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= Grip strength as a health and fitness proxy = Sexual dimorphism refers to behavioral and morphological traits, other than reproductive organs, that differ on average between the sexes. Humans are overall moderately sexually dimorphic, although this varies by feature. For instance, there is only about a 7-8% difference on average in height, 15% difference on average in weight, whereas musculature and body strength is highly dimorphic. Men have greater overall and regional skeletal muscle mass than women. On average, men have 61% more overall muscle mass and 78% more muscle mass in the upper arms. This concentrated muscle dimorphism in the arms and back translates to 90% greater upper body strength than women. There is less than 10% overlap in the distributions of upper body strength of men and women, meaning that the average man is stronger than 99% of women. Other traits which are dimorphic in humans include digit ratio, voice pitch, and facial features, canine length. However, associations between these traits and status, behavior, and health are less consistent than those of strength. Apicella et al., for instance, did not find a lower digit ratio among Hadza hunter-gatherer men.

Measurement of dimorphism based on weight alone can be misleading since overall weight is quite similar between the sexes but due to different pressures which have selected muscularity for men and fat mass for females. These reflect the different benefits to reproductive success offered to men with high levels of muscularity, namely the ability to win physical contests with other men and therefore achieve access to mates. Whereas women who had higher bodyfat gave birth to offspring with better nutrition and brain development. Lassek and Gaulin also discuss the tradeoffs in energy intake required to maintain high levels of muscle mass. These tradeoffs would be especially salient in contexts where highly quality foods were difficult to attain. Given the costs associated with maintaining high muscularity, higher benefits would need to result from having higher physical strength in order to maintain the selection for them.

Sexual selection refers to variability in fitness caused by variability in mating success, therefore leading to sexual dimorphism typically due to selection on males. Sexual selection can take the form of either intra- or intersexual selection. In the case of masculine trait evolution, intersexual selection refers to variation due to female choice; that is, the degree to which females can choose who they mate with, and how those choices affect male reproductive success. Intrasexual sexual refers to competition between males, the outcome of which determines access to mates. In both cases, males who achieve mating access have higher reproductive success, therefore their genes are overrepresented in following generations. Here, masculinity refers to dimorphic traits exaggerated in males.

Hand-grip strength is a commonly used measure of overall body strength. The grip strength metric is easily obtainable and is measured using a dynamometer. Typically, participants are asked to squeeze the dynamometer multiple times on each hand, and the highest output is taken. Dodds et al. show that standardized grip strength centiles are generalizable to populations in developed regions of the world, whereas grip strength values from developing regions tended to be lower. Grip strength is strongly influence by genetic factors and by androgenic hormone exposure, especially during development in utero. Individual variation in muscle strength is only partially explained by size and physical activity. Higher birthweight associated with later grip strength indicating importance of intrauterine growth and development.

Status
Individuals in both industrialized and non-industrialized societies seem to prefer to confer status and leadership to taller individuals. Strength and size associate with political influence in the Tsimane, which may be owing to their contribution to leader charisma and coordination ability. Physical strength and size also related to the acquisition of cooperation partners independent of Tsimane men’s status, perhaps because physical formidability is desirous in an ally in the event of conflict. Strength and size may also associate with production skill.

Allocating higher status to physically formidable individuals is likely also related to the evolution of group cooperation, such that people should be more likely to allocate higher status to those who can confer net benefits to the group. This may be through the specific knowledge, skills, or other traits that an individual possesses which benefit a specific group. Physically formidable men may represent a threat, given their ability to inflict physical harm and impose their will on others. It may seem that physically weaker men (and women) should choose strategies which check the power of more formidable conspecifics, in order to avoid the potential costs of exploitation from a formidable enemy. However, evidence from across a range of societies shows that physically dominant men tend to be afforded high status within cooperative groups. Since individuals within a coalition could leave if a leader imposed to many costs, or a group of coalition members could depose even the most formidable leader, Lukaszewski et al. argue that individuals who are allocated high status must confer at least some benefits to the group. They found evidence for this prediction in a study which asked participants to rate the perceived leadership abilities and perceived status based on pictures of men. The authors found that the effect of physical strength on perceived intimidation-based status was mediated by men’s perceived beneficial leadership skills.

In an experiment wherein adults rated photos of men described as part of a business, physical strength positively predicted ratings of their projected status within the organization, and this effect was mediated by perceptions that stronger men possessed greater leadership abilities of within-group enforcement and between-group representation (Lukaszewski et al., 2016). In this study, women’s physical strength did not predict their perceived leadership abilities, suggesting that the relationship between formidability and status is unique to men. In a causal analysis, the authors manipulated the physical strength of potential leaders in photographs. They found that physical strength has a causal impact on status allocation, and that this effect is mediated by the perception that strong men have important leadership skills, not from perceived generalized competence. In a causal manipulation of men’s heights and perceived status, the authors found that at least part of the association between height and perceived leadership is due to the perceived increase in strength and formidability that comes with increased height.

Importantly physical formidability in an individual may not directly correlate with that individual’s use of aggression or intimidation. Such behavior may be the very cues that an individual is likely to use their formidability to exploit others, and as such would not make a good leader or be likely to confer overall benefits to the group. Lukaszewski et al., argue that a major benefit from an individual to coalition are the very qualities of leadership which confer cooperation, and decision and action coordination within a group. Data show a consistent relationship between taller height and perceived leadership ability. While height is only partially correlated with upper body strength, it may be a cue to physical formidability. Lukaszewski et al. argue that leaders are valued for their contributions of within-group enforcement and between-group representation, and that physical formidability is a cue to these abilities. Other group members use this cue to allocate status to these individuals. Physical formidability is important in this arena as a dominant leader can inflict costs on others, but such that group members are benefited. In terms of within-group conflict, a physically strong leader may be likely to deter aggression from group members as well as punish free-riders and non-cooperators, whose actions impose costs on cooperative group members. Formidable leaders are better able to act as mediators by intervening in conflicts and enforcing a resolution. In terms of between-group benefits, stronger individuals have higher bargaining power in conditions where there may be conflict between groups. Stronger individuals may also be able to win allies who are likewise formidable, increasing the power of their coalitions.

Lukaszewski et al., found that physical strength did not associate with perceived task-related intelligence, meaning raters did not consider stronger men to be generically more competent than weaker men. There findings remained when controlling for ratings of physical attractiveness, meaning that strength predicted perceived leadership qualities of men, above and beyond its relationship to physical attractiveness. Stronger men were perceived as having both greater ability and willingness in within-group enforcement as well as between-group representation.

Overall, this evidence suggests that formidability is important as a cue for prestige-related status, that is individuals confer higher status to those who are perceived as having specific leadership qualities which are beneficial to the group. This is in contrast to dominance-based status model, which argues that formidable individuals are granted high status due to their willingness to aggressively pursue their own interests and intimidate rivals. Lukaszewski et al., argue that any relationship between this type of aggressive intimidating behavior must be mediated by its perceived beneficiality to the group in order for group members to willingly confer higher status to these individuals.

Personality traits
Grip strength has also been found to associate with personality traits. Kerry & Murray found negative correlations between grip strength and neuroticism in both men and women, but no correlation between grip strength and other Big Five traits. Perceived formidability correlated negatively with neuroticism and positively with extraversion in both sexes. Grip strength explains sex differences in neuroticism (sex differences in neuroticism completely disappeared when accounting for grip strength). Grip strength was negatively related to both pathogen disgust and sexual disgust in men and women, but not moral disgust. Fink et al., found that hand grip strength was positively correlated with sensation seeking, the tendency to seek new and intense experiences.

Broadly, the literature on associations between grip strength and personality traits supports the interpretation of grip strength as a strong marker of formidability. Stronger individuals are able to impose their will on others or prevent others from coercing or exploiting them. Neuroticism can be conceptualized as vigilance toward ecological and social threats. Stronger individuals are better able to protect themselves, and the costs of failing to detect a threat are lower than the costs for those who are physically weaker. Likewise, extraversion and sensation seeking are less costly for individuals who have less to fear from others, and more to gain from engaging in novel experiences. This is also related to evidence that individuals, men especially, have more to gain by engaging in risky behaviors, reflected in the sex difference in risk-seeking behaviors.

Health
Grip strength is associated with mortality such that lower grip strength (adjusted for sex and age) is associated with higher all-cause mortality. It is also a better marker of frailty than chronological age (among adults 64-74). Higher admission grip strength in older medical inpatients was associated with increased likelihood of discharge home independent of age and gender. Handgrip strength is also associated with survival time in adults with advanced cancer. Grip strength after hip fracture is predictive of physical functioning after rehabilitation.

Overall, grip strength is associated with mortality, length of hospital stay, physical functioning. Fukumori et al., found that lower grip strength was associated both cross-sectionally and longitudinally with depressive symptoms. Grip strength can also track nutritional status, especially in young adults, as muscle function reacts early to changes in nutritional status.

Fitness
Several studies have found that grip strength in men is positively correlated with lifetime number of sex partners or specific measures of promiscuity (reviewed in Gallup & Fink ). Among American men, Lassek and Gaulin found that fat free mass as well as limb muscle volume were positive predictors of past-year and lifetime sexual partners, as well as age at first sex. Among the Hadza, upper-body strength significantly predicted both hunting reputation (as measured by resident women) and self-reported reproductive success in terms of offspring production. Importantly, while the correlations between grip strength and health measures do not differ between the sexes, correlations between grip strength and measures of social and sexual competition are typically strong for men but not women.

Cues of upper-body muscularity and formidability seem to be important features of female mate choice among modern humans as they account for ∼70% of the variance in male bodily attractiveness. Lidborg et al. conducted a meta-analysis on a number of dimorphic traits and mating and reproductive success. Masculinity in all traits except facial morphology is associated with significantly greater mating success, however, this increased mating success does not appear to translate into greater reproductive success for any other trait than masculinity in men’s bodies. The authors conclude that since body masculinity was the only trait studied which was consistently associated with fitness across studies, it is the only one evidenced to be under current selection in populations with natural fertility. Since body masculinity is associated with physical formidability the authors also interpret this evidence in support of a history of male-male competition in the hominin lineage. It is also possible that this is mediated by important aspects of mate choice, since formidability is also associated with hunting ability for instance. The lack of effect of facial masculinity on fitness leads them to favor the male-male competition hypothesis, over the immunocompetence handicap hypothesis.

Current selection
Literature on the evolution of human sexual dimorphism has focused on the role of testosterone (and therefore masculine traits) on reproductive success. The immunocompetence handicap hypothesis proposes that only men able to pay the immune costs of high testosterone can display the traits linked to high testosterone. These men are preferred by women because these traits, such as facial masculinity, are cues as to the quality of their genes. However, two serious problems have emerged in relation to this hypothesis. For one, new evidence has shown that testosterone may not have a straightforward tradeoff with immunity but may instead modulate immunity. Furthermore, evidence that women prefer men with more masculine traits is mixed. Evidence has instead reiterated the importance of testosterone in producing other male traits, especially muscle development, which contributes to physical formidability. Given this association, some have argued that this dimorphism is better explained by male-male competition.

Competition between males has been an important factor in human evolution. Sexual dimorphism in the hominin lineage can be difficult to estimate. Evidence in terms of the size dimorphism of early australopithecines is mixed, estimates have varied from extreme dimorphism comparable to that of orangutans and gorillas, to more conservative estimates in line with modern human dimorphism. In Homo, early strong selection on male size due to intrasexual selection, followed by a reduction in sexual dimorphism due to an increase in female size in Homo erectus, however this picture is also unclear. This could be due to increased selection on female fecundity, or improved access to resources for females. Plavcan argues that modern human size dimorphism is derived from a common ancestor. Sexual dimorphism can be affected by a number of mechanisms, including mate competition, resource competition, intergroup violence, and female choice and is therefore its causal factors are difficult to establish. Furthermore, when male-male competition results in higher mating success for the winners, it lowers the effectiveness of other mechanisms such as mate choice and sperm competition.

Factors affecting current sexual selection are difficult to parse, at least in part due to ecological changes which affect the costs and benefits of reproductive behavior (e.g. effective contraception). Sexual dimorphism can be affected by a number of mechanisms, including mate competition, resource competition, intergroup violence, and female choice. Apicella et al. did not find a lower digit ratio among Hadza hunter-gatherer men. In an attempt to elucidate selection on male traits stemming from female choice and those under selection from male contests, Hill et al.,. They found that female choice showed positive directional selection on male height, whereas male contest showed positive directional selection on vocal masculinity and a combined measure of strength. Overall, dominance was a better predictor of mating success than attractiveness. Hill et al., interpret their results as supporting a stronger influence of male contest than female choice on the evolution of male traits.

Lidborg et al., find that facial masculinity has almost no association with reproductive success in high fertility contexts, and furthermore is unrelated to mating behaviors implying that facial masculinity does not influence female choice. These results seriously weaken arguments that facial masculinity functions as a reliable cue to women as to a man’s immunocompetence and therefore genetic quality. They also found that while digit ratio, voice pitch, height, and testosterone positively predicted mating success, they did not predict reproductive success, suggesting that these traits may not be under current selection since they do not appear to translate into higher reproductive success in natural fertility populations.

Relatedly, in an experiment in which raters were asked to estimate men’s strength and fighting ability from pictures of faces and bodies, both men and women could do so with remarkable accuracy. If cues of physical formidability were important only for men to accurately estimate their likelihood of winning physical contests with other men, it is unclear why women are just as accurate in this ability. Sell et al., argue that this is due to the direct benefits that are offered by a physically formidable mate, making it important for women to be able to determine this formidability. If so, it has been difficult to disentangle the ways that facial masculinity acts as a cue to women, as it may be a cue of strength rather than immune competence. Likewise, Fink et al ., found that men’s grip strength was correlated with perceived physical strength, dominance, and attractiveness as rated by women who viewed pictures of the men’s faces. This remained after controlling for age and weight.

Evidence from Sell et al., demonstrates that humans are remarkable good at assessing men’s strength and formidability. Furthermore, when estimating men’s strength, participants especially tracked upper body strength in their estimates. Not only does there appear to be a cognitive mechanism for the assessment of other’s strength based on bodies, but people are also able to accurately estimate strength from the face alone. The authors find that perceived fighting ability was not due to variation in size, but variation in perceived upper body strength, such that men’s actual upper body strength predicted both their perceived strength and fighting ability while controlling for height, weight, and age. Due to the large sex difference in upper body strength, sex is an effective cue of formidability. Although raters can estimate women’s strength from the same cues they do men’s, they are much less accurate.

Previous findings supported an interaction between ovulatory cycle phase and mate preference such that women who were ovulating preferred men with more masculine features as short term mates and men with less masculine features for long term mates. However other studies have found no effect, reporting that fertile women rated all men’s bodies as more attractive regardless of masculine features. Women preferred stronger men regardless of cycle phase. The data on female choice may be mixed partially because choosing mates based on masculine features may not always be straightforward for women. Choosing a mate with masculine features may be beneficial if he is better able to hunt and therefore provision, or to protect you and your offspring. However, these same features may also pose a threat if he is more likely to be violent and coercive and is more likely to turn that violence on her. Depending on typical rates of violence from partners, women might be better off choosing a mate who is slightly less able a hunter on average, but from whom she would be unlikely to fear potentially lethal violence. This tradeoff demonstrates important potential differences in dominance and female choice.