be universally employed in many cultures across the globe as dominant striking technique used in modern combatant arts and “street fights” as well (Horns et al. 2015; Morgan and Carrier 2013; Scoggin et al. 2010). Ancestrally, a man’s upper body strength was therefore a major component of his ability to inflict costs on others. Upper-body strength is considered as critical for physical encounters, particularly for wrestling, punching, or choking, which were the most common types of combats in ancestral populations according to analyses of skeletal remains (Walker 1997). Hence, greater strength should set the individual’s formidability higher.
Interestingly, hand morphology which is similar to modern humans was present in basal hominins (Horns et al. 2015) and although, proportions of the hand may improve manual dexterity, at the same time they make it possible for the hand to clench into fist and be used as a club against opponents. According to the protective buttressing hypothesis, bones of the human hand are proportioned in a way so that they provide a supportive buttress while clenched into fist which protects the hand from injuries and also that the human fist functions effectively to strengthen and stabilize the hand during punches, which allows competing males to strike with higher force while substantively reducing the risk of hand injury (Horns et al. 2015; Morgan and Carrier 2013). Nevertheless, the frequency of hand bone fractures resulting from fighting (Jeanmonod et al. 2011) has been used to argue that the hand is too fragile as a weapon (King 2013). Importantly, in fist fights the primary target is the face (Brink et al. 1998), and bones of our faces break much more frequently than do bones of the hands during fights, indicating that the fist is a sufficiently effective weapon (Carrier and Morgan 2014).
Studies focusing on the locations of injuries that result from assaults and interpersonal violence found that the face was the most common site (Brink et al. 1998; Shepherd et al. 1990) and fractures most frequently occurred in the mandibles, nasal complex, zygoma arches, maxillae, and supraorbital arches. Interestingly, the bone structures that suffer from the highest fracture rates are the parts of the skull that exhibit the greatest increase in robusticity during the evolution of genus Homo (Carrier and Morgan 2014) and are the most sexually dimorphic parts of the human skull (Enlow et al. 1996). Overall facial shape, expansion, and bunodont form of post canine teeth, increased robusticity of the orbits, excessively stronger masticatory system are the traits that tentatively represent functional features of protective buttressing of the face against injury during fighting (Carrier and Morgan 2014). It is also likely that masculine features of the face convey direct information about the degree to which the face is vulnerable to injury. A robust facial skeleton may make an individual stronger opponent simply because he is less susceptible to knockouts and serious injury.
Understanding others can help one to forecast others’ future behavior and adjust oneself accordingly. Being able to assess mating interests, trustworthiness, willingness to cooperate, or formidability of conspecifics is of high importance for individuals’ fitness, therefore to have neuro-cognitive mechanisms that work for perceiving and responding to these characteristics and intentions would be very effective. Such mechanisms should swiftly recognize adequate cues in others relying minimally on direct interaction and should be sensitive to cues that are related to fighting ability in this instance. This is carried out by first impressions – impressions that can be formed very quickly, based on whatever information is available. Such first impressions are often formed by using, among other features, the visual appearance of physical features of one’s face. A substantial body of evidence shows that people attribute various characteristics to others based on their facial appearances like physical strength (Holzleitner and Perrett 2016; Sell et al. 2009a) and aggressive behavior (Carré et al. 2009), among others.
Converging lines of evidence show that people infer various psychological traits based on static facial features. For instance, people judged as more powerful were reported being higher in assertiveness, aggressiveness, and power (Berry 1991), those who were judged as stronger and better fighters were physically stronger (Sell et al. 2009b), fighting success was congruently assessed (Little et al. 2015; Třebický et al. 2013, 2015), and even criminals who were judged as more violent were more likely to have been confined for violent crimes (Stillman et al. 2010). Majority of research on cues of threat potential in humans has investigated the relationships between measures of men’s threat potential such as body size, upper-body strength, or actual fighting ability and assessments of their facial traits (Arnott 2017; Sell 2017; Sell et al. 2012; Třebický et al. 2013). Several studies have reported positive correlations between measures of men’s upper-body strength and ratings of their facial dominance, strength, and ascribed fighting ability (Fink et al. 2007; Holzleitner and Perrett 2016; Windhager et al. 2011). Other studies found that faces of taller men are perceived as more dominant (Burton and Rule 2013; Re et al. 2013a; Watkins et al. 2010a). Ratings from distinct cultures including hunter-horticulturalists, people from industrialized regions, and pastoralists show that cues of physical strength and aggression are present in the face and that these cues can be extracted and assessed rapidly and accurately (Sell et al. 2010; Short et al. 2012; Třebický et al. 2015; Zilioli et al. 2014). A recent study by Han et al. (2017) showed that men’s perceived “facial threat potential” (a composite measure derived from dominance, strength, and weight ratings based on their faces) was positively related to their scores on the “actual threat potential” (constructed from men’s handgrip strength, weight, and height). Man’s handgrip strength (a proxy measure of upper body strength) is positively correlated with women’s ratings of their facial dominance (controlling for age and body weight) (Fink et al. 2007). Also, perceived strength from faces is correlated with actual muscle performance (Holzleitner and Perrett 2016; Hugill et al. 2009) and peer ratings of each other’s fighting ability are tracked by rating of dominance from facial images by strangers (Doll et al. 2014).
Threat Potential May Not Directly Relate to Formidability
It should be stressed, however, that previous studies concerning assessment and measures of formidability (Han et al. 2017; Sell et al. 2009b) rely on body strength or body size as a proxy for fighting ability. While physical strength is undoubtedly an important component of fighting ability, there are many additional sources of variation in the ability to fight. These studies are rather studying a threat potential; however, even high threat potential does not need to translate into actual fighting ability. Several researchers (Dixson et al. 2017; Kraus and Chen 2013; Little et al. 2015; Palmer-Hague et al. 2015; Pollet et al. 2013; Třebický et al. 2013) have overcome this issue by using available data on fighting ability from databases of mixed martial arts contests. Nevertheless, the generalization from these data might be limited as the sample consists of elite athletes highly skilled in hand-to-hand combat and individual bouts are fought only within restricted weight classes.
In most cases of hand-to-hand confrontation in general public, little-to-no experience or formal training can be expected, and therefore other factors like overall body size that influence the outcome of a physical fight must be at play. The fighting ability is undoubtedly a complex skill affected by various characteristics (i.e., ability to concentrate in stress, hand-to-eye coordination, overall stamina, or experiences) and our knowledge in this area is far from definite and contribution of other characteristics may vary according to the form of the fight.
A growing body of research aims to identify exact facial (and bodily) morphological traits and whether people use them as visual cues to estimate those characteristics from the face and body and subsequently test how accurately perceived characteristics relate to relevant morphological structures. For example, in the case of body size, taller people generally have more elongated faces than shorter ones (Windhager et al. 2011) and heavier men have wider and squarer lower facial regions (Coetzee et al. 2010). Available evidence also indicates that people use the abovementioned or related facial cues to assess others’ body weight and height, and they are fairly accurate in their judgments (Re et al. 2013b). It was found that low brows, large chins, wide noses, and narrow mouths are morphological traits related to perceived dominance and strength in both natural and computer-generated faces (Toscano et al. 2014), and that physical strength is associated with rounder faces with wider eyebrows and more prominent jaws (Windhager et al. 2011). While these studies demonstrate that there are specific facial morphological features associated with formidability-related traits and that people pay attention to these traits, they do not demonstrate whether these particular features are indeed present in more formidable individuals. Using the geometric morphometric methods, Třebický et al. (2013) showed systematic differences in structural configurations of facial features, perceived aggressiveness, and actual fighting success in sample of professional fighters (although, the association between facial configuration and fighting success was restricted to heavyweight fighters). Shape regressions revealed that aggressive-looking faces are generally wider and have broader chins, more prominent eyebrows, and larger noses than less aggressive-looking faces.
Facial Width-to-Height Ratio (fWHR)
Currently, the most frequently investigated facial measure is the facial width-to-height ratio (fWHR or relative facial width) (Weston et al. 2007). Previous studies found that fWHR is related to several behavioral and personality characteristics including aggressive behavior and perceived aggressiveness (Geniole and McCormick 2015; Haselhuhn et al. 2015), even in cross-cultural context (Short et al. 2012). Stirrat et al. (2012) found that narrow-faced men were more likely than wider-faced men to die by contact violence compared to other causes of death or homicide. In a sample of professional MMA fighters, the variation in fWHR is associated with actual fighting performance, perception of aggressiveness, fighting ability, body weight (but not height) (Třebický et al. 2015), and higher numbers and proportions of wins and longer fighting careers (Zilioli et al. 2014).
One, currently rather neglected aspect of formidability related research, is cohesiveness of coalitions that seems to influence the outcome of conflicts. Most of the studies conducted so far are aimed on only one specific scenario of physical confrontations, where only two opponents confront. Though, physical encounters among men are not limited solely to the two individual opponents and may frequently involve other, the allies. Of high adaptive importance would be the ability to assess the prowess of a potential opponent as that of a potential ally, in terms of deciding whether to enter the fight and pursue relevant joined activities. Assessments of formidability are therefore not relevant solely in the context of a potential opponent but also in the context of selection of allies. Some species, particularly social primates, assess alliances and cooperative value and show deference toward individuals with more allies or with the ability to deny benefits (Smuts et al. 1987). Men with higher fWHR also show some evidence of increased sensitivity to coalitional alliance (Stirrat and Perrett 2012).
Many factors were suggested to affect assessments of formidability; nevertheless, it is not yet fully understood how these assessments work in real-life situations. We propose a model of multi-level assessments of formidability that tries to cover processes responsible for the fight-or-flight decision when faced with a potential antagonist. In terms of decision-making models, this would correspond to “The fast and frugal tree” model where an individual follows a decision-tree with an exit option after each attribute (Martignon et al. 2003). We suggest that assessment of potential opponents acts on multiple levels. Based on the knowledge of one’s own formidability, a resource holding power (Parker 1974) of each competitor is evaluated regarding the potential costs he can inflict and benefits he can gain from confrontation, so that the welfare trade-off ratios (Tooby et al. 2008) which help to recalibrate appropriate level of anger of rivals could be calculated (Sell 2011). In general, judgments of fighting ability seem to focus on overall probability of winning a fight. The first step, in such a multilevel fight-or-flight decision-making process, might depend predominantly on the overall size of the opponent, as suggested by the ratings of fighting ability in our previous study (Třebický et al. 2015). If one of the competitors is substantially larger (e.g., taller, more muscular, or bulkier), the smaller one surrenders and withdraws from the conflict. When the rivals have roughly equal sizes, a further level of assessment takes place – which might be related to the perception of bargaining potential such as the assessment of aggressiveness from facial features (Třebický et al. 2013). If both competitors feel equally formidable, the confrontation may get escalated.
Antagonistic conflicts with conspecifics are a fundamental factor influencing fitness in many social species, including humans. Fighting ability had profound implications in our evolutionary past and, in some communities, continues to have significant implications in contemporary societies. Evolution of adaptations that facilitate decision-making in potentially agonistic interactions is therefore expected, as the individual must determine whether is it best to fight, flee, or appease the prospective foe. According to the recalibration theory of anger (Sell 2011), benefits of minimizing the costs of engaging in violent conflict are thought to have shaped adaptations for the assessment of others’ capacity to cause physical harm.
The main aim of this chapter was to review evidence whether morphological features served as reliable cues to the actual formidability and whether our visual perception is sensitive towards such cues. Current body of evidence support this notion and show that morphological features predicts which of two opponents are likely to withdraw from an aggressive encounter and based upon converging results of current studies investigating competition in men, we can conclude that our morphological features are valid predictive cues of actual performance in physical fights and that raters are sensitive toward these cues. These studies demonstrate that there are specific morphological features associated with formidability-related traits and that people pay attention to these traits. Raters are able to distinguish the winner of consequent fights with accuracy higher than chance (Little et al. 2015); the winners are rated as better fighters, as stronger, more dominant and attractive, compared to losers (Little et al. 2015). Holistic facial shape analyses and simple facial features ratios showed systematic differences in structural configurations of facial morphology in successful competitors, compared to unsuccessful ones and between individuals perceived as more aggressive compared to less-aggressive ones (Třebický et al. 2013, 2015).
At first glance, it seems obvious that winning and losing in a physical fight is determined by physical strength and body size. Height and bulk are two physical characteristics that are easily observable traits that can be used to provide a relatively crude, albeit somewhat accurate, appraisal of fighting ability among adolescent males (Beaver et al. 2015). Although assortment of other factors like health, skill, hand–eye coordination, proper technique, and armaments may come at play.
Currently, the research mainly focused on thin slices of information provided by a single cue rated on a single behavioral/personality characteristic. However, our perception is not limited to gathering information through just one perceptual modality, e.g., just with our sight and we do ascribe many characteristics based on a single slice of information. Utilizing multiple cues assessment and/or multiple characteristics rating, testing the agreement between modalities and scales, and their subsequent interplay remains a challenge for future studies.
We propose that the assessment of potential opponents acts on multiple levels, then simply based on first impressions. In this multilevel “fight or flight” decision-making model, individuals assess their potential competitors following a decision-tree with an exit option after each attribute. At the first level, the decision-making process might depend predominantly on the overall size comparisons.
Although, competition in humans becomes widely investigated area in behavioral sciences, we are still at realms of rather skin-deep understanding to how the assessments of potential opponents take place. Abovementioned evidence is an important contribution to emerging body of research concerning evolution of neurocognitive and behavioral processes fine-tuned toward perception and adequate behavioral responses toward potential opponents.
VT and JH are supported by the Czech Science Foundation GAČR P407/16/03899S, by Charles University Research Centre program No. 204056, and by the Ministry of Education, Youth and Sports NPU I program No. LO1611.
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