More-men-more-violence association holds particularly for male violence against other men, but is insignificant for violence against women; significant among childless men, but not fathers; robustness checks question causality of associations

Are skewed sex ratios associated with violent crime? A longitudinal analysis using Swedish register data. Andreas Filser et al. Evolution and Human Behavior, Volume 42, Issue 3, May 2021, Pages 212-222. https://doi.org/10.1016/j.evolhumbehav.2020.10.001

David Schmitt's take: more-men-more-violence association holds particularly for male violence against other men, but is insignificant for violence against women...significant among childless men, but not fathers...robustness checks question causality of associations

Abstract: There is widespread concern in both the popular and academic literature that a surplus of men in a population intensifies mating competition between men, particularly unpartnered men, resulting in increased violence towards both men and women. Recent contributions challenge this perspective and argue that male mating competition and levels of violence will be higher when sex ratios are female-skewed. Existing empirical evidence remains inconclusive. We argue that this empirical ambiguity results from analyses of aggregate-level data, which put inferences at risk of ecological fallacies. Our analysis circumvents such problems by using individual-level, longitudinal demographic register and police data for the Stockholm metropolitan area, Sweden (1990–2003, n = 758,498). These data allow us to investigate the association between municipality-level sex ratios and violent offending (homicide, assault, threat, and sexual crimes) while adjusting for sociodemographic factors. Results suggest that aggregated offending rates are negatively associated with male-skewed sex ratios, whereas individual-level violent offending correlates positively with male-skews. We find that the more-men-more-violence association holds particularly for male violence against other men, but is insignificant for violence against women. Moreover, the association is significant among childless men, but not among fathers. However, robustness checks question the causality of these associations. Female violent offending is positively, albeit due to a low number of cases, insignificantly associated with male-skews. Moreover, both male and female non-violent offending is higher in male-skewed municipalities. We discuss the implications with regard to the theoretical debate and problems of unobserved heterogeneity in the sex ratio literature.

Keywords: Sex ratioViolent crimeMating marketSweden

4. Discussion

In this study, we use Swedish register data to investigate the link between local sex ratios and violent criminal offending. Existing theoretical approaches are contradictory and empirical evidence remains inconclusive, suggesting both negative and positive associations between male-skewed sex ratios and violence (Schacht et al., 2014, Schacht et al., 2016; Schnettler and Filser, 2015, Schnettler and Filser, 2020). To our knowledge, this study is the first to use individual-level, longitudinal data to circumvent an ecological fallacy, a common problem of previous research on the issue (Filser & Schnettler, 2018; Pollet et al., 2017; Schacht et al., 2014). The detail of our data enable us to disentangle associations of intra- and inter-sexual violence with the sex ratio and to adjust for a number of socio-demographic confounders. Moreover, we investigate hypothesized, but largely untested differences in these associations by socio-economic, marital, and parental status.

On the surface, our results seem to provide additional support for the more-men-more-violence hypothesis. We find that male-on-male violence is positively associated with male-skewed sex ratios. Results for male-on-female violence suggest a similar positive association with male-skews, which is somewhat weaker and not statistically significant, due to a smaller sample size for these offenses. Both findings are compatible with the hypothesis that an abundance of men will particularly result in high levels male-male violence. Furthermore, our results suggest that intra-male violence correlates significantly with local sex ratios among childless men, but not among fathers. This may appear as further support for the hypothesis that local male-skews particularly instigate violent rivalry among men competing for partners.

However, the full set of results, including the results from models examining female-on-female violent offenses, and non-violent offenses, suggests caution before drawing any firm conclusions. First, when comparing our findings on male and female offending, we would expect either no association between sex ratios and female offending or one that is opposite to the association with male offending (cf. Stone, 2015). Yet, we find that the association between the sex ratio and female violent offending resembles that for male offending. The association for female offending is not significant, but this may be largely due to the low number of offenses by women in our data. The number of offenses is even lower for analyses of female offending split by victims' sex, which precludes any meaningful interference from results on female-on-male and female-on-female violent offending.

Second, we find that municipality-level sex ratios are not only associated with violent offending, but also with a general indicator of any non-violent offending. Property and white-collar offenses might correlate with sex ratios, as individuals are more pressurized to obtain resources and resort to scramble competition (Benenson & Abadzi, 2020; Edlund et al., 2013). However, given the broadness of the indicator, we would expect these associations to be weaker, compared to violent offenses. While this is only true for non-violent offending by men, we find that the association of sex ratios with female non-violent offending is even stronger than the one for violent offending. Furthermore, we find that the associations of sex ratios with non-violent offending are in the same direction for both male and female offenders. This contradicts theoretical expectations related to scramble competition as women should become less and not more likely to engage in non-violent offending as the sex ratio increases, that is, as the environment becomes less female-skewed.

In sum, these findings prompt us to suspect that there are still potential confounders that might drive the association between sex ratios and violent crime that we are not able to account for. We are able to adjust our models for contextual and individual-level socio-economic deprivation in a more comprehensive way than previous studies. Socio-economic deprivation is a key confounder of the association between sex ratios and violent crime, given that young women are more likely to migrate to more economically thriving regions (Leibert, 2016) and levels of violent crime are correlated with prosperity (Hooghe et al., 2011). We address this issue by including municipality-level fixed effects to account for time-constant unobserved heterogeneity on the municipality level. Moreover, we include a range of time-varying socioeconomic and demographic status variables on both the individual and context level. These adjustments should take care of socio-economic unobserved heterogeneity, yet some limitations remain.

Beyond socioeconomic factors, sex-selective migration patterns might be a source of unobserved heterogeneity. Empirical evidence suggests that women out-migrate from male-biased areas to more strongly female-biased areas than men (Uggla & Mace, 2017). With regard to violence and crime, one potential explanation could be that men are less concerned about falling victim to a crime (Jackson, 2009). Consequently, sex-selective migration might drive the association of sex ratios and violent crime independently of economic deprivation. Unfortunately, we are not able adjust our models for migration patterns in our analysis, particularly migration from outside our study area.

Another limitation of our study is the comparatively small geographical scope of our data. An underlying assumption of our analysis is that individuals are sensitive to cues of the municipality-level sex ratio and that municipalities meaningfully represents the local ecology which impacts individual behavior. However, individuals might have committed offenses in different contexts than their municipality of residence, resulting in a mismatch of the contextual sex ratio at the offense and our sex ratio measure. Moreover, municipalities might be too small entities to measure sex ratios in a way that also correlates closely with individuals' perceptions of partner markets (Filser & Preetz, 2020; Fossett & Kiecolt, 1991; Gilbert, Uggla, & Mace, 2016). Furthermore, our study area consists of a metropolitan area with an urban center, Stockholm city. Municipalities are a meaningful social entity in Sweden, because they organize schools and municipality centers serve as local hubs. However, individuals still commute and move between municipalities. With a size of 7150 km2, the area is well connected by public transport and roads. Consequently, municipalities are not as separate as calculating specific municipality-level sex ratios suggests them to be.

Nevertheless, the level of detail in our data allow us to elucidate a number of aspects previous studies have not been able to explore. A key contribution of our paper is to support concerns about studying the association of sex ratios and aggregated rates of individual social outcomes, as it is commonly done in the existing sex ratio literature (cf. Pollet et al., 2017 for an in-depth critique). Specifically, our findings demonstrate how, based on the same data, sex ratios and aggregated violent offending rates can suggest a negative association, even when individual probabilities for violent offending are actually positively associated with sex ratios. This illustrates the importance of individual-level analyses to further establish a coherent empirical basis in the sex ratio literature.

Moreover, our paper illustrates that detailed offending data are necessary to generate clearer evidence with regard to which types of violent offenses are associated with sex ratio skews. We wish to remain cautious with too much emphasis on the differential levels of significance due to diverging sample sizes in offenses. Yet, the weaker association for male-on-female offending compared to male-on-male offending puts predictions of increased male-on-female intimate partner violence (D'Alessio & Stolzenberg, 2010; Daly & Wilson, 1998; Uggla & Mace, 2015b; Vandello, 2007) or higher levels of sexual harassment in male-skewed environments into perspective (Malamuth et al., 2005; Trent & South, 2012; Trent, South, & Bose, 2015). However, our data do not differentiate between violence against (intimate) partners and other female victims and thus our results on male-on-female violence can only serve as a combined indicator of violence against women by both partners and other men. Additional research is necessary to disentangle these different types of male-on-female violence and their associations with the sex ratio.

Moreover, our results reveal that the association of skewed sex ratios with violence may differ across individual demographic characteristics, as has been shown for other outcomes (Uggla & Mace, 2017). Specifically, sex ratios are positively associated with violent offending in childless men, but not among fathers. While we outlined above that this finding should be treated with caution, it still serves as an illustration for the yet untapped potential of individual-level data for the literature on sex ratios and violent offending. Future studies should further explore this aspect.

In sum, our findings demonstrate the need for studies relying on more detailed data and advanced causal identification strategies when exploring the association of sex ratios with violence and aggression. Observational studies, no matter how detailed, might be unable to overcome unobserved heterogeneity problems at both the individual and the aggregate level. Experimental studies have generated evidence for sex ratio effects on aggression by human participants (Arnocky, Ribout, Mirza, & Knack, 2014). We encourage future research to evaluate whether these effects vary across parental status groups as our results indicate.

Finally, our findings should be considered within their specific socio-sexual context (see Schacht et al., 2014). Our study population fits the WEIRD definition (Henrich, Heine, & Norenzayan, 2010), with high levels of acceptance for uncommitted sexual relationships (cf. Widmer, Treas, & Newcomb, 1998). Therefore, our paper complements the literature in that it comes from a sexually liberal society, while previous individual-level analyses use data from more sexually restrictive contexts (Diamond-Smith & Rudolph, 2018; South et al., 2014). Such societies might not be suitable test cases for the more-men-more-violence hypothesis, since this perspective emphasizes uncommitted sexual relationships as a main mediator for the link between sex ratios and male violence (Schacht et al., 2014, Schacht et al., 2016). This limitation does not apply to our study. While we cannot provide evidence of a counterfactual causal effect of male-skewed sex ratios on violent crime, our findings at least cast doubt on the more-men-less-violence hypothesis vis-à-vis the more-men-more-violence hypothesis in this context.