Evolutionary demography

A substantial proportion of child deaths in sub-Saharan Africa are caused by diseases which could be prevented through greater levels of health-seeking behaviour. Socioeconomic and structural barriers in parental use and engagement with modern health technologies, such as bed net usage, vaccination and seeking medical treatment are well-established by population health scientists. However, understanding of how broader socio-demographic and child-level characteristics influence health-seeking behaviour remains limited. In this study, using data from the Demographic and Health Surveys, we explore the determinants of such behaviours from the perspective of human behavioural ecology (HBE), a framework notable for emphasizing the inherent resource allocation trade-offs parents face when investing in children, and for considering human behaviour as adapted to maximize lineage survival rather than individual health. Preliminary analyses confirm that wealth and education increase odds of health-seeking behaviour, but also demonstrate substantial differences in health-seeking behaviour by maternal age, marital status and child sex and birth order. Analyses to presented at IUSSP will also consider how these main effects interact as predicted by HBE theory.

We examine the mortality transition by placing country-specific trajectories on a landscape defined by demographic, social, and economic factors. We identify dimensions of a demographic ‘space’ that different countries have occupied through time. A population’s space is defined by life expectancy, GRR, latitude, and Gross Domestic Product per capita. We also quantify changes in the selection landscape by examining associations between mortality and Fisher’s reproductive value, Crow’s opportunity for selection, and Ryder’s demographic metabolism (the sum of the intrinsic birth and death rates). The large amounts of data (9 countries spanning 315 to 564 years) and the features of our demographic landscapes identify patterns among economic, selective, and historical factors that cannot typically be revealed in smaller scale analyses. For example, mortality reduction leads to changes in the relative variation of demographic traits, altering the potential for selection. We find that the potential for selection acting on longevity decreases dramatically as probability of survival to adulthood increases and that fertility selection is greatest when fertility is lowest. Likewise, countries often follow very different trajectories across the landscapes as they go through the mortality transition.

The diffusion of contraceptive use, as a proxy for the spread of low fertility norms, has long been of interest to researchers wishing to understand the fundamental drivers of fertility decline. However, few studies have incorporated community level influences with work on social networks and individual factors. I examine the multilevel determinants of contraceptive use in 22 villages undergoing late-stage demographic transition in rural southern Poland. I test individual and community level hypotheses about the diffusion of contraceptive use to examine the extent to which it is individually strategic and/or culturally transmitted between kin and social networks. I find evidence consistent with Cultural Evolutionary hypotheses that individuals imitate the behaviour of easily observable social models, and that contraceptive uptake may exhibit positive frequency-dependence (aka conformity). The results support hypotheses about social influence in demography. The findings are also consistent with theoretical modelling showing that even when conformity to group norms is relatively high, average education in a group can increase the rates of transmission of low fertility norms, and thereby the rate of contraceptive uptake. However some methods of contraception are more sensitive to traditional norms than others.

International inequalities in child health are well documented by population scientists, with children growing up in rural settings in sub-Saharan Africa at the greatest risk of poor health outcomes. In contrast, the factors that determine inequalities in child health within developing rural communities that share similar levels of service provision remain poorly understood. Building on theory and previous research by anthropological and evolutionary demographers, we examine family structure (i.e. family size, polygynous marriage) and alternative rural livelihoods (i.e. farming, pastoralism, wage-labour) as determinants of child health across an ethnically diverse sample of 56 villages in northern Tanzania. Out of 3400 children, WHO standards classify 41% as stunted, 16% as underweight and 4% as wasted. Preliminary analyses confirm marked differences in child health by family structure and livelihood. Presented analyses will use multilevel modelling to determine the relative importance of these factors amid a broader set of socioeconomic and demographic determinants, and furthermore consider hypothesised interactions between family structure and household characteristics. Findings will be discussed with regard to the cultural and socioeconomic factors that determine observed patterns, along with potential policy implications.

Some biologists have observed a close connection between game theory and evolution theory. Indeed, game theory allow biologists and economists to predict the outcome of some evolutionary processes. According to the Red Queen Hypothesis (Van Valen 1973), every species is a priori equally likely to get extinguished, regardless of the time during which it has existed. Each species is part of a zero-sum game against other species. This hypothesis refers to a kind of evolutionary arms race, where players constantly evolve together. We will consider its relevance for demographic development. Just as rat races among financial engineers can lead to a financial crisis (Aoki 2010) and competition without ceiling among lawyers can produce the bankruptcy of a law firm (Landers et al 1996), biotechnological competition can produce environmental crises. We will address these issues with the models of the theory of noncooperative games (arms races, rat races, etc).

Based on evolutionary theory, Trivers and Willard (TW) predicted that high-status parents display male-biased and low-status parents female-biased parental investment and sex ratios. In previous research we demonstrated that the TW effect does not show in parental gender preferences as expressed in differential fertility stopping (Kolk & Schnettler 2012) and likely also not in parental investment (Schnettler 2010). Research on the TW effect in sex ratios at birth remains mixed, however. Part of the reason for mixed results is that generally, the expected effect is small in human populations. Here we look at how sensitive the TW effect is to different status indicators and what role female employment status plays in mediating the TW effect. We use logistic regression on micro level data to examine the effect of socioeconomic status on the sex ratio at birth. We use individual data on births and individuals from Swedish administrative register data (N≈ 2 000 000 births). A wide arrange of socioeconomic traits are analyzed including income, wealth, occupational class and educational level. Early analysis reveals an ambiguous pattern that is contrary to the Trivers-Williard effect.

Son preference is a salient feature of many Asian populations, both contemporary and historical. Though sons are thought to experience a biological disadvantage in survival throughout the life-course, evidence has shown that daughters often fare worse, implicating preferential treatment of sons. In this paper, we examine evidence for gender-based differences in mortality among Taiwanese who were alive during the Japanese occupation. An interesting feature of this population is an unusually high rate of adoption, which has also been linked to higher levels of mortality among Taiwanese. Using event history analysis of longitudinal data collected from household registers, we explore these variables in an attempt to identify the factors associated with the hazard of death among colonial-era Taiwanese.

It is a common assumption that men desire more children than women do in high fertility countries. Evolutionary demographers have tried to explain females’ relatively lower desired fertility by invoking the larger costs faced by women, a woman’s desire to invest more in each child, and a woman’s certainty that her children are genetically related to her. There are known trade-offs between the quantity and quality of children, but no clear reason why women and men should experience these differently. Furthermore, in the simplest scenario, in the case of strict (i.e. life-long) monogamy, since a man’s lifetime reproductive success is tied to that of his reproductive partner, both individuals should have the same optimal family size. In this paper we develop a theoretical framework for examining the circumstances under which we expect conflicts of interest between partnered men and women in their fitness-maximizing optimal number of offspring. Using evolutionary simulations and dynamic programming to model behaviour, we examine the effects of paternity uncertainty, reproductive senescence, and the ability to switch or acquire more partners on the extent to which women and men will have different optima regarding the numbers of offspring and the pace of reproduction.

We present a theoretical model of behavior and nutrient allocation for organisms with discrete juvenile (larval) and adult stages, each with distinct diets, nutritional needs and foraging strategies. We predict optimal schedules of foraging and reproduction across a limited lifespan and examine the consequences for nutrient dynamics, paying particular attention to conditions under which it is optimal to tolerate starvation or to conserve resources and how these decisions affect mortality across the life cycle. We derive these optimal behavioral schedules assuming two alternative scenarios of environmental conditions, in which we do or do not impose deterioration in survival with age (senescence). We find that even in the absence of imposed senescence, terminal investment near the end of reproductive lifespans reduced survival probabilities of older individuals. We predicted earlier terminal investment and earlier senescence when reproductive overheads are low and when the survival costs of carbon depletion are small. Our findings suggest that resource allocation decisions alone are sufficient to produce signals of senescence and other age-patterns of mortality in animals optimizing decision schedules across their reproductive life cycles. We conclude that behavioral patterns of individuals can be strong drivers of observed d

We explain how upward transfers from adult children to their elderly parents might evolve as an interrelated feature of a deepening intergenerational division of labor. Humans have a particularly long period of juvenile dependence requiring both food and care time provided mainly by younger and older adults. We suggest that the division of labor evolves to exploit comparative advantage between young and old adults in fertility, childcare and foraging. Eventually the evolving division of labor hits a corner when the grandmother's fertility reaches zero (menopause). Continuing, it may hit another corner when the grandmother's foraging time has been reduced to her subsistence needs. Further specialization can occur only with food transfers to the grandmother, enabling her to reduce her foraging time to concentrate on additional childcare. We prove that this outcome can arise only after menopause has evolved. We describe the conditions necessary for both group selection (comparative steady state reproductive fitness) and individual selection (successful invasion by a mutation), and interpret these conditions in terms of comparative advantages.