We begin by evaluating the intensity of sexual selection in men. For sexual selection to shape a trait, the trait must be partly heritable, it must influence mating success, and mating success must influence reproductive success (Jones, 2009 ). Thus, we assess the association between mating and reproductive success using such conventional correlates as the operational sex ratio, parental investment, reproductive rate, and reproductive variance. We then examine correlations between male phenotypes and mating and reproductive success. Next, we use a comparative and functional approach to evaluate the extent to which, and in what ways, men's phenotypes were shaped by contest competition. Finally, we explore how contests may have contributed to male mating and reproductive success over human evolution.

In this chapter, we explore how men's phenotypes, including their psychologies, have been shaped by an evolutionary history of contest competition. Sometimes called intrasexual selection, contest competition is one of several mechanisms of sexual selection and involves the use of force or threat of force to exclude same‐sex competitors from mating opportunities (Andersson, 1994 ). Other mechanisms of sexual selection include mate choice (favoring ornaments and displays for attracting mates), sperm competition (occurring when multiple males' sperm occupy a female's reproductive tract during one fertile period), and sexual coercion. Multiple mechanisms of sexual selection can operate simultaneously in one species.

The preceding data suggest a positive and moderately strong relationship between men's mating success and reproductive success. The few datasets that assess this relationship, including data from foragers (Salzano et al., 1967 ), indeed indicate a positive relationship (Brown et al., 2009 ). However, sexual selection also requires mating and reproductive success to be associated with phenotypic variation (Jones, 2009 ; Klug et al., 2010 ). Several putative sexually selected traits have been associated with elevated mating success in men, including muscularity (Frederick & Haselton, 2007 ; Lassek & Gaulin, 2009 ), physical prowess (Faurie, Pontier, & Raymond, 2004 ; Smith, Bliege Bird, & Bird, 2003 ), masculine body shape (Hill et al., 2013 ; Hughes & Gallup, 2003 ; Rhodes, Simmons, & Peters, 2005 ), height (Mueller & Mazur, 2001 ), facial morphology (Johnston, Hagel, Franklin, Fink, & Grammer, 2001 ), and masculine and attractive voices (Hodges‐Simeon, Gaulin, & Puts, 2011 ; Hughes, Dispenza, & Gallup, 2004 ; Puts, 2005 ). In addition to assessing number of copulatory partners, these studies also variously assess number of wives, number of mates with whom a male has reproduced, number of extra‐pair copulations, number of affairs with mated women, age at first copulation, age at first reproduction, and the quality of a male's mates. Putative sexually selected traits such as physical formidability (Chagnon, 1988 ; Smith et al., 2003 ), height (Pawlowski, Dunbar, & Lipowicz, 2000 ), facial dominance (Mueller & Mazur, 1997 ), facial attractiveness (Jokela, 2009 ), and low voice pitch (Apicella, Feinberg, & Marlowe, 2007 ) have also been linked to men's reproductive success.

The strength of sexual selection also depends on the degree to which mates can be monopolized (Klug, Heuschele, Jennions, & Kokko, 2010 ). Temporal clumping of mates due to breeding synchrony tends to hinder the defense of multiple fertile females (Emlen & Oring, 1977 ). The fact that humans are not seasonal breeders, and that, contrary to early studies, women do not exhibit ovulatory cycle synchrony (Yang & Schank, 2006 ), should thus increase the potential for polygyny. However, some female characteristics decrease the degree to which estrous females can be monopolized. Although observers can detect phenotypic changes associated with women's ovulatory status in laboratory studies (Havlíček, Dvoráková, Bartos, & Flegr, 2006 ; Puts et al., 2013 ), such changes are extremely subtle relative to the dramatic genital swellings of chimpanzees and increased proceptivity of estrous great apes (Graham, 1981 ). Indeed, there appears to have been selection to conceal ovulation in women (Gangestad & Thornhill, 2008 ). Permanently enlarged breasts also obscure the cessation of lactational amenorrhea and resumption of ovulation after weaning. A consequence of such cryptic fertility is that men might be expected to compete more intensely to monopolize long‐term mates rather than for single copulations, as occurs in chimpanzees and other primates with advertised estrus (Wrangham & Peterson, 1996 ). The monopolizability of females also depends on how widely females are dispersed in the environment, and hence the costs of locating, courting, or defending multiple females (Emlen & Oring, 1977 ). If females are social and thus spatially clumped, they may be defensible by a single male, as occurs in gorillas (Harcourt, Stewart, & Fossey, 1981 ), or by a group of males, as occurs in chimpanzees (Morin, 1993 ) and humans (see below).

Sex differences in reproductive variance are also often used to assess the strength of sexual selection (Bateman, 1948 ; Jones, 2009 ). In traditional societies, men's reproductive variances are approximately 2–4 times those of women (Brown, Laland, & Borgerhoff Mulder, 2009 ). These reproductive disparities are substantial but are likely far smaller than those among elephant seals and even gorillas, in which more males fail to reproduce, and successful males are able to monopolize more mates. In the average forager society, only 21% of married women are married polygynously (Marlowe & Berbesque, 2012 ). Still, more men than women remain unmarried, divorce is common (Blurton Jones, Marlowe, Hawkes, & O'Connell, 2000 ), and men are likelier than women to reproduce with a new mate—all of which effectively increase the level of polygynous mating and reproduction (Daly & Wilson, 1988 ). Among the Ache of Paraguay, marriages are sequentially monogamous, and men have 4.2 times the reproductive variance of women (Hill & Hurtado, 1996 ). Moreover, throughout the world, the transition to stratified state‐level societies pushed harem sizes and reproductive disparities to extremes far exceeding those found in gorillas and even elephant seals in some cases (Betzig, 1986 ).

Kokko and colleagues (Kokko & Jennions, 2008 ; Kokko et al., 2012 ) demonstrate via mathematical models that the relationship between the OSR and the strength of sexual selection is complex, and conclude that variables that strongly influence the OSR—the lengths of time that individuals spend in and out of the mating pool—rather than the OSR itself, directly influence the extent to which individuals of a given sex will benefit from increased mating opportunities. Less investment in offspring tends to increase maximum potential reproductive rate (number of offspring per unit time), as well as time spent in the mating pool (Clutton‐Brock & Vincent, 1991 ; Trivers, 1972 ), which should increase the benefits of investing in traits that augment mating success (Kokko et al., 2012 ). Unlike most mammals and all nonhuman apes, humans exhibit significant paternal investment, which slows male reproductive rates, removing men from the mating pool. However, parental investment is decidedly unequal between the sexes. Women, but not men, invest in offspring via gestation and nursing for up to several years in foraging societies (Eibl‐Eibesfeldt, 1989 ), and women provide more parental care on average than men do in all known societies (Geary, 2000 ). Combined with menopause, the human sex difference in parental investment leads to a sex difference in potential reproductive rates. Across societies, the ratio of male‐to‐female maximum achieved reproductive rates varies but always exceeds 1, often by a large margin. The highest recorded male lifetime reproductive output across human societies is over 1000, whereas the female maximum is 69 (Glenday, 2013 ). Among traditional societies, sex differences in reproductive rates are smaller, but considerable. For the Yanomamö of Venezuela, Chagnon ( 1992 ) reports a male lifetime maximum of 43 offspring and a female maximum of 14. Among Xavante Indians from Brazil, the male reproductive maximum was 23, and the female maximum was 8 (Salzano, Neel, & Maybury‐Lewis, 1967 ).

The intensity of sexual selection is frequently estimated using the operational sex ratio (OSR), the average ratio of sexually active males to fertilizable females (Emlen & Oring, 1977 ). The OSR quantifies the ratio of competitors to contested resources (mates) and describes the potential difficulty in achieving mating opportunities (Kokko, Klug, & Jennions, 2012 ). At any time, a sizeable proportion of women are removed from the mating pool because they are pregnant, lactating, or postmenopausal, making men the OSR‐majority sex. Marlowe and Berbesque ( 2012 ) estimate the OSR among human foragers to be between 11.7 for the physiologically possible OSR and 8.6 for the behavioral OSR (reflecting realized reproductive behavior). This places humans above chimpanzees (OSR = 4.5) and most of the other 17 anthropoid primates evaluated by Mitani, Gros‐Louis, and Richards ( 1996 ), but well below orangutans and gorillas, which had by far the most male‐biased OSRs.

Evidence of Design for Contests

If the competing sex can obtain mates by force, then other mechanisms of sexual selection, such as mate choice and sperm competition, are limited (Puts, 2010). Contest competition tends to evolve when mates, or the resources necessary to win mates, are localized in space or time and are thus economically defensible (Emlen & Oring, 1977). Generally, mate and territory defense appear more feasible in “one‐dimensional” mating environments (burrows or tunnels) or “two‐dimensional” mating environments (land or floors of bodies of water) than in three‐dimensional environments (air, open water, or trees) (Emlen, 2008; Puts, 2010; Stirling, 1975). For example, males engage in more fighting over mates in terrestrially breeding seals (Stirling, 1975) and turtles (Berry & Shine, 1980) relative to aquatically breeding species. The fact that humans are terrestrial rather than arboreal primates should, therefore, facilitate the evolution of male contests.

Grafen (1987) emphasized the difference between selection in progress and adaptation, suggesting that trait‐related approaches are most useful in demonstrating past sexual selection. Whereas sexual selection may not always produce sex differences (Hooper & Miller, 2008), the presence of large secondary sex differences suggests an evolutionary history of strong sexual selection. Some of the most conspicuous products of sexual selection are sex differences in life history variables, body size, muscularity, aggression, sexual and threat displays, weaponry and ornamentation (Andersson, 1994)—all of which are present in humans. For example, men mature later and senesce and die sooner, a life history suggesting an effectively polygynous mating system (Daly & Wilson, 1983). The presence of secondary sex differences suggests past sexual selection, but a functional analysis of these traits is required to determine their possible roles in mating competition and the mechanisms of sexual selection that shaped them.

A thorough functional analysis of men's phenotypes indicates an evolutionary history of moderate‐to‐strong contest competition. Men exhibit all of the hallmarks of contests: same‐sex aggression, greater size and strength than females, weapons, and threat displays (Andersson, 1994), as we discuss next.

Fighting and Physical Aggression Rates of lethal violence in forager societies are similar to those in chimpanzees (Wrangham, Wilson, & Muller, 2006), and from an early age, human males are more physically aggressive than females. In studies spanning many cultures and time periods, males compared to females have engaged in more rough and tumble play and other types of physical aggression, fantasized more about violence, and more frequently committed violent offences (Ellis et al., 2008). Across societies, the vast majority of murderers and murder victims are men, particularly young men (Archer, 2004, 2009; Daly & Wilson, 1990; Walker & Bailey, 2013). Sex differences in homicide are most extreme for same‐sex homicide; men have killed other men far more frequently than women have killed other women in every society and time period for which data are available (Daly & Wilson, 1988). Excluding war killings, about 95% of same‐sex homicides are committed by men (Daly & Wilson, 1988). Including war killings, the proportions of same‐sex killings perpetrated by men would surely approach 100%. Across all 70 preliterate societies surveyed by Whyte (1978), men were more likely than women to engage in warfare. Male intrasexual violence is responsible for a significant proportion of deaths, especially in males, in many natural fertility populations (Keeley, 1996). Violent death through homicide or warfare accounts for approximately one in two deaths among the Waorani of Ecuador (Beckerman et al., 2009), one in three deaths among the Dugum Dani of New Guinea and the Yanomamö of Venezuela and Brazil (Chagnon, 1988), and one in four deaths and one in five deaths, respectively, among the Mae Enga and Huli of New Guinea (Chagnon, 1988). In a study of 10 small‐scale Amazonian societies, the percentage of violent deaths ranged from 6% to 56%, with an average of 30% (Walker & Bailey, 2013). Among the !Kung San of Botswana, per capita homicide rates are approximately four times those in a typical year in the United States (Lee, 1984). Archaeological evidence also indicates extensive male‐male aggression over human evolutionary history. This evidence includes a lack of female skeletons at gravesites where individuals apparently died in a massacre (Bamforth, 1994), missing bones in male skeletons consistent with warfare‐related trophy taking (Andrushko, Latham, Grady, Pastron, & Walker, 2005; Bamforth, 1994), and evidence of traumatic injuries on male skeletons (Milner, Anderson, & Smith, 1991; Walker, 2001). As well as influencing predispositions toward physical aggression, contest competition may have shaped other aspects of men's psychology and behavior. For example, men's pain tolerance systems are calibrated in ways predicted from a history of male‐male fighting. Across studies, males generally have higher pain tolerance than do females (Ellis et al., 2008). Importantly, men's and women's pain systems are influenced by different stimuli. Though both male and female competitive athletes experience analgesic effects after athletic competition, men but not women experience analgesia from competition without exercise, whereas women but not men experience analgesia from exercise without competition (Sternberg, Bokat, Kass, Alboyadjian, & Gracely, 2001). That competition should reduce pain in men but not women is consistent with the hypothesis that men's psychologies are designed to be prepared for potentially injurious competition. Winning (versus losing) a video game simulation of male‐male combat increased men's preferences for facial femininity in women, suggesting that ancestral men's ability to obtain and defend high‐quality mates was dependent upon their success in male–male competition (Welling, Persola, Wheatley, Cárdenas, & Puts, 2013). Males also take more risks of physical injury than do females, especially when peers are present (Ginsburg & Miller, 1982; Morrongiello & Dawber, 2004) and when these peers are same‐sex individuals of similar status (Ermer, Cosmides, & Tooby, 2008). In addition, the development of group‐level competitive activities in boys may subserve male coalitional intrasexual competition in adulthood (Geary, Byrd‐Craven, Hoard, Vigil, & Numtee, 2003). Boys spend more time than girls in group activity by age 3, and this difference grows by age 6 (Benenson, Apostoleris, & Parnass, 1997). Boys form denser social networks (Benenson, 1990) and participate in higher levels of competitive and organized play (Rose & Rudolph, 2006). Male dominance behavior is most common when men are first introduced (Savin‐Williams, 1987) and decreases thereafter as intergroup competitive behaviors increase. Additionally, men show higher levels than women on several measures of tolerance of same‐sex peers (Benenson et al., 2009). Paradoxically, the circumstances under which men act kindly toward each other may be understood partly as consequences of selection for group‐level aggression. In a public goods game, men but not women increased cooperativeness after being primed for intergroup competition (van Vugt, De Cremer, & Janssen, 2007). When a defector in an economic game was punished, empathy‐related brain responses were reduced, and reward‐related brain responses were increased, in men but not women (Singer et al., 2006). Men also show higher testosterone increases following between‐group competitive victories than within‐group competitive victories (Oxford, Ponzi, & Geary, 2010; Wagner, Flinn, & England, 2002).

Size and Strength Male contests also tend to favor greater male size and strength. A number of studies have suggested that early hominids such as Australopithecus afarensis (3.6–2.9 million years ago [mya]) were characterized by large‐size dimorphism approaching or even surpassing that of orangutans and gorillas (Gordon, Green, & Richmond, 2008; Lockwood, Richmond, Jungers, & Kimbel, 1996; McHenry, 1991). However, recent work that has attempted to avoid potential methodological problems of previous studies, such as small samples and size‐based sex assignment, have rendered more modest estimates of skeletal sexual dimorphism, comparable to that of modern humans (Reno, McCollum, Meindl, & Lovejoy, 2010; Reno, Meindl, McCollum, & Lovejoy, 2003; Suwa et al., 2009). Modern human skeletal size dimorphism is intermediate between that of chimpanzees and gorillas, reflecting the moderate difference between male and female maturation rates (Leigh & Shea, 1995). In total body mass, men are approximately 20% heavier than women (Archer, 2009; Marlowe & Berbesque, 2012). This is below the body mass dimorphism of polygynous primates (averaging over 60% greater male size), above that of monogamous primates (averaging less than 10% greater male size), and comparable to that of species with multimale groups (Clutton‐Brock & Harvey, 1984). However, humans are more sexually dimorphic than overall body mass alone suggests. This is because, unlike other primates, human females store more body fat than do males (Wells, 2012), perhaps for producing highly encephalized offspring (Lassek & Gaulin, 2008). In estimating the role of male contests in humans, it is thus more appropriate to consider sexual dimorphism in fat‐free mass, which is 31%–43% greater in men than in women (Lassek & Gaulin, 2009; Wells, 2012). Men also put on 61% more lean muscle mass than women, including 50% more lower body muscle mass and 75% more arm muscle mass (Lassek & Gaulin, 2009). Men possess about 90% greater upper‐body strength, so that the average man is stronger than more than 99.9% of women (Abe, Kearns, & Fukunaga, 2003; Lassek & Gaulin, 2009). In addition, men have 65% greater lower body strength (Lassek & Gaulin, 2009), which translates into greater speed and acceleration (Mayhew & Salm, 1990). Even controlling for body mass and proportion of lean body mass, men are stronger, in part because their muscles have shorter fibers and greater angles of pennation (Chow et al., 2000). These sex differences in muscularity are comparable to those of gorillas (Zihlman & MacFarland, 2000).

Weapons Sexual selection often endows the more competitive sex with weapons such as horns, antlers, or canines. Humans lack significant sexual dimorphism in canine size, with both sexes having relatively smaller canines than our closest relatives (Wood, Li, & Willoughby, 1991). The trend toward canine reduction in our lineage can be traced back more than 6 million years to Sahelanthropus tchadensis in Central Africa (Brunet et al., 2002) through a largely continuous fossil record including well‐represented genera such as Ardipithecus (5.8–4.4 mya) and Australopithecus (4.2–2.5 mya) to Homo after 2.5 mya (Suwa et al., 2009). A number of theories have been proposed to account for this shift in canine morphology including dietary adaptations, selection against threatening displays, or replacement by handheld weapons (Greenfield, 1992). A dietary explanation appears unlikely as there are not other indications of a dramatic dietary shift in Sahelanthropus and Ardipithecus (Brunet et al., 2002; Suwa et al., 2009). Another hypothesis is that canine reduction indicates reduced intermale contest competition, possibly resulting from selection for cooperative hunting, and/or female choice for less competitive mates more likely to engage in parental care (Halloway, 1967; Lovejoy, 2009). If so, the moderate levels of size dimorphism observed in early hominids may reflect ecological selection and/or female mate choice rather than contest competition (Gordon, 2013; Lovejoy, 1981). A nonmutually exclusive alternative is that canine weaponry was supplanted by handheld weapons and forelimbs freed by bipedal locomotion (Carrier, 2011; Darwin, 1874; McHenry, 1991). Across societies, the manufacture and use of weapons against same‐sex rivals is ubiquitous among men and rare among women (Archer, 2004; Ellis et al., 2008; Smith & Smith, 1995; Warner, Graham, & Adlaf, 2005). Proficiency at weapons use also shows large sex differences. For example, men are more than 1.5 standard deviations more accurate at targeting and intercepting projectiles, and this difference remains large after controlling for experience (Watson & Kimura, 1991). Clubs, spears and hurled stones may have kept enemies at a distance, making biting ineffectual. Similarly, large maxillary canines appear to have been replaced in several deer species with the evolution of antlers, which also keep enemies' mouths at a distance (Barrette, 1977). Male chimpanzees use branches in dominance displays (but not as offensive weapons), suggesting that tools have been used since the last common ancestor of Pan and Homo. However, the first evidence of stone‐tool cut marks on human bones, probably due to postmortem butchering, occurs around 800,000 years ago, and the first evidence of attack with a weapon (a spear thrust through the lower limb and pelvis) does not appear until just over 100,000 years ago (Walker, Hill, Flinn, & Ellsworth, 2011), so the temporal relationship between canine reduction and the use of handheld weapons remains speculative.