Marriage markets and mating aggression help explain societal differences in violent crime

Aggression and Violent Behavior 16 (2011) 420–427

Contents lists available at ScienceDirect

Aggression and Violent Behavior

Marriage markets and mating aggression help explain societal differences in violent crime Nigel Barber ⁎ 4229 Silver Ct., Birmingham, AL 35213, United States

a r t i c l e

i n f o

Article history: Received 22 December 2010 Accepted 18 January 2011 Available online 1 February 2011 Keywords: Homicide Sex ratio Marriage Markets Mating Effort

a b s t r a c t Violent crimes (murders, rapes, and assaults) are higher in countries with a relative scarcity of men according to research using both United Nations and INTERPOL data and controlling for economic development, income inequality, urbanization, population density, police presence, and drug trafficking (Barber, 2009a). This is an apparent contradiction given that males are more criminally violent and I argue that the most plausible explanation is that there is more direct mating effort, and hence more violent crime, in countries having a relative scarcity of men (or a low sex ratio). Alternative explanations that are discussed and found wanting include cultural determinism. Causal links connecting the marriage market and violent crime include possible sociological, physiological, and developmental mechanisms that offer exciting prospects for future researchers. © 2011 Elsevier Ltd. All rights reserved.

Contents 1. 2.

Adaptation in modern societies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Defining adaptation in a way that suits socially-learned behavior . . . . . . . . . . . . . . . . 2.1. Evolutionary Social Science: a causal evolutionary model for explaining societal differences 3. Marriage markets, sex ratios, and social behavior . . . . . . . . . . . . . . . . . . . . . . . . 4. Violent crime and direct mating effort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. The anthropology of mating aggression and violent crime . . . . . . . . . . . . . . . . . . . . 6. How to make sense of data linking marriage markets and violent crime . . . . . . . . . . . . . 7. Alternative theoretical explanations for variation in violent crime . . . . . . . . . . . . . . . . 8. Filling in the causal mechanisms through which mating competition increases violence. . . . . . 9. Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Evolutionary theory rationalizes the great complexity of life for all species and there is no reason why it cannot also illuminate the causes of variation among human societies (Barber, 2008; Richerson & Boyd, 2004, p. 94–96). All of the complexity of human societies must have emerged from the same process of natural selection that fashioned the social behavior of other species on planet Earth. Admittedly, recent human societies, at least from the agricultural revolution on, are qualitatively different from earlier societies due to their complexity, level of technological development, modes of information transmission, capacity to store food and concentrate power and wealth, etc. The concept of adaptation is nevertheless

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useful for analyzing variation in violent crime even in contemporary societies. This claim rests upon two theoretical innovations. The first is an expanded definition of adaptation to include social learning, as well as genetically-determined variation, so as to render the concept applicable to modern societies. The second is the establishment of a chain of causality that describes how natural selection may produce modern societal variation. In particular, varying marriage markets affect violent crime in systematic ways because those societies having an excess of females (or a low sex ratio, defined as the number of men per 100 women) elicit more direct mating effort thereby increasing male–male violence and violent crimes. This paper discusses the role of adaptive mechanisms in accounting for society-wide differences in violent crime rates with an emphasis on proximate mechanisms.

N. Barber / Aggression and Violent Behavior 16 (2011) 420–427

1. Adaptation in modern societies A gene-based evolutionary adaptation is a design-like match between features of the organism and the way that it makes a living (Reeve & Sherman, 1993; Williams, 1966). An adaptation is often thought of as providing the solution to an adaptive problem. The long neck of a giraffe solves the problem of reaching into the higher branches of trees where these animals browse. Adaptations must increase biological fitness (or the probability of surviving and reproducing) so that individuals manifesting them are biologically successful and transmit the alleles that underlie them into future generations at the expense of rival alleles. After many generations of such genetic selection, favorable alleles get fixed in the population. According to the gene-centered view of adaptation shared by many evolutionary psychologists an adaptation is found in virtually all members of a species, at least at the appropriate age or developmental stage and if gender appropriate (Buss, Haselton, Shackelford, Bleske, & Wakefield, 1998). This criterion of universality is far too restrictive if the concept of adaptation is to be applied to behavior, or to societal differences in behavior, for behavior is not only contingent upon the immediate context but is also affected by the developmental environment. According to gene-centered adaptationism, the current fitness impact of some characteristic is deemed irrelevant to its status as an adaptation: what is required instead, is a demonstration that the characteristic was selected for over a long enough period in the evolutionary past to drive it to fixation in the species-typical genotype (Buss et al., 1998; Symons, 1992). These two criteria of adaptations having to be universal and requiring deep roots in evolutionary history are too stringent to be applied to behavior (as opposed to anatomy that may be studied in the fossil record over millions of years). Indeed, insisting upon universality and evolutionary antiquity makes the concept of adaptation peripheral to the social and behavioral sciences. The argument that selection removes genetic variability with respect to behavior — that is pivotal to the universality argument — flies in the face of much personality research. In general, personality variation partly reflects genetic polymorphisms that are maintained due to the trade-off between varied fitness costs and benefits associated with the Big Five dimensions of personality, for example (see Nettle, 2006), or individualism versus collectivism (Fincher, Thornhill, Murray, & Schaller, 2008). It is one thing to criticize the adaptationist program advanced by evolutionary psychologists. How can adaptation be defined in a better way that allows us to comprehend societal differences in violent crime in adaptative terms? To begin with, we must include behavioral phenotypes as well as morphological ones and must therefore include a role of society-specific learning processes. 2. Defining adaptation in a way that suits socially-learned behavior In summary, the definition of adaptation taken from evolutionary biology is more suited to phenomena that can be observed over tens of millions of years in the fossil record. It is less relevant to behavioral evolution that occurs too rapidly and leaves little or no fossil record for comparative analysis. Defining behavioral adaptations as species typical is far too restrictive to include socially learned adaptations that vary among local populations. For, different local populations encounter varied challenges to survival and reproduction and behavioral solutions to such problems that are partly dependent on social learning traditions should be considered adaptations (Richerson & Boyd, 2004) because they serve the same function as genetically evolved solutions to the same problem. In either case, an adaptation is defined as a fitness-enhancing phenotype (whether geneticallybased, learned, or both) that helps an organism to adjust to varied habitats in ways that promote survival and/or reproductive success.

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Adaptations have design-like features in the sense that the phenotype is explicitly suited for such adjustment, just as a key is suited to turn a particular lock. In the past, evolutionary psychologists coped with the lack of a fossil record of behavior in an ingenious, but unsatisfactory, way. They assumed that humans are adapted to the environment of evolutionary adaptedness (or the ecological conditions that prevailed during the two-million-year history of Homo) rather than current conditions. Further, they assumed that the brain comprised numerous specialpurpose information processing devices, or modules, each of which was designed to solve some recurrent problem of the remote evolutionary past, such as selecting a mate, detecting cheaters on social contracts, or acquiring language (Cosmides & Tooby, 1987). The structure of these modules was believed to be inherited via a genetic program that was driven to fixation through persistent positive selection so that all humans have essentially the same psychology (with exceptions for gender differences and developmental changes). These assumptions are each disputable (Barber, 2008; Laland & Brown, 2002). Evolutionary psychology has nevertheless stimulated much empirical research predicated on the assumption that if we understood what problems our ancestors faced, and imagined possible components of the relevant solutions (or reverse engineering), that we could conduct tests on modern populations to see whether their minds work as would be predicted using such logic. It seems preferable that a natural-science approach to our species would focus on observable behavior rather than “the mind” that presupposes subjective self-reports, not to mention a complex system of intervening variables, such as modular information processing mechanisms that are generally difficult to observe, or to reconcile with known mechanism of neuroscience or gene expression (Barber, 2008). Moreover, this approach could not be applied to nonhuman animals. By that criterion, it is inherently unsuitable as a naturalscience approach and thus falls into a human-uniqueness trap that has bedeviled evolutionary analyses of human behavior over the past century-and-a-half. I argue that in addition to pan-human adaptations, we should also consider that behavioral differences among local populations can be adaptations to local conditions. This logic is quite general and draws on the literature of animal behavior. Examples of socially-learned adaptations include the song complexity of birds, rats specializing on pine cones in some local communities, or shellfish in others, and the Mauritius kestrel developing a novel habit of nesting on cliffs to elude predators (Avital & Jablonka, 2000; Barber, 2008; Colias & Colias, 1984). The prevailing definition for an adaptation among evolutionary psychologists excludes such phenomena, leaving us with an impoverished account of animal behavior (much less that of humans) in which learned solutions to adaptive problems that make the difference between survival and extinction for individuals, or entire communities, are ignored as irrelevant to adaptation. Consequently, we are left to conclude that most modern human behavior is adaptively neutral or irrelevant. If we are to avoid that conclusion — one that has been all too popular among opponents of evolutionary approaches to human behavior — then we must redefine adaptation in a manner that is more relevant and useful for behavioral scientists whose subject matter is behavior, including criminal behavior, and its underlying evolutionary biological mechanisms. Here are the main criteria for a more useful definition of behavioral adaptations: 1. Behavioral adaptations crop up among local populations and do not have to be species typical. 2. Behavioral adaptations need not be observable in the fossil record over thousands of generations to be accepted as valid. 3. Current (or comparatively recent) fitness consequences are relevant for defining behavioral adaptations (contrary to the arguments of Buss et al., 1998 and Symons, 1992 among others).

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At a more general level, it is simply wrong to imagine that human adaptations have been fixed over the past two million years, if only because our ancestry extends back over many different species during that time period who varied in size, habitat, diet, cranial capacity, tool complexity, encephalization, and so on. Phenotypes that enhanced fitness in the evolutionary past may continue to do so, of course, although that is not always true. The human capacity for storing food energy — an adaptation in past environments — causes obesity and metabolic illnesses and is quite maladaptive today, for instance. By the same token, behaviors that enhance fitness today, such as nonviolence, would not have had the same consequence in the past when assaultive violence might have brought immediate benefits rather than swift and long-lasting incarceration. Such changes in the fitness consequences of behavior do not invalidate an adaptationist approach to modern behavior: they make it indispensable (when appropriately modified). 4. Societal differences in behavior may be adaptive so that societal variation in criminal violence is part of an adaptive response to varied marriage markets as explained in more detail below. In general, one finds that men emphasize direct mating effort in societies, or communities, where women are sexually active outside marriage, and this behavioral pattern bespeaks adaptive variation in male sexual behavior. A formal theoretical model that contemplates societal variation from an adaptationist perspective is known as Evolutionary Social Science (Barber, 2007, 2008, 2009b). This specifies a chain of causation whereby changes in local conditions, such as varied marriage markets affect how evolved behavioral predispositions play out.

2.1. Evolutionary Social Science: a causal evolutionary model for explaining societal differences Evolutionary Social Science (not abbreviated to avoid confusion with evolutionarily stable strategies) shares features with both human behavioral ecology and evolutionary psychology. The similarities and differences are outlined in Table 1, which is loosely based on Winterhalder and Smith (2000). The table highlights many differences in these related approaches. The key points are that behavioral ecology focuses on human adaptations to life in subsistence societies whereas Evolutionary Social Science focuses on modern societies. Evolutionary psychology focuses primarily on cross-societal universals that are interpreted in terms of pan-human adaptations, although most evolutionary psychologists recognize that psychological development proceeds differently in response to varied environments, such as varied levels of psychological stress during childhood (Belsky,

Steinberg, & Draper 1991; Del Giudice, 2009), an approach that is often referred to as “life history theory”. Modern human social behavior can be causally linked to ecological pressures using four necessary key assumptions that form the core of Evolutionary Social Science (Barber, 2007, 2008). These assumptions are: (1) that modern societies owe their character to an interaction of hunter–gatherer adaptations with the modern environment (interactionism); (2) that some changes in societies are due to changes in individuals (methodological individualism); (3) that historical changes and cross-societal differences can be due to similar adaptational mechanisms (counter cultural relativism); (4) and that different social contexts modify individual development in adaptive ways (adaptive development). These four assumptions must be made if evolutionary explanations are to be applied to the problem of explaining societal differences in violent crime (or anything else). A corollary is that if any of these assumptions is wrong, evolutionary explanation for societal differences may be impossible. Each of the above mechanisms is relevant to variation in violent crime. Interactionism means that human adaptations relevant to direct mating effort (which increases violent crime, Minkov, 2009) are influenced by novel economic factors, such as the relative independence of some working women from economic support by their sexual partners, particularly in the context of generous state provisions for child care, such as those in modern social democracies. If more individual women opt for sexual intercourse prior to marriage this increases the pool of women available for extramarital sex which, in turn affects the viability of direct mating effort by men as an alternative to marriage (methodological individualism). Such phenomena can account both for societal differences and for changes in crime rates over time (counter cultural relativism, Barber, 2003a). Abusive parenting has been identified as a cause of antisocial behavior and crime (see below). Evolutionary Social Science reinterprets such phenomena through the prism of adaptive development. Does parental behavior facilitate the kind of reproductive strategy that is likely to succeed in a particular social context? For instance, children maturing in an urban slum might learn to be tough, suspicious, and street smart thereby protecting themselves from being taken advantage of by others. Specific parenting practices, such as corporal punishment, may promote this phenotype (Nightingale, 1993). This approach to adaptive development is within the mainstream of evolutionary psychology (Del Giudice, 2009). Given this theoretical backdrop, variation in violent crime over generations, and among nation states, can be thought of as one manifestation of ancient adaptations for mating competition as they play out in varied modern environments. One aspect of differential expression of mechanisms for mating competition is adaptive development, the process through which the violent tendencies of young people are affected by their familial and community surroundings. With this theoretical framework in place, it is time to review

Table 1 Evolutionary Social Science compared to closely related fields. Evolutionary Social Science

Behavioral ecology

Evolutionary psychology

Adaptations Expected current adaptiveness Explanatory focus

Short-term, phenotypic Intermediate

Short-term, phenotypic High

Long-term, genetic Low

Modern behavior

Behavioral strategies of foragers

Methods Hypothesis generation

Compare societies, compare socialization Behavior fits differences in developmental, adult social context

Favored topics

Individual differences, societal differences, reproductive strategies

Ethnographic observation Foraging optimality, evolutionarily stable strategies Subsistence, reproductive strategies

Evolved psychological mechanisms Surveys, experiments Reverse engineering

Key constraints Key assumptions

Economic, ecological, marriage market Interactionism, methodological individualism, adaptive development, counter cultural relativism

Ecological, material Ecological determinism

Mate choice, sex differences Cognitive, genetic Universals, cognitive modules

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evidence that violent crime varies predictably with marriage market conditions such that high levels of direct mating effort are correlated with high rates of violent crime. 3. Marriage markets, sex ratios, and social behavior The marriage market is most often measured using sex ratios, although this approach can be criticized as being overly simplistic. In general, though, the logic is that in societies having more women than men, (i.e., a low sex ratio) it is more difficult for women to marry resulting in higher levels of non marriage for women, and higher levels of non marital reproduction (Guttentag & Secord, 1983). Any of these variables might be substituted for the sex ratio as a measure of the marriage market. When using sex ratios as a measure of marriage market conditions, it is worth pointing out that there are many relevant control variables. Income is one given that unemployment, or having a minimum-wage job effectively disqualifies men from marriage in many societies (Staples, 1985). Age is critical because the sex ratio is relevant only for persons who are in an age range where marriage is likely (e.g., 15–64 years). Some scholars prefer to restrict their analysis to the peak ages of first marriage for males and females and this makes sense in societies where most people marry once only. Marital status is important as well because married people are theoretically removed from the marriage market. Nevertheless, there is some evidence that young married women play the field in the sense that they are more likely to divorce and remarry if there is a good supply of marriageable men (see Barber, 2003b). There are many different ways of measuring the marriage market and there are many different kinds of sex ratios that may be used for this purpose. Sex ratios based on narrow age ranges (e.g. b5 years) and that consider only single people are suitable for studying secular changes in the marriage market within a particular society but the numbers are highly unstable given that they vary not only with changing birth rates (given that women prefer to marry men who are a few years older than themselves) but also with changing ages of marriage both within and between the sexes. For this reason, many researchers prefer to use a broad sex ratio covering most of men's and women's reproductive lives (or marriage careers in the case of women) because this is more stable over time and therefore preferable for comparative purposes. So what is the empirical relationship between the marriage market and violent crime? Analysis of INTERPOL crime data (murders, rapes, and assaults) found that violent crime is a predictable feature of countries with a relative scarcity of men (ages 15–64 years, Barber, 2000a) in regressions controlling for level of economic development. This is a counter-intuitive result given that most of the violent crimes in any country are committed by men. O'Brien (1999) also reported that during years when there were more males, American rape rates were lower but Avakame (1999) concluded that a greater supply of males in the U.S. augments female homicide victimization. Walsh (2003) reviews several studies conducted within the U.S. finding higher violent crime in locations with relatively fewer males. Barber (2009a) confirmed and extend Barber's (2000a) INTERPOL finding by using better-quality data sources (UN murders and WHO homicides) and considering additional control variables as possible alternative explanations. Lim, Bond, and Bond (2005) replicated the effect using the WHO data for a smaller sample. The effect of sex ratios on violent crimes is quite robust and fairly substantial as assessed through different kinds of comparative study. Given that men have much higher violent crime rates compared to women, it is counterintuitive that societies having a greater proportion of males should have lower crime rates. This apparent contradiction can be resolved in terms of alternative mating strategies of men in societies that vary in their sex ratios. When there is an excess of females in the population, women are more likely to be sexually active outside marriage. This phenomenon

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may be illustrated by prevalence rates of HIV/AIDS that are much higher in sub-Saharan Africa where adult sex ratios are comparatively low compared to the Middle East where sex ratios are high, and HIV/ AIDS infection rates are low, indicating lower levels of extramarital sexuality. Such regional differences in sexual behavior as a function of the sex ratio are fairly easily explained. Where there is a substantial excess of females in the population, many of them have difficulty in marrying and are therefore more likely to be sexually active outside marriage. The existence of a pool of women who are sexually active outside marriage means that men may emphasize mating effort rather than competing over spouses. In such societies, direct mating competition between men increases, which has the consequence of increasing male–male violence (the possible mechanisms being sketched out below). Conversely, when there is a scarcity of females, women are in greater demand as marriage partners and are less likely to be sexually active outside marriage for various reasons. One reason is that premarital sexuality damages a young woman's sexual reputation so that she is less desirable as a marriage partner on the logic that she provides less paternity confidence that men look for in a potential spouse (Barber, 2002). Another factor is that women in sexually restrictive societies are chaperoned, or protected by their male relatives from the possibility of contact with sexual partners. Because few women are sexually active outside marriage, this invalidates a masculine strategy of direct mating effort. Men are thus constrained to compete over marriage partners rather than sex partners (Guttentag & Secord, 1983). Such indirect (nonviolent) reproductive effort usually takes the form of competition over economic resources that women prefer in a potential husband (Barber, 2002). Empirical evidence shows that the distinction between both types of reproductive competition is psychologically, and behaviorally, salient. Research on American college students, for example, found that if women perceive their campus to lack men who would make committed husbands, they emphasize their own sexuality and engage in brief relationships with various men, for instance (Cashdan, 1993). Cross-national research also found that both men and women in countries with a relative scarcity of men are more interested in casual sex (as measured by the Sociosexual Orientation Inventory, Schmitt, 2005). Such direct mating effort is implicated in violent crimes. 4. Violent crime and direct mating effort A great deal of evidence bolsters the conclusion that violent crime is produced by direct mating competition (Barber, 2007, 2009a,b). Such evidence includes analysis of contextual factors in assault and homicides, characteristics of offenders and victims, cross-national research, and time series analyses. Characteristics of the criminal are revealing: criminal violence in the form of assaults and homicides is overwhelmingly a masculine activity and it is more likely to be perpetrated on male victims (Campbell, Muncer, & Bibel, 2001; Daly & Wilson, 1988). Men are most violent during the ages when they are actively dating, a phenomenon that could be partially mediated by testosterone levels (Barber, 2002). Time series analyses of violent crimes in the U.S., England, and Scotland, also found that crime increases during years when women found it more difficult to marry (and are thus likely to be sexually active outside marriage, Barber, 2003a). One of the more intriguing cross-national patterns in violent crime is the fact that murders, assaults, and rapes are significantly higher in the Americas than they are in the rest of the world (Barber, 2006; Neapolitan, 1994). This difference is statistically explainable in terms of higher proportions of single parenthood in these countries. (As an index of the number of women who are sexually active outside marriage, single parenthood is a convenient measure of direct mating effort.) Lim et al. (2005) also reported that the effects of the sex ratio (and economic development) on WHO homicides were mediated by

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individual-level preference of love over status in a prospective mate, indicating that violent crime increases where a higher premium is placed on love (or direct mating effort) than social status (suggesting indirect competition via status enhancement). Single parenthood is a convenient index of direct mating effort in a country but it has some problems including co-residence of ostensibly single parents. Although “single” mothers may well live with the “single” father of their offspring, cohabiting relationships are briefer than marriages (Smock, 2000). Given that paternal investment in offspring is thereby reduced, men can engage in more direct mating effort (Barber, 2002). The fact that more sexually active women are without a permanent mate due to rapid dissolution of unions also creates more opportunities for brief sexual relationships than would be true if they were formally married. For this reason direct mating effort is likely to be correlated with reduced paternal (and parental) investment. This raises the question of whether mating effort, as such, is the important factor in violent crime, or whether crime is a product of reduced parental investment. These factors can be teased apart in time-series analyses by looking for an immediate effect of single parenthood on crime (tapping direct mating effort) versus one that is delayed by a generation (tapping parental investment during childhood of the criminal). The immediate effect is far more important and when this is statistically removed, the lagged effect has no predictive power according to Barber's (2004a) analysis using data for 39 countries. Such ecological correlations are provocative but they do not fill in the causal mechanisms through which mating effort might increase violent crime. Before attempting to fill in the causal links between mating effort and violent behavior, it is helpful to think about these phenomena in a broad anthropological perspective. 5. The anthropology of mating aggression and violent crime In subsistence societies lacking frequent warfare, the primary motive for homicide seems to be sexual competition among men including the jealous aggression associated with lover's triangles where men kill unfaithful spouses as well as sexual competitors (Daly & Wilson, 1988). This generalization prevails around the world from the !Kung of the Kalahari Desert in South Africa, to the Inuit of North America, the Tiwi of North Australia, the Siriono of Bolivia, and the Arapesh of New Guinea (Symons, 1979). Moreover, sexual jealousy is the primary motive for violence against women in nation states around the world (Barber, 2002; Daly & Wilson, 1988). There has been little systematic cross-societal investigation of the association between mating competition and violent crime. Nevertheless, several plausible indices of mating competition are positively correlated with violent crimes in cross-national research. Violent crime correlates positively, if weakly, with divorce rates in developed countries, for instance (Neapolitan, 1999). In societies with high rates of divorce, there are more sexually active unmarried reproductive-age women, and hence a rationale for increased direct mating effort by men. A similar logic applies to societies with a scarcity of males having more violence given that such scarcity promotes premarital sexuality (Barber, 2000a,b,c, 2001, 2002, 2007) because it is more difficult for women to marry creating a pool of sexually active single women. Conversely, a scarcity of women promotes premarital (and extramarital) chastity. These ideas suggest that violent crime should be linked to the marriage market in predictable ways. If such patterns exist what is the best way of interpreting them, bearing in mind that such evidence is correlational? 6. How to make sense of data linking marriage markets and violent crime Considerable evidence thus shows that violent crime rates are higher in countries where males make up a lower proportion of the

population. These data are counterintuitive given that crime rates are higher in males than females in all nations and thus call for explanation and discussion. The results are quite general showing up in several different data sets for different populations, various time periods, and using different analytic approaches (cross-national, ecological comparisons within countries, individual-level, crosssectional, time-series, and so on. It is reasonable to suspect that such results might be due to the effects of various confounding variables. Yet, marriage-market effects on crime persist despite controlling most or all of relevant control variables. This task is most easily accomplished in cross-national comparisons because there are rather few reliable cross-national predictors of violent crime (Barber, 2000a; Neapolitan, 1997). The effect could not be attributed to economic development, income inequality, urbanization, level of policing, or the presence of a major international drug trafficking network (Barber, 2009a). (Warfare could also be ruled out because none of the countries in this study was actually at war and violent military deaths are not classified as murders or homicides in any case.) We may thus conclude that violent crime rates are indeed higher in countries where there is a relative scarcity of males, despite the fact that males are considerably more prone to criminal violence than females. Although other possible third variables might explain this effect, it is difficult to imagine what they might be, given that there is no other known factors with a sufficiently large effect size in crossnational research on violent crime to qualify as alternative explanations (Neapolitan, 1997). Admittedly, most researchers have paid insufficient attention to the role of single parenthood and the lack of current data is a real problem here. When Barber (2009a) controlled for teen parenthood as a proxy for the unavailable data on single parenthood, he found that this had little effect on the conclusion that countries with a greater proportion of adult men in the population have lower crime rates. See Schmitt (2005) for analysis of other possible third variables connecting sex ratios and sexual behavior, that might also be relevant to violent crime. Moreover, this counterintuitive effect of the sex ratio on violent crime is larger, more consistent, and better established perhaps than any other factor in comparative research (Walsh, 2003). Much data thus confirm Barber's (2000a) finding of crime decreasing as masculinity of the population declined. These data are correlational, of course, and subject to various interpretations apart from the causal conclusion that the level of violence is determined by direct mating competition in countries experiencing a scarcity of men. As to homicides, for instance, it is theoretically possible that the direction of causality is reversed and that a high murder rate simply serves to remove more males than females from the population. There are two good reasons for doubting this interpretation. First, other crimes, such as rapes and assaults, that do not subtract males from the population have similar correlations with the sex ratio (Barber, 2000a, 2004). Second, homicides are too rare to have much of an impact on the sex ratio. Based on cross-national data, homicide rates generally average fewer than 8 per 100,000. Over the adult life of the individual, this amounts to less than 0.5% and would not be expected to have a detectable impact on sex ratios expressed as whole percentages (Central Intelligence Agency, CIA, 2006). Lim et al. (2005) also suggest that violence in low-sex-ratio societies is due to the greater resource competition among women unable to find mates, which would be a rival explanation to the mating aggression approach developed in this paper. Yet, when one considers that women's rates of violent crime are an order of magnitude lower than those of men (Daly & Wilson, 1988), this is an almost absurdly inadequate alternative explanation. How, then, are we to explain why low-sex-ratio countries, and societies in general, are so violent according to analysis of various crime data (Barber, 2000a, 2009a,b)? A plausible interpretation is that when men are in greater demand in the marriage market than women, more females are sexually active outside marriage, either

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because they are unable to marry, or because they use sexual intimacy as a technique for competing over the romantic attentions of desirable men (Guttentag & Secord, 1983). This sets the scene for more extensive direct mating competition by men — a process that is inherently violent due to a great variety of behavioral, physiological, and societal mechanisms. Conversely, in societies having a scarcity of females, women are more sexually restrictive and are less likely to be sexually active outside marriage as this harms their otherwise excellent marriage prospects. Because so few women are sexually active outside marriage, direct mating competition is not a viable strategy for men given that their sexual options are determined by the sexual proclivities of women. Although the marriage market is currently the most plausible explanation for societal variation in violent crime based on the empirical research, it is certainly not, at this point, the preferred interpretation among criminologists and social scientists. 7. Alternative theoretical explanations for variation in violent crime The evidence implicating direct mating competition among men in violent crime is thus rich, if mostly indirect, but it is not the only current interpretation of the evidence, or even the one most widely used. Many social scientists and other scholars prefer to discuss the association between violence and sexuality in terms of declining moral values that would permit both extramarital sexuality and impulsive violence to occur in the same societies. This approach is inherently circular and cannot be taken seriously as a scientific explanation, however, and the same is true of other cultural determinist approaches that refer to a culture of violence (Barber, 2004, 2005, 2007, 2008). Although expectations about sexual behavior, and about violent behavior, do clearly vary from one society to another, such phenomena can be explained in terms of objective differences in the marriage market and its economic influences (Abrahamson, 1998; Barber, 2000b, c, 2001, 2002; Guttentag & Secord, 1983). The same argument applies to antisocial impulses more generally, including interpersonal violence (Barber, 2007). Many scholars who are not interested in evolutionary explanations of violent crime also prefer to discuss various psychological factors (lack of social trust, frustration, etc.) that mediate the connection between country differences in economics or sex ratios and violent crime (reviewed in Lim et al., 2005). Ideally, such approaches could be reconciled with the evolutionary one developed here but doing so is beyond the scope of this paper. Societal variation in violent crime is partly a manifestation of male reproductive competition — based on evolved psychological predispositions — playing out in modern social contexts. Greater understanding of the means through which social contexts either reduce, or increase, direct sexual competition is thus of central importance for understanding societal variation in criminal violence. Such work holds out the promise of providing a natural-science explanation for variation among modern societies that would replace moralistic, or cultural-determinist alternatives (Barber, 2007). It is also important to grasp the anthropological and sociological contexts of violence as these affect the form of marriage — even whether it persists — with implications for mating effort and violence. How does increasing mating effort on the part of men boost violent crime rates? 8. Filling in the causal mechanisms through which mating competition increases violence How can direct male mating competition increase violent crime? At least three levels of mechanisms may be involved: sociological, behavioral, and physiological. At a sociological level, patterns of association play a clear role. Thus, (mainly) young men assemble wherever sexually active women may be encountered. Venues such as

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night clubs and bars, are characterized by exceptionally high levels of male–male violence (Peterson, Krivo, & Harris, 2000). Apart from the obvious fact that crimes of violence are bound to be more probable at locations where violence-prone individuals meet, why are such venues particularly likely to evoke crimes of violence? This question gets at the underlying behavioral mechanisms. At the behavioral level, violence frequently breaks out at night clubs, bars, and other such dating venues over issues of “face,” or dyadic social status among possible mating competitors. Such incidents may be provoked by one person accidentally jostling another in a crowded room, or getting into a dispute over who is next to play on a pool table. When these disputes come to the attention of police, due to property damage or injury, they are classified as “trivial altercations,” because they appear to lack adequate motivation. Yet, there is a loss of status by men who back down in such confrontations. The view that such violence is really a form of mating aggression rests on the claim that combatants in these altercations are often known to each other, implying a plausible scenario of repeated encounters in which status-related conflict recurs. Loss of face impairs their reputation and status among peers and thereby indirectly reducing their appeal as dating partners (Barber, 2002; Daly & Wilson, 1988). Such hot-headedness is particularly characteristic of young adulthood and reflects physiological influences (the third category of causes), particularly levels of testosterone and other androgens (Archer, 2006). This is a complex relationship because testosterone levels are temporarily increased by competitive situations, and even by sexual relationships (Archer, 2006). It may be over simplistic to attribute criminal violence to any single hormone. However, in view of the comparative evidence for other species, this caveat is generally overstated. In other words, anyone who denies that androgens play an important role in violent behavior, including violent crime runs the risk of ignoring a pattern of evidence supportive of the contrary position. Thus, the time course of testosterone production is linked in interesting ways to both mating competition and crime. Young men experience peaks of criminal incidence and testosterone production at around the same ages (Archer, 2006; von der Pahlen, Sarkola, Seppa, & Eriksson, 2002). Criminal violence is associated with testosterone levels in other data. With marriage, men experience a simultaneous decline in criminal offending and testosterone levels (Booth & Dabbs, 1993; Mazur & Michalek, 1998) compared to single men of the same age. The relationship between testosterone and violence might be causal, although the true experiments necessary to draw this conclusion have not been conducted and could not be conducted on humans for ethical reasons. Divorce is something of a natural experiment in this respect, however, because when previously married men begin dating again, their testosterone rises against the normal tendency to decrease with age and they are more likely to commit crimes of violence (Mazur & Michalek, 1998). Both phenomena may reflect a tendency to begin staying out late at night in bars and other dating venues as well as increased alcohol consumption (Waite & Gallagher, 2000). In other words, it may be that testosterone boosts violent crime but there are many possible third variables in this relationship, several having to do with resumption of dating behavior and frequenting high-crime dating locations, such as bars. Increased alcohol consumption is yet another possible third variable connecting masculine divorce with increased risk of violent behavior. Alcohol consumption disinhibits aggressive behavior. The disinhibiting effects of alcohol and other recreational drugs is widely recognized as a prominent factor in the physiology of violent crime (Giancola, 2002; Norris, Davis, George, Martell, & Heiman, 2002). By reducing serotonin activity in the brain, alcohol contributes to impulsive aggression. (Alcohol actually reduces testosterone levels.) There is thus a rich network of factors associated with direct sexual competition that mediate the connection between scarcity of males in

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the population and violent crime. Investigation of these probable links opens up many avenues for future research. In establishing such linkages, it is important to emphasize that crimes of violence are quite rare in our society: most dating men do not engage in violent behavior, although this might have been more common in forager communities (Symons, 1979). However rare crimes of violence are, contemporary societies with more extramarital sexuality do have more violence on account of direct reproductive competition and the psychological adaptations underlying it. Such variation in sexual behavior is a key factor in accounting for societal differences in violent crime. 9. Discussion and conclusions The impact of direct mating competition on violent crime committed by males is substantial in terms of the variance explained. This key empirical role has been largely ignored by criminologists and other social scientists to date (but see Walsh, 2003). The reasons for this relative neglect are probably complex but three reasons seem particularly obvious. First, the study of crime is compartmentalized separately from the study of sexuality in an academic world that encourages, and rewards, specialization and discourages synthesis. Second, criminology is a branch of sociology, a discipline that has been particularly slow to adopt the sort of evolutionary analysis that is best suited for integrating research on sexuality with research on violent crime (Barber, 2008; Lopreato & Crippen, 1999). Third, sociologists have been more interested in accounting for violent crime (and crime more generally) in terms of economic factors, such as poverty, or income inequality, rather than in terms of sexual behavior. Such factors are related to crime rates in comparative analyses (Pridemore & Trent, 2010) but that may be because poverty and inequality undermine marriage as a reproductive strategy (Barber, 2006, 2008). Although the empirical support for a connection between marriage markets and violent crime is very good, skeptical readers might question the practical importance of such findings. To begin with, if academics who study violent crime are unwilling to be receptive to the key explanatory role of mating aggression, then its clear relevance, and predictive value may not matter greatly. Taking an optimistic perspective, criminologists of the future may become more receptive to evolutionary ideas. There is an even more sobering reason that connecting marriage markets with mating aggression and violent crime may prove to be of limited value. That is that marriage itself is on the decline so that the orderly relationship between marriage markets and crime may no longer be discernible. Marriage is declining in many developed countries by many different measures such as declining marriage rates and increasing divorce rates. A century ago, divorce rates in the U.S., and in Europe were very low (typically well below 5% of marriages) whereas divorce rates in these countries today are typically around 50% of marriage rates (Martinson, 2006; OECD, 2010; Shorter, 1975). The proportion of births outside wedlock has followed a similar trajectory. Another revealing statistic is the proportion of young women who cohabit. In some European countries this number had exceeded 50% of women aged 20–25 years by the end of the 20th Century (Chesnais, 1996), implying that sexual relations outside marriage is now the majority practice there. On the surface, these data suggest that marriage is on the verge of disappearing. Yet, marriage rates are much higher in some developed countries than others, e.g., the U.S. Another qualification is that formal marriage can be mostly replaced by de facto marriage without much apparent change in living arrangements, or in the quality of life of children, as illustrated by secular change in Swedish marriage (Popenoe, 1988). Although young Swedes are mostly uninterested in formal marriage, children are very likely to spend their early years under the same roof as their fathers and formal marriage may have

minimal practical consequences so far as reducing male–male mating competition (and violent crime) is concerned. Moreover, the association between single parenthood and child poverty found in Britain, the U.S., and other countries is not seen in Sweden where households with children are more affluent, on average, than households without children, reflecting state subsidies for raising children (Home sweet home, 1995). Consequently, there is little economic motive for formal marriage and marriage rates in Sweden are indeed low. What is more, Swedish children born to single mothers are as healthy as children of married parents and no more likely to be involved in serious crime, drug dependency, or unplanned teenage pregnancy. One of the key practical differences between formal marriage and cohabitation is that common-law unions are significantly shorter. The most common explanation for this difference is that cohabitation arrangements are entered into without as much of an emotional commitment, so that the union is more likely to break up (Smock, 2000). Another issue is that legal marriages have more inertia because they are more difficult and expensive to dissolve, often with adverse economic consequences for either, or both, parties, such as payment of alimony, or a reduced standard of living following the divorce due to maintaining two residences rather than one. Ancestral marriage contracts came into being among foragers because men who stayed with their spouse and helped her to raise children were more reproductively successful than men who attempted to father children with multiple mates without investing substantially in any of them. Similarly, women who opted for permanent marriages were more reproductively successful than those who mated with several men without securing investment in their offspring. This basic bargaining arrangement between the sexes was maintained following the Agricultural Revolution in which men continued to play a key role in food production, particularly given that they frequently owned land, provided needed labor, or maintained control over technologies of food production, such as by fashioning plows, raising draught animals, or organizing crop rotations (Sakala, 2000). It is, however disrupted by modern economic conditions. As women became more economically independent, thanks to increased participation in paid employment, and increasing wages relative to men, there was an increase in the proportion of births to unmarried women, as reflected in Barber's (2003c) cross-national analysis. Women's economic independence is also bolstered by welfare states having generous child support provisions and this is one reason that more births are to single women in social democracies as opposed to countries like the U.S., where social welfare programs relevant to women of reproductive age are targeted more narrowly at lowincome individuals. Although female economic independence might appear threatening to the continued existence of marriage (Popenoe, 1988), this is not necessarily true. One reason is that marriage provides appreciable household economies so that married couples may accumulate more capital than single people having the same combined income (Waite & Gallagher, 2000). Marriage can thus be considered a social mobility strategy as well as a reproductive strategy. That helps explain why single parenthood remains rare among middle class women in the U.S. whereas it is much more common among low-income women (Abrahamson, 1998) suggesting that middle class people are more interested in social mobility. If marriage promotes social mobility, this would also explain other puzzling phenomena, such as a tendency for women who begin their families as single mothers to marry subsequently, a pattern that is fairly common among African Americans, for instance (Burton, 1990). There is a certain irony in seeking to establish the connection between marriage markets and violent crime at a time when marriage is becoming much less frequent and could actually disappear. Even if marriage is about to disappear (and this is highly uncertain)

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