Download Title: Too Many Men: The Violence Problem? Authors: Ryan Schacht

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Title: Too Many Men: The Violence Problem?
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Authors: Ryan Schacht (1*), Kristin L. Rauch (1) & Monique Borgerhoff Mulder (1-3)
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(1) Department of Anthropology, University of California at Davis, Davis, CA 95616, USA
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(2) Graduate Group in Ecology, University of California at Davis, Davis, CA 95616, USA
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(3) Center for Population Biology, University of California at Davis, Davis, CA 95616, USA
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*Corresponding author: Schacht, R. ([email protected])
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Abstract (120/120 words)
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Are males more likely to engage in violent forms of competition when mates are limited or abundant? We
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review the theory behind parental investment and mating market models, emphasizing recent reformulations
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within sexual selection theory integrating both groups of models. The relationship between sex ratios, the
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intensity of sexual selection, and parental investment is clarified, particularly regarding violent behavior in
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human populations. We explore conflicting trends within the evolutionary, sociological and economic
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literatures, and evaluate empirical evidence for the different models of sexual selection. In short, we find that
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the “more men more violence” expectation derives from a simplistic interpretation of Trivers’ original paper, a
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failure to appreciate recent theoretical developments, and the very loose use of the term ‘competition’.
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Main Text (3500/3500 words)
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More men more violence
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Recent high-profile cases of violence, such as the notorious 2012 Delhi gang rape case, have the public,
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journalists and researchers alike all searching for explanations. A popular explanation centers on male-biased
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sex ratios, with specific examples coming from both India and China where there are growing numbers of extra
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men (often termed “bare branches”) due to son preferences and daughter-biased infanticide and elective
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abortion [1]. Because, on average, men are more violent than women it is argued that more men will necessarily
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lead to more violence. This argument is not new to the scientific literature and has become central to how many
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understand sexual selection. Essentially, when there are more males than females in a population, males are
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expected to compete vigorously for the limited number of mating opportunities available. In applying this idea
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to humans, it is appealing to attribute elevated rates of violent crime to high sex ratios and a shortage of women
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[e.g., 2, 3]. While this reasoning is intuitive, we question both its underlying theoretical basis and its empirical
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support. Put plainly, are males more likely to engage in violent forms of competition when mates are limited or
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when they are abundant? We tackle this issue by focusing primarily on human societies. We explore conflicting
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trends within the evolutionary, sociological and economic literatures, and in particular we highlight recent
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reformulations within sexual selection theory that challenge our intuitions and generate very different
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predictions, rather more in line with recent empirical findings.
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The “more men, more violence” prediction is derived from a long-standing model of sexual selection, laid out
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by Trivers [4] and developed by Emlen and Oring [5] and Clutton-Brock and Vincent [6], the former with the
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concept of Operational Sex Ratio (OSR) and the latter with Potential Reproductive Rate (PRR) (see Box 1 for
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Acronyms Defined). Essentially, according to the traditional parental investment (PI) model, when one sex is
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tied up with parental care, or more generally with activities that lower its PRR, the other sex competes over this
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limited resource, leading to the prediction that the sex in abundance competes more intensely for mates than
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does the rarer sex, and the ancillary expectation that this will generate more violence in the more abundant sex.
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There is however a different model for thinking about mate competition – the mating market model – that
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emerged within the social sciences decades before the traditional parental investment model [e.g., 7, 8]. Its roots
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lie in the supply and demand models of economics. Becker [9] argued that an increase in the sex ratio raises the
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demand for wives, giving women greater bargaining power and requiring men to invest more efficiently in the
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traits that make them attractive as long-term partners [see also 10, 11]. Mayr [12] first raised this idea in
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biology, in his discussion of sex ratios and mating systems in birds, and Noë & Hammerstein [13, see also 14]
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reintroduced these ideas into behavioral ecology.
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These two perspectives have coexisted for over four decades yet make strikingly different predictions, under
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many conditions, regarding the mating strategies of the more numerous sex. Given the apparent logic, utility
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and empirical support for both perspectives, the persistent disconnect between these approaches is troubling.
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This paper will review the theory behind PI and mating market models and evaluate empirical (primarily
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human) evidence for both. We outline innovations in sexual selection theory [15-18], which offer promising
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ways to integrate the two groups of models and develop a sharper understanding of the relationship between sex
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ratios, the intensity of sexual selection, and parental investment, with a view to shedding more light on the
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occurrence of violence in humans.
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Parental Investment Models
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Building on Bateman’s [19] demonstration of greater sexual selection in males than females, Trivers [4]
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proposed that the relative PI of the sexes is a key variable controlling the operation of sexual selection. The
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higher-investing sex becomes a limiting resource for the sex that invests less, leading to escalated levels of mate
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competition in the latter. Often, and especially for mammals, females invest more in parental care than do
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males, therefore males face higher levels of competition for access to the limited number of females.
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Emlen and Oring [5] added the concept of OSR. The OSR is the ratio of sexually active males to sexually
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receptive females and is highly influenced by patterns of parental investment. Higher investment by females
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decreases the amount of time they are ‘receptive’ to fertilization. Such sex differences in the availability of
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gametes skew the OSR towards males leading to the claim that males face a greater intensity of sexual selection
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on the traits that make them competitive for relatively scarce females. Insofar as relative PI is typically an
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important factor contributing to variations in OSR between species and/or populations, Clutton-Brock and
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Vincent [6, see also 20] proposed that PRRs of males and females can be used to predict patterns of competition
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between the sexes.
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According to this perspective, when males are in abundance they are expected to compete vigorously for mating
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opportunities, often with the implications of intense physical struggles and violence. Insofar as some males are
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more successful than others in monopolizing these opportunities as a result of heritable traits [e.g., 21], this is
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expected to lead to intensified levels of sexual selection on males.
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Mating Market Models
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Mating markets operate by the principle of supply and demand. The rarer sex has more bargaining power in the
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marketplace than the more common sex, and can leverage their scarcity to realize their preferred mating
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strategies. The more common sex, as a consequence of their abundance, must cater to the preferences of the
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rare sex in order to acquire a mate.
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Economists and sociologists regularly employ mating market theories. While much of their research is not
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explicitly based on sexual selection it is, nonetheless, consistent with evolutionary reasoning. In a famous book
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entitled “Too Many Women” Guttentag and Secord [11] draw from historical accounts and quantitative
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analyses to demonstrate how sex ratios affect the many aspects of the relationships between men and women.
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They show that in societies with a surplus of women, men find themselves in demand and can leverage their
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scarcity, behaving promiscuously and offering little parental investment; whereas when women are in short
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supply, marriage and a commitment to family are highly valued. A more recent example comes from Colombia
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[22], where high male mortality rates in some regions have led to an abundance of women, decreased marriage
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rates and high incidences of men involved in concurrent relationships. Much cross-cultural work corroborates
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these findings and has shown that female-biased sex ratios are associated with lower levels of male parental
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investment and higher rates of female headed households [23]. Conversely, when there are too many men the
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nature of relationships change. For example Angrist [24] found that among immigrants to the U.S., high sex
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ratios had a large positive effect on the likelihood of female marriage and a large negative effect on female
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labor force participation due to higher rates of male investment. In general, male biased sex ratios are
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associated with less promiscuity [23] and greater conjugal stability [25].
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A Reformulation with implications for violence and sex ratios
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In a recent model Kokko and Jennions [18] implicitly integrate the two veins of thought, by including both the
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importance of PI and the frequency-dependent nature of mating strategies. Accordingly, the adult sex ratio
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(ASR) becomes a key predictor of sex-differentiated behavior. For example, due to poor odds of remating in a
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crowded market, a male-biased ASR discourages male desertion in search of additional matings and instead
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favors increased male care [18] and less intense sexual competition [26]. According to this reformulation the
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intensity of sexual selection is driven by intrasexual variability in quality [16], paternity certainty [15, 17], and
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crucially for our argument here, the relative abundance (or shortage) of either sex [18], such that when one sex
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is in oversupply both sexes respond by strategically altering their preferences and behavior, as in a mating
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market [14]. Central to understanding this shift in thinking is the reevaluation of several variables and their
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relationship with the intensity of sexual selection (Box 2), including the concept of OSR which has until
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recently structured our understanding of sexual selection [26].
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While some of the empirical findings consistent with the traditional model are recovered in the reformulated
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model, albeit by employing the new logic, [27], in general predictions are strikingly different. All else being
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equal (to which we return), traditional models predict an abundance of men to lead to greater sexual
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competition, violence, crime and paternal absenteeism, while reformulated models, in general, contradict these
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expectations. According to the reformulated perspective men, when they are in abundance (and not therefore in
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demand), will cater to women’s preferences for investment and fidelity in order to attract and retain limited
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mating opportunities.
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Turning to humans
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To obtain a clearer understanding of how mate competition varies within species we need to know about the
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relationship between sexual selection and ASR. To take an empirical approach to this question in humans we
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worked through two networks of anthropologists [28, 29], using both published sources and personal
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communications, to examine across 15 populations the relationship between ASR and the opportunity for sexual
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selection (Is; Box 3). The relationship between the Is of males and sex ratio of the mating pool is negative,
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suggesting greater sexual competition among men in female-biased than in male-biased populations. Clearly
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this is a preliminary empirical demonstration, of primarily illustrative significance, but it does raise the
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possibility that, across this small subsample of human populations, when men are scarce they show greater
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variance in fitness than when they are in abundance.
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How does this relate to the patterning of violent competition across societies? We turn now to an examination of
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the relationship between the sex ratio and different forms of behavior that might be seen as indicators of
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competition to determine whether or not they are negatively associated, as the reformulated model would
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predict. As is quickly observed from Table 1, the results are mixed. In some cases male-biased sex ratios are
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associated with higher rates of violence and crime and in some cases with lower. Why might this be? While
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methodological differences play a role (see notes in Table 1), this table reveals there is no simple pattern of
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violence in relation to mating competition.
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A closer look at ‘competition’ and how it varies with ASR
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Given the variety of forms that mate competition can take (see Box 4), it is incorrect to assume that violent
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competition is the only male response to a shortage of opposite sex mates – both theoretical [a male-biased ASR
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favors male care; e.g. 18] and empirical [male mate guarding in response to mate shortage; e.g. 30] findings
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counter this assumption. This is perhaps the most fundamental reason why violence is not necessarily
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attributable to “too many men”. More generally, competition is used with a variety of distinct meanings, both in
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the human and nonhuman sexual selection literature, and it is often unclear how it relates to mating effort (e.g.,
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are males competing to be selected as long-term or short-term mates?). Clarifying this relationship is key to our
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understanding of the role of sexual selection and for making predictions of patterns of violence and criminal
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activity.
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We can venture explanations for some of the variable patterning of violence with sex ratio shown in Table 1. In
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line with the reformulated model, comparative studies in the US and cross-nationally find an abundance of
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males associated with lower rates of rape and sexual assault [31, 32], and a shortage of men associated with
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higher rates of male-male homicide victimization [33]. On the other hand, female homicide victimization rates
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seem to counter predictions of the reformulated model: there is a positive relationship with ASR in the U.S. [34,
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35]. However, when women have the advantage in the mating market this may favor female sexual strategies,
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such as mate sampling [36], that lead to male sexual jealousy [37]. Mate guarding, in response, is one strategy
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used by males when in abundance to ensure paternity certainty [30, 38] and may be carried to extremes in some
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cases. These examples highlight a key point - not all violence is due to sexual competition, mate retention
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strategies may also be violent. Therefore, “violence” may be present in both high and low sex ratio conditions,
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making identifying a particular violent act and how it relates to mating competition of critical importance for
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understanding the strength and direction of sexual selection in a particular ASR.
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Our consideration of how violence is patterned in relation to ASR points to three things. First, the patterning is
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complex, and likely reflects multiple interacting factors not yet modeled by theoreticians. Key factors may be
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the degree of intrasexual variation in quality, and the shape of the Bateman gradients that capture the marginal
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returns to agonistic competition [39]. Second, violent crime statistics need to be more precisely disaggregated in
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order for the ideas suggested above to be testable (e.g., avoid combining multiple measures). We need to know,
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for example, whether a particular type of crime, or specific violent act, is: sexual effort or competition over
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resources for parental care [e.g., higher rates of property crime when men are in abundance, 40]; an
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exemplification of within or between sex conflict; restricted to certain sectors, or socioeconomic brackets, of the
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population. For example one man might commit theft and another man may go to the office, with both gaining
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the resources to attract a mate; similarly some men might display their good genes through violence, others
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through artistic expression. In both examples, the motivation and outcomes may be the same, but the context
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can impose constraints on behavioral options and influence patterns of crime. Third, social scientists have not
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yet properly delineated the behavioral options available. For example, in response to high ASR, unmarried men
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might migrate to regions with more women, patronize prostitutes, resort to polyandrous marriage, or even set up
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bachelor households and “bachelor villages” as reported for contemporary China [41].
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In short, our understanding of how men compete for women, how women’s reproductive strategies vary, and
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how these are affected by sex ratio, is greatly under-theorized. It is clear, however, that the uncritical
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acceptance of the “more men, more violence” expectation of traditional theory is unwarranted.
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More theory to the rescue
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Modelers have already anticipated the variation in the relationship between sex ratio and violence we see in
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Table 1, and from them we can identify potential avenues towards a more precise understanding of the
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patterning of violence across human societies. First, Kokko & Jennions [18] show that, counter intuitively, a
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particular behavior may be selected for even when it increases mortality rates. This is because frequency
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dependent selection selects against care in the rarer sex, and mortality patterns will influence which sex is rare.
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So, if competition entails a higher risk of mortality, then the competing sex will remain the rare one, favoring
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competition among those who survive. If caring (whether as a form of mate competition or not) brings a higher
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mortality risk, the caring sex will become the rare sex and thus the opposite sex is now abundant and favored to
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provide care due to poor odds in the mating pool [27]. Thus empiricists committed to explaining the patterning
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of human violence should be quantifying the relative mortality costs associated with caring and mate
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competition across different human societies, a difficult task to be sure.
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Second, modern sexual selection theory identifies two additional factors other than ASR that influence male
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strategies, therefore complicating simple (“all else equal”) predictions about the effect of ASR on violent
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behavior: the extent of variation in quality among males and females, and paternity certainty [15]. Where there
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are differences in male quality that affect the future likelihood of mating, paternal care may be unlikely for two
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reasons. First, already mated males are likely to desert because they face very good odds of remating by virtue
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of the qualities that gained them mating success in the first place. Second, unmated males have no choice but to
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remain in the mating pool since they have no offspring to care for. Similarly where males have low paternity
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certainty, even at high ASR high paternal care will not be favored. These factors can vary independently of
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ASR between populations, and can interact, as when low paternal care favors female strategies that reduce
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paternity certainty.
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The third important factor that intersects the relationship between ASR and male violence is the criteria on
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which females choose mates. If females choose males on their provisioning qualities, and provisioning does not
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entail violence, then consistent with Kokko & Jennions’ [18] model the lowest levels of violence would indeed
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be observed at highest ASRs. However, if successful provisioning depends on the control of resources, and
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resource access entails physical competition, high ASRs may indeed be associated with violence. Furthermore,
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male violence might be selected even at high ASRs if females choose mates based on indirect benefits to
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offspring, affording their sons a potential advantage in violent competitive environments [42, see also 43].
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Finally, much of the above reasoning assumes that females are able to exercise choice. Model predictions about
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levels of violence must be modified accordingly when females are not able to express their mate preferences
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[44]. Additionally, recent findings have shown that assuming a tradeoff between parenting effort and mating
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effort is problematic [45]. Indeed, parental care can be a mate competition strategy [two-spotted goby,
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Gobiusculus flavescens; 46].
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Conclusion
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Humans are a good species in which to investigate how violent competition and other traits are related to sex
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ratio because we have such variable mating systems, from harem polygyny attained through violence among
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men against women [e.g., Yanomamo, 47], through resource defense polygyny attained through economic
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competition among men who are chosen by women or their kin [e.g., Kipsigis, 48], to situations where men and
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women choose each other on the basis of individual qualities [e.g., Tsimane, 49]. This review has suggested that
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violence is not structured according to simple predictions from the traditional parental investment model, nor
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intuitive reasoning. A major reason is that measures of violence are simply proxies for what the models are
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actually trying to gauge: the intensity of sexual selection. When we measured the intensity of sexual selection
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directly (using Is), the findings assembled in Box 3 are in line with reformulations in sexual selection theory,
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pointing to the importance of ASR as a predictor of the relative opportunities for sexual selection.
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The belief that violence and crime are exacerbated in human populations by an excess of males is
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oversimplistic. We show in Table 1 that the patterning of violent crime shows no simple association with the
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sex ratio, the patterning is complex. We discuss reasons why current understandings of sexual selection are as
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yet inadequately articulated to deal with a number of the critical intervening factors we have identified. We also
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recognize that empiricists have failed to quantify some of the key parameters needed for such modeling, such as
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the relative costs of care competition and the role of violence in attaining mates. Finally, we point to a need for
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a much richer ethology of human violence – the data is primarily drawn from police reports and/or national
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statistics that, for the most part [for a remarkable exception see 50], combine inter and intra sexual attacks,
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crime directed at people and property, and crime emanating from different sectors of the population.
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The simple message to take from this review is that the often related claim that when males are more numerous
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than females (high ASR) males will create a potential social problem [e.g., 51], rests (as we can now see) on a
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very specific set of assumptions about the nature of male-male competition and the extent to which females can
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make choices over mating. There are policy applications of this research, with serious practical implications for
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people’s lives. Recommendations that a low sex ratio will alleviate problems of male violence, while well-
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intentioned, could actually exacerbate the problem [e.g., attempting to reduce bullying by lowering a
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classroom's sex ratio, 52]. Likewise, “tough on crime” policies that incarcerate increasing numbers of men may
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actually be contributing to rates of violence, rather than alleviating them, through the resulting sex ratio
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imbalance in highly policed communities [e.g., 53]. Similarly appeals to abolish polygyny because of the
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dangerous emergence of a class of unmarried men rely on equally flawed logic [2]. In short, the “more men
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more violence” expectation derives from a simplistic interpretation of Trivers’ original paper, a failure to
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appreciate more recent theoretical developments, and the very loose use of the term competition.
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Acknowledgements
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For financial support we thank the University of California, Davis (RNS) and the Wissenschaftskolleg zu Berlin
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(MBM); for discussions and comments on the manuscript our colleagues in the HBE and Cultural Evolution
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labs at UCD, and for secondary data the generous anthropologists whose populations are included in Box 3.
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TABLE 1. Sex ratio and violence: a literature review*
Reference
Sample
Sex Ratio Measure
(1)
Measure of
violence (2)
[33]
100 countries; UN
Complete sex ratio
Homicide
[34]
U.S.; FBI
70 countries; INTERPOL
Female homicide
victimization
Murders, rapes &
violent assaults
Positive
[32]
Men and Women
(18+)
Men and Women
(15-64)
[54]
Review
Mixed
Historical accounts
Positive
[55]
Massachusetts; NIBRS
U.S.; FBI
[56]
India; Government data
Simple &
aggravated assault
Male-on-female
partner violence
Homicide
Unassociated
[37]
Men and Women
(18-23 & 24+)
Men (18+)/Women
(18-34)
Complete sex ratio
[40]
China; Government data
HRAF
Violent and
property crime
Warfare mortality
Positive
[57]
Men and Women
(16-25)
Complete sex ratio
[58]
Review
Mixed
Historical accounts
Positive
[59]
U.S.; State data
Complete sex ratio
[60]
56 countries; WHO & UN
Complete sex ratio
Homicide & suicide
rates
Homicide
[53]
153 U.S. cities; FBI &
Census
45 nation sample; WHO &
UN
U.S.; Census & FBI
Men and Women
(15-59)
Complete sex ratio
Murder & robbery
Negative
Homicide
Negative
Five-year
groupings
Rape
Negative
India; Crime in India
database
46 nations; WHO & UN
Complete sex ratio
Homicide
Complete sex ratio
Homicide
Negative
Men and Women
(18+)
Unmarried men &
women (18-44)
Homicide
Negative
[65]
46 nations; World Values
Survey
U.S. Counties; FBI & Census
Homicide
Unassociated
[35]
U.S. Cities (n=217); FBI
Complete sex ratio
[66]
Chinese Cities (n=37);
community level data
[61]
[31]
[62]
[63]
[64]
Female homicide
victimization
Men (17-23), women Forced sex
(15-21)
Relationship
SR/Violence
Negative
Negative
Positive
Positive
Negative
Mixed
Negative
Positive
Positive
Positive
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*We performed a literature search for violence and sex ratio in humans on Web of Science (April 3, 2013,
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n=64). Some search results were excluded due to redundancies, lack of empirical data (e.g. book reviews), or
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irrelevance to the question at hand (e.g. studies looking at the sex ratio of criminal offenders without reference
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to the population sex ratio). This table summarizes the results of the remaining papers (n=21), highlighting
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inconsistencies in the relationship between the sex ratio and violence as well as critical methodological
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differences.
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(1) Measures of sex ratio vary widely, and there is no evidence that the scale of the sex ratio measurements
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(ranging from national level to village level data) is appropriate to capture the relevant mating pool.
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(2) Measures of violent crime range quite widely: in some cases all homicides are included, some just female
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victims and others include a mix of physical assault and property crime.
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BOX 1: Acronyms and Definitions
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Operational Sex Ratio (OSR): the ratio of sexually active males to sexually receptive females in a population
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Potential Reproductive Rates (PRR): the hypothetical maximum number of independent offspring produced by
males and females per unit time
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Adult Sex Ratio (ASR): the ratio of adult males to adult females in a population
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High sex ratio: more males than females in a population
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Low sex ratio: more females than males in a population
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BOX 2. Changing the direction of the causal arrow between parental investment and sexual selection
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The traditional PI model has been influential in the development of sexual selection theory but it is logically
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flawed. The reasoning goes that because females produce large, costly eggs, male fitness is constrained by
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access to mates, producing (in most cases) female-biased care and male-biased competition. However, shortly
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after Trivers [4] presented this argument, criticisms of the model began to mount. (I) Sex differences in PI
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cannot be taken as a determinant of the intensity of sexual selection as this entails committing the faulty logic of
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the “Concorde Fallacy” [18, 67]. Past investment alone is irrelevant to decisions about future behavior. (II) As
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with Maynard Smith’s [68] classic model relating parental care evolution to sex differences in mating
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opportunities, Trivers’ verbal model lacks internal consistency, violating the requirement of equal average
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fitness for females and males and effectively making females exogenous to the model [16]. While males do
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have higher PRRs [6], it is actual and not potential rates that matter in terms of selection [69]. To make the
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model self-consistent the additional paternity of deserting males must be accounted for, and comes at a cost to
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the paternity of other males [i.e. the extra mates of successful males must come from somewhere; 17]. (III) In
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the traditional PI model a male-biased-OSR leads to more intense intrasexual selection and greater competition
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among males due a shortage of females [5]. However, male biased OSRs do not necessarily lead to greater
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intensity of sexual selection. Klug et al [70] show how OSR only accurately predicts sexual selection under a
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limited set of circumstances, most specifically when mate monopolization is strong. In fact a wise strategy for a
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male who may face a long wait time in between reproductive events if he were to desert would be to instead
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stay with his current partner [18]. Thus the OSR can equally be thought of as a frequency dependent mechanism
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that selects for care in the sex that is in abundance.
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In sum, the relative abundance of gametes (i.e. more sperm than eggs) generates the conditions for sexual
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selection. If selection occurs, patterns of care and competition are an outcome [15]. Therefore, sexual selection
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is not an outcome of patterns of PI as posed in traditional models, but instead the arrow of causality must be
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flipped [69].
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BOX 3. The Sex Ratio and Opportunity for Sexual Selection across 15 populations
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We calculated the opportunity for sexual selection (Is) of males against the sex ratio of the mating pool for each
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population, selected from the work of human behavioral ecologists working in non-industrial societies. The Is is
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a standardized measure of variance in reproductive success (RS) and is calculated by dividing the variance in
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RS by the squared mean of mating success [71-73]. It represents the upper limit of the potential strength of
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sexual selection in a given population (importantly, not the actual strength of sexual selection on specific traits).
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The Is is useful for cross-population comparisons because it is standardized by mean fitness and describes the
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variation in mating success that drives sexual selection within a population. The Is provides a valuable
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summary statistic to describe mating systems [74] because it offers a concise description of the distribution of
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fertilizations [75] and does not rely on vague descriptions of variation in the distribution of matings that labels
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such as ‘monogamy’ and ‘polygyny’ often do [76]. Below we show how the opportunity for sexual selection is
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associated with the sex ratio of the local mating pool across 15 populations.
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The regression line (calculated using maximum likelihood estimation) shows a negative relationship between
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the sex ratio of a population’s mating pool and the Is among males (dashed line) and the 95% confidence limits
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(displayed in pink for female-biased sex ratios and blue for male-biased sex ratios). The slope is negative,
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suggesting that traditional assumptions regarding a positive relationship between the abundance of males and
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the intensity of sexual selection is flawed. Rather, as the sex ratio decreases the opportunity for selection among
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males increases [see also (77) for a similar conclusion for human populations but based on normative mating
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system categorizations].
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BOX 4. Empirical Ambiguities: The Term ‘Competition’, and ‘Violence’ as Proxy
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The specific form that mate competition takes is central to the question of sex ratios and violence. Here we
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propose a working taxonomy:
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(1) Demographic competition: In a simple demographic sense, there is ostensibly a higher degree of mate
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competition among the more common sex when sex ratios are imbalanced; this is indeed the inference from
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OSR. The term “competition” here is used simply to depict the fact that, under most conditions, not all
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individuals find a partner due to the number of competitors. Note that the term is neutral with respect to
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competitive behavior, and with respect to the implications for the intensity of sexual selection (Box 3), even
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though it corresponds to OSR (see Box 2).
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(2) Courtship competition: These behaviors are usually directed at opposite sex individuals, and are
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“competitive” due to mate selection. Females in some species (including humans) have considerable autonomy
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with respect to their choice of mates [see 78]. Accordingly competition among males may take the form of
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providing what females and their relatives want [e.g., a bridegroom’s labor 79] , and among females in
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providing what males want [chastity, 80]. This might entail accumulating resources [48], advertising paternal
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proclivities and abilities [81], or demonstrating genetic fitness [82].
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(3) Agonistic competition: This type can, of course, also occur behaviorally among and between both sexes
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irrespective of ASR. Focusing here just on males it can take many forms, as in bellowing displays in bison
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[83], fighting over the control of breeding or nesting territories as in red-winged blackbirds [84], or forced
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copulations as in orangutans [85]. In humans this kind of competition might entail physical attacks to eliminate
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(or character attacks to denigrate) male competitors [86], direct sexual attacks on women [44], or property
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crimes [40]. In many cases this type of competition is directed at same sex individuals, but not always, as in
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rape and sexual assault [87].
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It is clear from these simple categories and examples that not all mate competition is violent, and not all
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violence is mate competition – a problematic reality for empirical analyses that use violence as a proxy for the
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intensity of sexual selection. For instance, a female shortage might decrease violence by favoring more
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courtship competition among males. Similarly, a male shortage could increase violence by diminishing the
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influence of female choice for caring males.
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