Evolutionary Demography

Kruger, D.J., Aiyer, S.M., Caldwell, C.H., & Zimmerman, M.A. (2013). Local scarcity of adult men predicts youth assault rates. Journal of Community Psychology, 42, 119-125.

Father involvement reduces risky youth behavior at the individual level. We examine the association between the scarcity of adult men and youth violence at the Census Tract level across a small Midwestern city experiencing decades of economic adversity and high rates of violence. We calculated the ratio of men to women aged 25-64 and indicators of concentrated disadvantage across residential Census Tracts with 2000 US Decennial Census data and the average monthly assault rates for those aged 10-24 between June 2006 and December 2008 with data from the local police department. Adult male scarcity and the proportion of individuals 25 or older who had less than a high school degree were the two unique predictors of youth assault rates, together explaining 69% of the variance. Interventions promoting effective social, material, and protective support from fathers and other adult male role models may ameliorate risk for youth violence.

Kruger, D.J., Clark, J., & Vanas, S. (2013). Male scarcity is associated with higher prevalence of premature gestation and low birth weight births across the USA. American Journal of Human Biology, 25, 225-227.

Objectives: Modern adverse birth outcomes may partially result from mechanisms evolved to evaluate environmental conditions and regulate maternal investment trade-offs. Male scarcity in a population is associated with a cluster of characteristics related to higher mating effort and lower paternal investment. We predicted that modern populations with male scarcity would have shorter gestational times and lower birth weights on average.

Methods: We compared US Centers for Disease Control and Prevention county-aggregated year 2000 birth records with US Decennial Census data. We combined these data in a path model with the degree of male scarcity and known socio-economic predictors of birth outcomes as exogenous predictors of prematurity and low birth weight, with single mother households as a proportion of families with children as a mediator (N = 450).

Results: Male scarcity was directly associated with higher rates of low birth weight. Male scarcity made significant indirect predictions of rates of prematurity and low birth weight, as mediated by the proportion of families headed by single mothers. Aggregate socio-economic status also indirectly predicted birth outcomes, as mediated by the proportion of families headed by single mothers, whereas the proportion African American retained both direct and indirect predictions of adverse birth outcomes.

Conclusions: Male scarcity influences life history tradeoffs, with consequences for important social and public health issues such as adverse birth outcomes.

Hassinger, B.E. & Kruger, D. J. (2013). The polygyny paradox: Several male biased populations exhibit a high prevalence of polygyny. Journal of the Evolutionary Studies Consortium, 5, 131-137.

Polygyny is the most common mating system in mammals and is widespread in human cultures. The population sex ratio shapes human social patterns and mating strategies. When women are scarce relative to men, one might imagine that a relative surplus of men would predict a greater presence and extent of polyandry, as one woman could find more than one husband. However, we predict that high sex ratios, indicating a relative surplus of men, will instead be more likely to be associated with a greater extent of polygyny, where some men have multiple wives. Although the opposite pattern is numerically intuitive, we base our prediction on the divergence in reproductive strategies between men and women. These sex differences shape how men and women leverage advantages associated with numerical scarcity for different reproductive goals. In support of our hypothesis, five countries with high proportions of men to women (ages 15-64) and a combined population of 33 million individuals have relatively high levels of polygyny, even when controlling for GDP per capita. We demonstrate the power of and evolutionary theoretical framework for understanding behavioral, social, and demographic patterns.

Kruger, D.J., & Vanas, S.B. (2012). Local scarcity of women predicts higher fertility among married couples and more single father households. Letters on Evolutionary Behavioral Science, 3, 17-20.

Influences of the sex ratio on the intensity of mating competition and selectivity for partners produce different outcomes in female biased and male biased populations because the reproductive strategies of men and women are somewhat divergent. Male scarcity enhances male mating opportunities and incentives for long-term commitment are diminished, encouraging serial and simultaneous polygyny. Paternal investment is lower in these populations, as indicated by higher divorce rates, more out-of-wedlock births, and a greater proportion of single mother households. Scarce females are more effective at securing commitment from partners and obtaining higher levels of resource investment. Women marry earlier in male biased populations. Although single father households are relatively uncommon, we expect to see higher proportions of households with children headed by single fathers where women are scarce. We also expect to see higher fertility among married couples, both because women may have greater bargaining power in reproductive decision-making and the role of woman in childbearing may be more salient and more highly valued. Data from the U.S. Census 2009 American Community Survey across 318 Metropolitan Statistical Areas supported these hypotheses.

Kruger, D. J., Munsell, M.A., & French-Turner, T. (2011). Using a life history framework to understand the relationship between neighborhood structural deterioration and adverse birth outcomes. Journal of Social, Evolutionary, and Cultural Psychology, 5, 260-274.

Life History Theory is a powerful framework for understanding how evolved functional adaptations to environmental conditions influence variation in significant life outcomes. Features indicating relatively high extrinsic mortality rates and unpredictability of future outcomes are associated with relatively faster life history strategies. Regulatory mechanisms that facilitated reproductive success in ancestral environments may contribute to adverse birth outcomes in modern technologically advanced populations. Adverse local environmental conditions may reduce maternal somatic investment in gestating offspring, consistent with long-term maternal interests. In this study, we demonstrated a relationship between neighborhood structural deterioration and adverse birth outcomes in Flint, Michigan, USA. We used Geographical Information Systems software to calculate the density of highly dilapidated structures, premature births, and low birth weight births in .25 mile square areas. Controlling for parental education and type of health coverage, the degree of structural deterioration was associated with the concentration of premature births and low birth weight births.

Garcia, J. R. & Kruger, D. J. (2010). Unbuckling in the Bible Belt: Conservative sexual norms lower age at marriage. Journal of Social, Evolutionary and Cultural Psychology, 4, 206-214.

Sociosexual norms, as well as marital patterns, vary widely across human history and culture. Yet, humans share an evolved psychobiology of sexual motivation that reliably leads to procreation through incentivizing sexual activity. We predicted that in socially monogamous societies with conservative norms for sexual behavior, the interaction of sexual motivation and strong discouragement of non-marital sex would lead to relatively earlier marriages on average, as young adults seek a socially sanctioned way of expressing their sexuality. We examined this hypothesis with United States Census data from the year 2000. We found that cities in the "Bible Belt," a region characterized bystrong evangelical Christian sentiment, had significantly lower mean and median marital ages for both men and women, supporting our prediction. These relationships held when controlling for median household income, median family income, and even the operational sex ratio. We discuss our findings in terms of individuals minimizing discourse discrepancies and cognitive dissonance, to ultimately coincide with regional cultural norms.

Kruger, D.J., Fitzgerald, C.J., & Peterson, T. (2010). Female scarcity reduces women's marital ages and increases variance in men's marital ages. Evolutionary Psychology, 8, 420-431.

When women are scarce in a population relative to men, they have greater bargaining power in romantic relationships and thus may be able to secure male commitment at earlier ages. Male motivation for long-term relationship commitment may also be higher, in conjunction with the motivation to secure a prospective partner before another male retains her. However, men may also need to acquire greater social status and resources to be considered marriageable. This could increase the variance in male marital age, as well as the average male marital age. We calculated the Operational Sex Ratio, and means, medians, and standard deviations in marital ages for women and men for the 50 largest Metropolitan Statistical Areas in the United States with 2000 U.S Census data. As predicted, where women are scarce they marry earlier on average. However, there was no significant relationship with mean male marital ages. The variance in male marital age increased with higher female scarcity, contrasting with a non-significant inverse trend for female marital age variation. These findings advance the understanding of the relationship between the OSR and marital patterns. We believe that these results are best accounted for by sex specific attributes of reproductive value and associated mate selection criteria, demonstrating the power of an evolutionary framework for understanding human relationships and demographic patterns.

Kruger, D.J., & Schlemmer, E. (2009). Male scarcity is differentially related to male marital likelihood across the life course. Evolutionary Psychology, 7, 280-287.

If marriage markets were only subject to the influences of numerical supply and demand, one would expect that the scarcer sex in a population would have a greater proportion married. Previous research has demonstrated that when males are scarce, they are actually less likely to be married, presumably because their market scarcity enhances their short term mating success and decreases incentives for commitment. However, males in modern societies appear to shift from mating effort to parental investment across the life course. Also, women preferentially value indicators of phenotypic quality for short term relationships, and these signals may be increasingly difficult to display with progressive physiological senescence. We predicted that men in low sex ratio populations would use market scarcity to their advantage for mating effort when young, but would shift towards commitment strategies when older. Data from the 50 largest Metropolitan Statistical Areas in the USA confirmed that a female biased sex ratio was associated with a lower proportion of men married between ages 20 and 29, but a higher proportion of men married between ages 35 and 74.

Kruger, D.J., & Schlemmer, E. (2009). When men are scarce, good men are even harder to find: Life history, the sex ratio, and the proportion of men married. Journal of Social, Evolutionary, and Cultural Psychology, 3, 93-104.

We used a life history framework to clarify the relationship between the operational sex ratio (OSR) and the proportion of men who are married across adulthood. Previous studies have found a direct relationship between the OSR and the likelihood of male marriage, although these analyses did not distinguish among age groups. We predicted that more women than men would be marred at younger ages, but women would be less likely to be married than men in later adulthood, reflecting age related trends in male and female reproductive values. We predicted that men would use scarcity in a low sex ratio population to their advantage differentially by age, withholding from marriage while young but having higher marital rates than women in older age. This would be consistent with the shift from mating effort to parental investment across the male life course in modern societies. Census data from the ten largest metropolitan areas in the United States supported these hypotheses.

See also: Mortality patterns