A meta-analysis of the association between male dimorphism and fitness outcomes in humans
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Evaluation Summary:
This work evaluates the strength of the evidence that human sexual dimorphism is the product of sexual selection. As a meta-analysis of studies that connect various measures of masculinity to various measures of reproductive success, this paper represents a synthesis of what this vast literature can show thus far. The work will be of general interest to evolutionary social scientists from a variety of disciplines, and it does a good job of clearly and concisely presenting the current state of sexual selection research on human males. The data are well presented, but the interpretation of the results is currently limited by some gaps in the theoretical framework guiding the manuscript.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewers #1-4 agreed to share their names with the authors.)
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Abstract
Humans are sexually dimorphic: men and women differ in body build and composition, craniofacial structure, and voice pitch, likely mediated in part by developmental testosterone. Sexual selection hypotheses posit that, ancestrally, more ‘masculine’ men may have acquired more mates and/or sired more viable offspring. Thus far, however, evidence for either association is unclear. Here, we meta-analyze the relationships between six masculine traits and mating/reproductive outcomes (96 studies, 474 effects, N = 177,044). Voice pitch, height, and testosterone all predicted mating; however, strength/muscularity was the strongest and only consistent predictor of both mating and reproduction. Facial masculinity and digit ratios did not significantly predict either. There was no clear evidence for any effects of masculinity on offspring viability. Our findings support arguments that strength/muscularity may be sexually selected in humans, but cast doubt regarding selection for other forms of masculinity and highlight the need to increase tests of evolutionary hypotheses outside of industrialized populations.
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Reviewer #4 (Public Review):
This article describes the results of an impressive meta-analysis based on a high number of published effects investigating the relationship between sexual dimorphism in men and their mating and reproductive success.
The article is very well written and covers a vast amount of literature.
Most of my comments are not corrections, but rather subjective ideas on how the text could be restructured. In my opinion, the article is clearly written and the rationale behind research questions and methodology is well explained. I appreciate how the authors present the entire analysis, adding multiple robustness tests and presenting their results in an easy to follow manner (which was not easy, due to the complexity of the methodology implemented).
I cannot criticise any major issues in this manuscript.
The main …
Reviewer #4 (Public Review):
This article describes the results of an impressive meta-analysis based on a high number of published effects investigating the relationship between sexual dimorphism in men and their mating and reproductive success.
The article is very well written and covers a vast amount of literature.
Most of my comments are not corrections, but rather subjective ideas on how the text could be restructured. In my opinion, the article is clearly written and the rationale behind research questions and methodology is well explained. I appreciate how the authors present the entire analysis, adding multiple robustness tests and presenting their results in an easy to follow manner (which was not easy, due to the complexity of the methodology implemented).
I cannot criticise any major issues in this manuscript.
The main outcomes of the article not only present a robust test of previously mixed results, but also provide a strong recommendation of how future studies should be conducted (i.e. how to use mating success proxies, and what samples to include).
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Reviewer #3 (Public Review):
As the authors lay out, there are a number of theoretical perspectives that expect that male features that are sexually dimorphic and, hence, vary in their levels of "masculinity" (or perhaps less sex-anchored, vary along a male-female dimension) within human males, to have been under sexual selection historically (if not now), which may in part explain their sexual dimorphism. The target article examines associations between a number of such traits that have been examined-bodily strength and muscularity, facial masculinity, vocal pitch, 2nd to 4th digit ratio (2D:4D), height, and testosterone levels-with measures of mating "success" (e.g., sexual partner number) and reproductive outcomes (e.g., reproductive success). With traits keyed such that more positive values reflect greater "maleness," virtually all …
Reviewer #3 (Public Review):
As the authors lay out, there are a number of theoretical perspectives that expect that male features that are sexually dimorphic and, hence, vary in their levels of "masculinity" (or perhaps less sex-anchored, vary along a male-female dimension) within human males, to have been under sexual selection historically (if not now), which may in part explain their sexual dimorphism. The target article examines associations between a number of such traits that have been examined-bodily strength and muscularity, facial masculinity, vocal pitch, 2nd to 4th digit ratio (2D:4D), height, and testosterone levels-with measures of mating "success" (e.g., sexual partner number) and reproductive outcomes (e.g., reproductive success). With traits keyed such that more positive values reflect greater "maleness," virtually all associations with putative fitness components were found to be positive, though not all associations had confidence intervals that do not cross the zero-point (i.e., not all are "significant").
The strongest associations were with body masculinity. Specific measures included strength, body shape, and muscle or non-fat body mass (though the associations are not broken down by indicator type). In the mating domain, the overall correlation was .13 (.14 in the behavior domain, perhaps most related to mating "success"). In the reproductive domain, the mean correlation was .14, and .16 in high fertility samples (a subset of which may represent natural fertility populations). Especially when strength (e.g., grip strength) was used as the measure of body masculinity, these associations are likely underestimated, due to imperfect validity of the masculinity/muscularity indicator.
Associations with voice pitch were, on average, nearly identical to those involving body masculinity: .13 overall in the mating domain and .14 overall in the reproductive domain. But due to smaller sample size, the confidence interval around the correlation in the reproductive domain included zero.
The next grouping of traits, in terms of strength of association, contains facial masculinity and testosterone levels. There, associations were .09 and .08 in the mating domain and .09 and .04 in the reproductive domain, respectively. Once again, not all confidence intervals were exclusively above zero.
Associations with both 2D:4D and height were weaker: .03 and .06 in the mating domain and .07 and .01 in the reproductive domain, respectively.
I offer a few observations.
First, the meta-analysis, to my mind, offers some interesting data. We need to be aware of its limitations. Many samples are drawn from WEIRD populations (Henrich et al., 2010). It remains unclear to what extent fertility and reproductive success in these samples, even when drawn from high fertility populations, reflect processes that would have operated in ancestral human groups. It makes sense that some of these features may well have been variably associated with fitness components in ancestral populations, but potential key moderator variables (e.g., pathogen prevalence, level of paternal provisioning, level of intergroup violence, degree of female choice [vs. arranged marriages]) may not be available to examination here. To the extent moderation exists, mean levels in this meta-analysis are less meaningful (though not meaningless), as we do not know whether the distribution of moderators in this sample of samples is representative of populations of interest. (E.g., due to advances in modern medicine, these samples may be much healthier than ancestral populations in which these features were subject to selection.) And that is just a partial list of caveats we need to keep in mind. Nonetheless, with those limitations kept in mind, these findings are interesting to reflect upon.
Second, the associations of course do not tell us what processes drive them. They are correlations. Indeed, we do not know whether the traits themselves were directly implicated in the processes leading to their associations with fitness outcomes. (2D:4D surely wasn't-it's a marker of other causal variables-but its associations are among the weakest seen here.) It makes some sense that the stronger the associations, the more likely the trait in question was directly causally implicated in these processes. And again, that may be particularly true of body masculinity, as associations with it may be underestimated due to fallible indicator validity. But even then, we cannot rule out other mediating traits. Perhaps more muscular men exhibit greater confidence and gain leadership roles more readily than less muscular men, giving them an edge in intrasexual competition or intersexual choice due to associated behavior or status. Or maybe they ultimately gain greater control of resources, giving them advantages in competition for mates or provisioning of offspring. This is not to deny that muscularity may well have been (and be) under sexual selection; but it may have been selected along with other traits rather than the direct target of selection itself.
Third, then, we do not know what intrasexual or intersexual selection processes may have been involved historically, even if these traits have directly been under sexual selection. To what extent are these associations due to advantages in intrasexual competition? To what extent might they be due to female preferences and choice? Naturally, as the authors note, these processes are not mutually exclusive. After all, in lekking species, males compete with one another for a symbolic spatial position, which, because it represents the outcome of the competition, leads to mating success via female choice. Still, we might be interested in knowing what processes led to the associations found, and how they speak to sexual selection and mating processes in humans.
Once again, however, the associations reported are interesting to reflect upon. And they could, either directly or indirectly (by stimulating additional research), lead to better answers to issues raised above. One key outcome that relatively little data currently speak to, for instance, is mortality rate of offspring. As the authors note, men who are more successful with respect to mating effort may invest lower amounts of parental investment in offspring. In theory, then, their greater offspring number could be offset to an extent by lower survival rates. In the relatively few data the authors aggregated from the literature, that was not clearly the case. But more data may be needed, especially with respect to the strongest predictors of mating success, and especially in more traditional societies.
Paternal investment in offspring, however, need not pay off just in terms of offspring survival rates; paternal provisioning may permit greater rates of reproduction via shortening of interbirth intervals in traditional societies. The data here show that, at least with respect to body masculinity, more masculine men have greater mating success and greater reproductive success. Yet the data do not necessarily tell us that the female partners of these men have greater reproductive success. More masculine men's rates of offspring production could be spread over more female mates than that of less masculine men. Knowing whether female partners of more masculine men benefit reproductively by mating with masculine men is pertinent to addressing whether the reproductive success of masculine men has been mediated, in part, by female mate choice.
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Reviewer #2 (Public Review):
In this manuscript, the authors set out to provide a comprehensive meta-analysis of associations between masculinized phenotypes and fitness-relevant outcomes (mating, reproduction, and offspring viability), so as to assess the current state of evidence for hypotheses of sexual selection on human males across high- and low-fertility populations. I enjoyed reading this manuscript, which is well organized and very clearly written. I also appreciated the depth of the analyses reported by the authors. Overall, I am pleased with this research and think it will make a valuable contribution to the literature on human sexual selection and masculinity more generally.
I do not have any major concerns regarding the methods and results. However, I think the paper would greatly benefit from introducing greater nuance …
Reviewer #2 (Public Review):
In this manuscript, the authors set out to provide a comprehensive meta-analysis of associations between masculinized phenotypes and fitness-relevant outcomes (mating, reproduction, and offspring viability), so as to assess the current state of evidence for hypotheses of sexual selection on human males across high- and low-fertility populations. I enjoyed reading this manuscript, which is well organized and very clearly written. I also appreciated the depth of the analyses reported by the authors. Overall, I am pleased with this research and think it will make a valuable contribution to the literature on human sexual selection and masculinity more generally.
I do not have any major concerns regarding the methods and results. However, I think the paper would greatly benefit from introducing greater nuance into the theoretical framework and conclusions, which I believe will meaningfully change some of the takeaways presented in the discussion. I have provided references throughout to aid the authors in this effort during revision, though they should certainly not feel compelled to cite each reference provided. I would also appreciate that the authors provide some estimates of (a priori) statistical power when they make claims regarding statistical power in the interpretation of results.
Major comments:
The authors have done a very nice job of efficiently introducing the reader to mainstream hypotheses regarding sexual selection on human male phenotypes, particularly those emphasized within evolutionary psychology. I recognize that the authors' primary contribution is empirical and that they have in large part followed the typical presentation of these hypotheses in previous literature. However, given that this paper may be an important point of reference for future research in this area, I would like to encourage the authors to address some important nuances in greater detail that are frequently overlooked.
(i) The authors argue that "Sexual selection is commonly argued to have acted more strongly on male traits as a consequence of greater variance in males' reproductive output (3) and male-biased operational sex ratio, i.e. a surplus of reproductively available males relative to fertile females (e.g. 4)". This argument then leads to a discussion of why formidability as indexed by strength and other potential indicators of physical dominance are expected to be under selection in males. However, recent work in sexual selection theory has begun to emphasize the importance of the co-evolution of male offspring care and reproductive competition, leading in many cases to opposite predictions compared to classical models of OSR. In particular, more recent models predict that males should often increase rather than decrease offspring care relative to mating effort when men are in relative abundance. These predictions have received support in recent empirical studies in human populations, and help to explain otherwise puzzling patterns such as e.g. the association between male-biased sex ratios and monogamy + low reproductive skew across many taxa. Please see
Kokko, H., & Jennions, M. D. (2008). Parental investment, sexual selection and sex ratios. Journal of evolutionary biology, 21(4), 919-948. Schacht, R., Rauch, K. L., & Mulder, M. B. (2014). Too many men: the violence problem?. Trends in Ecology & Evolution, 29(4), 214-222. Schacht, R., & Borgerhoff Mulder, M. (2015). Sex ratio effects on reproductive strategies in humans. Royal Society open science, 2(1), 140402.
Considering these models, one might expect that a variety of behavioral and psychological phenotypes would be under male-specific sexual selection that are simply not considered in the present study. One might also expect that appropriate proxies of male fitness will also vary across populations, independently of the presence/absence of contraception. The authors argue that they selected mating-based proxies of reproductive behaviors and attitudes under the assumption that "preferences for casual sex, number of sexual partners, and age at first sexual intercourse (earlier sexual activity allows for a greater lifetime number of sexual partners)... correlated with reproductive success in men under ancestral conditions". Yet, in large-scale industrialized societies that have undergone a demographic transition, high status males are often observed to invest more in offspring care and the production of intergenerationally transferable wealth at the expense of greater fertility, which may be an adaptive response to shifting demands in relation to competition for status.
Shenk, M. K., Kaplan, H. S., & Hooper, P. L. (2016). Status competition, inequality, and fertility: implications for the demographic transition. Philosophical Transactions of the Royal Society B: Biological Sciences, 371(1692), 20150150.
In general, long-run fitness may often not map so simply onto promiscuous sexual behavior in such a straightforward way. Measures such as age at first intercourse may also be confounded with environmental heterogeneity among participants, which could instead indicate environmentally induced plasticity within individuals' lifetimes toward a faster pace of life.
(ii) Related to this point, the authors discussion of the relationship between testosterone and male phenotypes is somewhat over-simplified, although again in keeping with much of the previous literature in evolutionary psychology. While it was long emphasized that testosterone is a mechanism of aggression per se, recent work has shown that testosterone is better understood as a mechanism for increasing status-seeking, competitive behavior, which can greatly vary in form across socioecological contexts.
Eisenegger, C., Haushofer, J., & Fehr, E. (2011). The role of testosterone in social interaction. Trends in cognitive sciences, 15(6), 263-271.
Unfortunately, most of the fWHR and 2D:4D literature has ignored these findings and continues to focus solely on aggression even in WEIRD student samples, where we can be certain that aggression is generally not a viable strategy for attaining and maintaining social status. To my knowledge, only a few studies have explicitly tested this more nuanced hypothesis regarding associations between masculinized phenotypes and differing forms of status-seeking behavior, both of which have found support for ecologically contingent effects in regards to fWHR. Martin et al. (2019) predicted and found support in bonobos for higher fWHR predicting higher scores on an affiliative measure of social rank among both males and females, consistent with the importance of relationship strength and social network centrality for competitive advantage among bonobos. Similarly, Hahn et al. (2017) found that fWHR in human males consistently predicts prosocial behavior and leadership in large-scale institutions. This is consistent with the fact that leadership traits, rather than aggression and formidability per se, are often important predictors of status in human societies (and in contexts of relatively higher SES within those societies).
Hahn, T., Winter, N. R., Anderl, C., Notebaert, K., Wuttke, A. M., Clément, C. C., & Windmann, S. (2017). Facial width-to-height ratio differs by social rank across organizations, countries, and value systems. PLoS One, 12(11), e0187957. Martin, J. S., Staes, N., Weiss, A., Stevens, J. M. G., & Jaeggi, A. V. (2019). Facial width-to-height ratio is associated with agonistic and affiliative dominance in bonobos (Pan paniscus). Biology Letters, 15(8), 20190232.
In regard to the male-male competition hypothesis, as noted in the previous comment, we might therefore expect sexual selection to occur on a variety of male traits other than formidability related measures, as well as to be highly population-specific-rather than there being some universal optimum for "masculine" traits-given that what constitutes an adaptive male phenotype likely varies across populations in regard to both male-male competition and female choice. Finally, it should be noted that testosterone is by no means the only sex hormone relevant to considering patterns of human sexual dimorphism. Please see Dunsworth (2020) for a discussion of the centrality of estrogen in proximally explaining sexual dimorphism in body size
Dunsworth, H. M. (2020). Expanding the evolutionary explanations for sex differences in the human skeleton. Evolutionary Anthropology, 29, 108-116.
(iii) The authors should provide more references to (and brief discussion of) mixed results regarding the degree of sexual dimorphism in facial and digit ratio metrics. While they cite a few studies in the introduction, one might leave the text with the impression that there is clear enough evidence for 2D:4D being influenced by (pre-natal) sex hormones and being a sexually dimorphic phenotype. However, these results have been strongly challenged, not only be ref 14 and 20 in the main text, but also various other studies e.g.
Barrett, E., Thurston, S. W., Harrington, D., Bush, N. R., Sathyanarayana, S., Nguyen, R., ... & Swan, S. (2020). Digit ratio, a proposed marker of the prenatal hormone environment, is not associated with prenatal sex steroids, anogenital distance, or gender-typed play behavior in preschool age children. Journal of Developmental Origins of Health and Disease, 1-10. Richards, G. (2017). What is the evidence for a link between digit ratio (2D: 4D) and direct measures of prenatal sex hormones?. Early Human Development. Richards, G., Browne, W. V., Aydin, E., Constantinescu, M., Nave, G., Kim, M. S., & Watson, S. J. (2020). Digit ratio (2D: 4D) and congenital adrenal hyperplasia (CAH): Systematic literature review and meta-analysis. Hormones and Behavior, 126, 104867. Richards, G., Browne, W. V., & Constantinescu, M. (2021). Digit ratio (2D: 4D) and amniotic testosterone and estradiol: An attempted replication of Lutchmaya et al.(2004). Journal of Developmental Origins of Health and Disease.
Similarly, not all metrics of facial masculinity are equally valid given current empirical evidence. In a recent longitudinal study, only cheekbone prominence was found to show consistent evidence of sexual dimorphism across age groups.
Robertson, J. M., Kingsley, B. E., & Ford, G. C. (2017). Sexually dimorphic faciometrics in humans from early adulthood to late middle age: Dynamic, declining, and differentiated. Evolutionary Psychology, 15(3), 1474704917730640.
Overall, I found the authors' discussion of how they selected the specific facial metrics lumped together in their analyses to be underspecified. Please note in the discussion as well that BMI is a well-known confound in studies of facial masculinity and may be a cause of null results in the present study (unless I happened to miss this in the regard to the moderation results - if so, my apologies!).
Geniole, S. N., Denson, T. F., Dixson, B. J., Carré, J. M., & McCormick, C. M. (2015). Evidence from meta-analyses of the facial width-to-height ratio as an evolved cue of threat. PloS one, 10(7), e0132726.
(iv) Finally, please provide reference to and potentially brief discussion of the current state of the literature as regards "good genes" hypotheses of female choice, which is relevant for determining how useful previous studies are for directly addressing this hypothesis. Please see:
Achorn, A. M., & Rosenthal, G. G. (2020). It's not about him: Mismeasuring 'good genes' in sexual selection. Trends in Ecology & Evolution, 35, 206-219.
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Reviewer #1 (Public Review):
This manuscript is a meta-analysis of literature, predominantly that from evolutionary psychology. The background seems well-explained, and the discussion and literature review well-written. The authors have done an impressive job of collating and synthesising a truly vast amount of literature that (as they demonstrate) is often pretty ambiguous in its results. The results are well-presented and well-reasoned, without overstating the evidence. The entire manuscript is clear and easy to read and follow. Table 1 makes it particularly easy to follow. I appreciate their emphasis that the various hypotheses about sexual dimorphism are not mutually exclusive, and that this study does not seek to explicitly test either one of them.
There is enough evolutionary anthropology inserted here to see that the authors have …
Reviewer #1 (Public Review):
This manuscript is a meta-analysis of literature, predominantly that from evolutionary psychology. The background seems well-explained, and the discussion and literature review well-written. The authors have done an impressive job of collating and synthesising a truly vast amount of literature that (as they demonstrate) is often pretty ambiguous in its results. The results are well-presented and well-reasoned, without overstating the evidence. The entire manuscript is clear and easy to read and follow. Table 1 makes it particularly easy to follow. I appreciate their emphasis that the various hypotheses about sexual dimorphism are not mutually exclusive, and that this study does not seek to explicitly test either one of them.
There is enough evolutionary anthropology inserted here to see that the authors have a passing familiarity with it, although I would encourage them to dig much more deeply into this literature in framing their work. In short, there is a tension between evolutionary psychology and evolutionary anthropology that can be very fruitfully explored with the results of this analysis, and the authors only scratch the surface of this at the end of the manuscript.
Something that seems crucial here, and in this literature more generally, is the likelihood that men have a number of different effective strategies. The background and discussion do a good job of discussing the various possibilities, and how combinations of possibilities that include both female choice or male-male competition could explain human mating behavior. However, it does not really dig into what the implications might be for how multiple, distinct strategies could impact different aspects of the data. What comes to mind is orangutans, in which the large, masculine males appear to obtain mating opportunities primarily through female choice, while the smaller males that have not developed the large body sizes and facial flanges may obtain additional mating opportunities through sexual coercion. In a large sample or meta-analysis like this, a combination of strategies in human males that are at odds with one another, yet both highly effective, may have results that tend to cancel one another out - is there any evidence of this? Getting more into the primate literature here could be useful.
The authors point out that there could be a strong confounding effect with the way testosterone operates developmentally. Testosterone during adolescence translates well to the development of masculine characteristics, but does not necessarily predict testosterone later in life (hence, the expression of masculine features may not actually relate well to circulating testosterone that could be at least partially drive male-male competition). The authors did an excellent job of discussing these potential confounding effects, but I would have found potential issues like this (and like the one above) to be presented usefully in a table that lays out the different potential confounding issues, and then discusses what the predictions should be in the meta-analysis results for each one.
The meta-analysis seems well-designed, and the methods appropriate. However, it did feel a bit like data mining with so many different variables run against one another. I do not think this is actually the case, and the authors do justify each of their decisions. In fact, one of the main outcomes of this work is that they show how few of these parameters actually relate strongly to one another. However, the authors might want to be aware that this study could be read as data-mining because of the search for significance amongst so many different variables, and offset this with explicit discussion and framing up front that they intend to examine how effective the various study parameters actually are at uncovering the relationships they seek to uncover. This is something the authors discuss very articulately at the end, but I would appreciate seeing this up front as one of the goals of the paper.
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Evaluation Summary:
This work evaluates the strength of the evidence that human sexual dimorphism is the product of sexual selection. As a meta-analysis of studies that connect various measures of masculinity to various measures of reproductive success, this paper represents a synthesis of what this vast literature can show thus far. The work will be of general interest to evolutionary social scientists from a variety of disciplines, and it does a good job of clearly and concisely presenting the current state of sexual selection research on human males. The data are well presented, but the interpretation of the results is currently limited by some gaps in the theoretical framework guiding the manuscript.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private …
Evaluation Summary:
This work evaluates the strength of the evidence that human sexual dimorphism is the product of sexual selection. As a meta-analysis of studies that connect various measures of masculinity to various measures of reproductive success, this paper represents a synthesis of what this vast literature can show thus far. The work will be of general interest to evolutionary social scientists from a variety of disciplines, and it does a good job of clearly and concisely presenting the current state of sexual selection research on human males. The data are well presented, but the interpretation of the results is currently limited by some gaps in the theoretical framework guiding the manuscript.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewers #1-4 agreed to share their names with the authors.)
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