Sex differences in human mortality rates emerge from a complex interaction of genetic heritage and developmental environment. Although mortality is not in itself a behavior, it is an indirect product of behavior and physiology and thus responsive to life history variation in resource allocation, behavioral tendencies, and relevant environmental conditions. The explanatory framework of Tinbergen's Four Questions is sufficiently powerful in generalization to promote understanding of this phenomenon. Excess male mortality is a result of a trade-off between competitiveness and longevity. Male life history gives greater emphasis to reproductive effort at the expense of somatic effort, and mating effort at the expense of longevity compared to female life history. Men exhibit riskier behavioral patterns and greater physiological susceptibility, dying at higher rates from behavioral and most non-behavioral causes across the lifespan. The magnitude of the sex difference in mortality in developed nations peaks when males sexually mature and enter into mating competition. Social and environmental conditions intensifying male competition for resources, status, and mates lead to increased male mortality.
Kruger, D.J., & Polanski, S.P. (2011). Sex differences in mortality rates have increased in China following the single-child law. Letters on Evolutionary Behavioral Science, 2, (1) 1-4.
Male behavior and physiology is designed for enhanced competitiveness at the expense of longevity, resulting in higher mortality rates compared to females in most species. These differences vary across populations consistent with factors indicating the intensity of male mating competition. Reproductive dynamics are strongly influenced by the relative proportions of potentially reproductive males and females in a population. Because the reproductive strategies of men and women are somewhat divergent, market influences on the intensity of mating competition and selectivity for partners produce different outcomes in female biased and male biased populations. The single-child law implemented in China in 1979 has led to increasing proportions of men in the Chinese population. Using historical mortality data, we found a trend for increasing sex differences in Chinese mortality rates from 1982 to 2000. This increase was most prevalent in young adulthood, when male mating competition is most intense as males reach sexual maturity and seek female partners. In contrast, males exhibited more survival gains than females in infancy and early childhood.
Kruger, D.J. (2010). Socio-demographic factors intensifying male mating competition exacerbate male mortality rates. Evolutionary Psychology, 8, 194-204.
Sex differences in mortality rates stem from a complex set of genetic, physiological, psychological, and social causes whose influences and interconnections are best understood in an integrative evolutionary life history framework. Although there are multiple levels of mechanisms contributing to sex based disparities in mortality rates, the intensity of male mating competition in a population may have a crucial role in shaping the level of excess male mortality. The degree of variation and skew in male reproductive success may shape the intensity of male mating competition, leading to riskier behavioral and physiological strategies. This study examines three socio-demographic factors related to variation in human male reproductive success; polygyny, economic inequality, and the population ratio of reproductively viable men to women across nations with available data. The degrees of economic inequality and polygyny explained unique portions in the sex difference in mortality rates, these predictors accounted for 53% of the variance. The population ratio of reproductively viable men to women did not explain any additional variance. These results demonstrate the association between social conditions and health outcomes in modern nations, as well as the power of an evolutionary life history framework for understanding important social issues.
Kruger, D.J. (2008). Human life history variation and sex differences in mortality rates. Journal of Social, Evolutionary, and Cultural Psychology, 2, 281-288.
Sex differences in mortality rates stem from multiple causes that are best understood when integrated in an evolutionary life history framework. This study investigates the relationship of sex differences in mortality rates across nations to indicators of the life history characteristics of populations. Controlling for gross national income per capita, the overall sex difference in mortality rates was directly related to the adolescent fertility rate, the percentage of newborns with low birth weight, and was inversely related to the average mother's age at birth of first child. Sex differences for behavioral (external) causes of mortality were also directly related to the difference between the average age of males and females at first marriage. These findings indicate that the sex difference in mortality rates is an important life history indicator, and is related to reproductive patterns. Greater sex differences in mortality may reflect greater degrees of male competition for resources, social status, and mates.
Kruger, D.J., & Nesse, R. M. (2007). Economic Transition, Male Competition, and Sex Differences in Mortality Rates. Evolutionary Psychology, 5, 358-374.
Sex differences in mortality rates stem from a complex set of genetic, physiological, psychological, and social causes whose interconnections are best understood in an integrative evolutionary framework. We predicted that the transition from centrally planned to market economies in Eastern Europe inflated the discrepancy between male and female mortality rates, because economic uncertainty and increasing variation and skew in social status and resources should increase risky male behavior and the impact of stress on physiological susceptibility to internal causes of death. We computed the ratio of the male mortality rate to the female mortality rate separately for 14 Eastern European countries and for the combined population of 12 Western European countries in the pre-transition (1985-1989), transition (1990-1994), and post-transition (1995-1999) periods. We found that the Male to Female Mortality Ratio (M:F MR) for 14 Eastern European nations increased during the years of economic transition, most prominently during early adulthood. Larger sex differences in mortality rates occurred in both young adulthood, reflecting a shift towards riskier behavioral strategies, and middle adulthood, indicating greater physiological susceptibility to stress. For 12 of the 14 Eastern European nations, the increase was substantially larger than the slight increase in the overall Western European M:F MR. The impact of the transition on the magnitude of mortality discrepancy across countries varies considerably and likely reflects conditions particular to each country. These findings illustrate how traits shaped by natural selection interact with environmental conditions to influence male psychology and ultimately mortality patterns.
Kruger, D.J., & Nesse, R. M. (2006). An evolutionary life-history framework for understanding sex differences in human mortality rates. Human Nature, 17, 74-97.
Sex differences in mortality rates stem from genetic, physiological, behavioral, and social causes that are best understood when integrated in an evolutionary life history framework. This paper investigates the Male-to-Female Mortality Ratio (M:F MR) from external and internal causes and across contexts to illustrate how sex differences shaped by sexual selection interact with the environment to yield a pattern with some consistency, but also with expected variations due to socioeconomic and other factors.
Nesse, R. M. & Kruger, D.J. (2006, January 30). The Vulnerable Sex. Invited lead article for The House Magazine, the weekly magazine for the British Parliament, 4-5.
Kruger, D.J., & Nesse, R. M. (2006). Understanding sex differences in Croatian mortality with an evolutionary framework. Psychological Topics, 15, 351-364.
Invited, peer reviewed article for the special issue on Evolutionary Psychology.
Being male is the strongest demographic predictor of early mortality in Croatia. For every woman who dies between the ages of 15 and 34, three men die. Between the ages of 15 and 54, men are four times as likely as women to die from behavioral causes of death, such as accidents, homicides, and suicides. A causal explanation for sex differences in mortality must be based on an understanding of how sex differences were shaped by natural selection, and how those differences interact with environmental factors to create observed patterns and variations. In brief, males have been selected for riskier behavioral and physiological strategies than women, because of the greater variance and skew in male reproductive success. This paper examines the sex difference in Croatian mortality in three parts. First, we quantify the Croatian Male to Female Mortality Ratio (M:F MR) for 9 major causes of death across age group to provide a richer understanding of the sex difference in mortality from a life history framework. Second, we compare the Croatian M:F MR from behavioral, internal, and all causes with that of the available world population to demonstrate how Croatian mortality can be understood as part of a universal pattern that is influenced by unique environmental context. Third, we investigate how the War of Independence in 1991-1995 affected mortality patterns though its impact on behavioral strategies and the physical embodiment of distress.
Kruger, D.J., & Nesse, R. M. (2004). Sexual selection and the Male:Female Mortality Ratio. Evolutionary Psychology, 2, 66-77.
This paper extends the evolutionary understanding of sex differences in mortality rates by quantifying and graphically examining the overall Male to Female Mortality Ratio (M:F MR) for 11 specific leading causes of death across age groups in the USA, over the course of the lifespan in 20 different countries, and across the past 70 years in 5 countries. The resulting quantitative descriptions of rates, trends, and the relative contributions of various proximate causes of death to the M:F MR provide an initial exploration of the risks associated with being male. This analysis also illustrates how sex differences shaped by sexual selection interact in complex ways with multiple aspects of culture and environment to yield a pattern that has some consistency across decades and societies, but also has variations arising from differences among cohorts and cultures. The results confirmed our expectations of higher mortality rates for men than for women, especially in early adulthood, where three men died for every woman who died. For external causes the ratios were even higher. Historical mortality data reflect an epidemiological transition in which discrepancies between male and female mortality rates increase as general mortality rates fall. Cross-national variation in the modern M:F MR further suggests a universal pattern that is influenced by cultural and environmental context. Being male is now the single largest demographic risk factor for early mortality in developed countries.