Life Expectancy and the Timing of Life History Events in Developing Countries

Mortality risk predicts global, local, and individual patterns of human reproduction

BACKGROUND

Human reproductive dynamics in the post-industrial world are typically explained by economic, technological, and social factors including the prevalence of contraception and increasing numbers of women in higher education and the workforce. These factors have been targeted by multiple world governments as part of family policies, yet those policies have had limited success. The current work adopts a life history perspective from evolutionary biology: like most species, human populations may respond to safer environments marked by lower morbidity and mortality by slowing their reproduction and reducing their number of offspring. We test this association on three levels of analysis using global, local, and individual data from publicly available databases.

RESULTS

Data from over 200 world nations, 3,000 U.S. counties and 2,800 individuals confirm an association between human reproductive outcomes and local mortality risk. Lower local mortality risk predicts "slower" reproduction in humans (lower adolescent fertility, lower total fertility rates, later age of childbearing) on all levels of analyses, even while controlling for socioeconomic variables (female employment, education, contraception).

CONCLUSIONS

The association between extrinsic mortality risk and reproductive outcomes, suggested by life history theory and previously supported by both animal and human data, is now supported by novel evidence in humans. Social and health policies governing human reproduction, whether they seek to boost or constrain fertility, may benefit from incorporating a focus on mortality risk.

Attitudes and Laws About Abortion Are Linked to Extrinsic Mortality Risk: A Life-History Perspective on Variability in Reproductive Rights

Abortion policy is conventionally viewed as a political matter with religious overtones. This article offers a different view. From the perspective of evolutionary biology, abortion at a young age can represent prioritization of long-term development over immediate reproduction, a pattern established in other animal species as resulting from stable ecologies with low mortality risk. We examine whether laws and moral beliefs about abortions are linked to local mortality rates. Data from 50 U.S. states, 202 world societies, 2,596 adult individuals in 363 U.S. counties, and 147,260 respondents across the globe suggest that lower levels of mortality risk are associated with more permissive laws and attitudes toward abortion. Those associations were observed when we controlled for religiosity, political ideology, wealth, education, and industrialization. Integrating evolutionary and cultural perspectives offers an explanation as to why moral beliefs and legal norms about reproduction may be sensitive to levels of ecological adversity.

Using sequence analysis to test if human life histories are coherent strategies

Life history theory, a prominent ecological model in biology, is widely used in the human sciences to make predictions about human behaviour. However, its principal assumptions have not been empirically tested. We address this gap with three research questions: (1) do humans exhibit coherent life history strategies; (2) do individuals adopt strategies along a slow-fast continuum; and (3) are socioeconomic circumstances during childhood associated with the pace of the life history strategy that an individual adopts? Data from the Wisconsin Longitudinal Study is used to reconstruct the life histories of US women including information on puberty, fertility, menopause and death. We introduce a novel methodological approach to evolutionary anthropology, sequence analysis, to assess if human life histories are coherent strategies and how these strategies are patterned. In subsequent analyses we used multinomial logistic regressions to test whether childhood socioeconomic status predicts the life history patterns women follow. Results provide little evidence that humans follow coherent life-history strategies; Wisconsin women are clustered by the number of children they have but not by ages at life events. Socioeconomic status does not predict which cluster women fall into, suggesting that less well-off women do not have higher fertility, as predicted.

Effects of Individual Mortality Experience on Out-of-Wedlock Fertility in Eighteenth- and Nineteenth-Century Krummhörn, Germany

Life history theory predicts that exposure to high mortality in early childhood leads to faster and riskier reproductive strategies. Individuals who grew up in a high mortality regime will not overly wait until they find a suitable partner and form a stable union because premature death would prevent them from reproducing. Cox proportional hazard models were used to determine whether women who experienced sibling death during early childhood (0–5 years) reproduced earlier and were at an increased risk of giving birth to an illegitimate child, with illegitimacy serving as a proxy for risky sexual behavior. Furthermore, we investigate whether giving birth out of wedlock is influenced by individual mortality experience or by more promiscuous sexual behavior that is clustered in certain families. Models are fitted on pedigree data from the eighteenth- and nineteenth-century Krummhörn population in Germany. The results show a relationship between sibling death in early childhood and the risk of reproducing out of wedlock, and reproductive timing. The risk of giving birth out of wedlock is linked to individual mortality experience rather than to family-level effects. In contrast, adjustments in connubial reproductive timing are influenced more by family-level effects than by individual mortality experience.

Individual-specific mortality is associated with how individuals evaluate future discounting decisions

How organisms discount the value of future rewards is associated with many important outcomes, and may be a central component of theories of life-history. According to life-history theories, prioritizing immediacy is indicative of an accelerated strategy (i.e. reaching reproductive maturity quickly and producing many offspring at the cost of long-term investment). Previous work extrapolating life-history theories to facultative calibration of life-history traits within individuals has theorized that cues to mortality can trigger an accelerated strategy; however, compelling evidence for this hypothesis in modern humans is lacking. We assessed whether country-level life expectancy predicts individual future discounting behaviour across multiple intertemporal choice items in a sample of 13 429 participants from 54 countries. Individuals in countries with lower life expectancy were more likely to prefer an immediate reward to one that is delayed. Individuals from countries with greater life expectancy were especially more willing to wait for a future reward when the relative gain in choosing the future reward was large and/or the delay period was short. These results suggest that cues to mortality can influence the way individuals evaluate intertemporal decisions, which in turn can inform life-history trade-offs. We also found that older (but not very old) participants were more willing to wait for a future reward when there is a greater relative gain and/or shorter delay period, consistent with theoretical models that suggest individuals are more future-orientated at middle age.

Is the timing of menarche correlated with mortality and fertility rates?

Timing of menarche has largely been studied in the context of a secular trend. However, since mortality and fertility rates are fundamental demographic factors linked to a population’s developmental and reproductive characteristics, we expect that the timing of menarche, a precondition to reproduction, is also associated with these vital rates. We conduct an analysis of 89 countries and 21 demographic, socioeconomic, nutritional, and educational variables selected for their known influence on menarche. Model results predict that a country’s fertility and adult female mortality rates are significant predictors of mean age at menarche, while other covariates are not. Specifically, menarche is delayed in countries with high mortality and high fertility, which may be proxies for assessing overall environmental quality. We emphasize that, for a comprehensive understanding of the timing of menarche, it is critical to take into account both individual- and population-level influences.

Impact of HIV/aids epidemic on human capital development in West Africa

West Africa occupies the third position with respect to the burden of human immunodeficiency virus/acquired immune deficiency syndrome (HIV/AIDS) globally, after Southern and East Africa. About 5 million adults and children are infected with the disease in the subregion, while HIV prevalence in the general population hovers around 2% and 5%. This paper attempts to investigate the impact of HIV/AIDS epidemic on human capital development in 11 West African countries over the period 1990 to 2011. The study used a dynamic panel data modeling approach, using first difference, difference generalized methods of moment, and system generalized methods of moment estimating techniques. Four measures of HIV/AIDS and 2 human capital measures were used in the study. The findings revealed that HIV/AIDS pandemic had negative and significant impact on human capital in West Africa. However, the statistical significance was more pronounced on life expectancy (a measure of human capital), while the negative impact on school enrolment (another human capital measure) was not significant. It is therefore recommended that the spread of HIV/AIDS disease in West Africa should be effectively controlled, while the number of infected persons undergoing antiretroviral therapy in the subregion should be increased to a near 100% coverage.

What can cross-cultural correlations teach us about human nature?

Many recent evolutionary psychology and human behavioral ecology studies have tested hypotheses by examining correlations between variables measured at a group level (e.g., state, country, continent). In such analyses, variables collected for each aggregation are often taken to be representative of the individuals present within them, and relationships between such variables are presumed to reflect individual-level processes. There are multiple reasons to exercise caution when doing so, including: (1) the ecological fallacy, whereby relationships observed at the aggregate level do not accurately represent individual-level processes; (2) non-independence of data points, which violates assumptions of the inferential techniques used in null hypothesis testing; and (3) cross-cultural non-equivalence of measurement (differences in construct validity between groups). We provide examples of how each of these gives rise to problems in the context of testing evolutionary hypotheses about human behavior, and we offer some suggestions for future research.

Increased mortality exposure within the family rather than individual mortality experiences triggers faster life-history strategies in historic human populations

Life History Theory predicts that extrinsic mortality risk is one of the most important factors shaping (human) life histories. Evidence from contemporary populations suggests that individuals confronted with high mortality environments show characteristic traits of fast life-history strategies: they marry and reproduce earlier, have shorter birth intervals and invest less in their offspring. However, little is known of the impact of mortality experiences on the speed of life histories in historical human populations with generally higher mortality risk, and on male life histories in particular. Furthermore, it remains unknown whether individual-level mortality experiences within the family have a greater effect on life-history decisions or family membership explains life-history variation. In a comparative approach using event history analyses, we study the impact of family versus individual-level effects of mortality exposure on two central life-history parameters, ages at first marriage and first birth, in three historical human populations (Germany, Finland, Canada). Mortality experience is measured as the confrontation with sibling deaths within the natal family up to an individual's age of 15. Results show that the speed of life histories is not adjusted according to individual-level mortality experiences but is due to family-level effects. The general finding of lower ages at marriage/reproduction after exposure to higher mortality in the family holds for both females and males. This study provides evidence for the importance of the family environment for reproductive timing while individual-level mortality experiences seem to play only a minor role in reproductive life history decisions in humans.

Relative importance of demographic, socioeconomic and health factors on life expectancy in low- and lower-middle-income countries

Background

We attempted to identify the pathways by which demographic changes, socioeconomic inequalities, and availability of health factors influence life expectancy in low- and lower-middle-income countries.

Methods

Data for 91 countries were obtained from United Nations agencies. The response variable was life expectancy, and the determinant factors were demographic events (total fertility rate and adolescent fertility rate), socioeconomic status (mean years of schooling and gross national income per capita), and health factors (physician density and human immunodeficiency virus [HIV] prevalence rate). Path analysis was used to determine the direct, indirect, and total effects of these factors on life expectancy.

Results

All determinant factors were significantly correlated with life expectancy. Mean years of schooling, total fertility rate, and HIV prevalence rate had significant direct and indirect effects on life expectancy. The total effect of higher physician density was to increase life expectancy.

Conclusions

We identified several direct and indirect pathways that predict life expectancy. The findings suggest that policies should concentrate on improving reproductive decisions, increasing education, and reducing HIV transmission. In addition, special attention should be paid to the emerging need to increase life expectancy by increasing physician density.

Hierarchical regression and structural equation modeling: two useful analyses for life course research

This article reviews the life course perspective and considers various life course hypotheses such as trajectories, transitions, critical periods, sequencing, duration, and cumulative effects. Hierarchical regression and structural equation modeling are suggested as analyses to use in life course research. Secondary analysis was performed on the Early Head Start Research and Evaluation Study, 1996-2010, to illustrate their strengths and challenges. Models investigated the influence of mother and infant characteristics and of parent-child dysfunction at 14 and 24 months to children's cognitive outcomes at 36 months. Findings were interpreted and discussed in the context of life course hypotheses.

Resource availability, mortality, and fertility: a path analytic approach to global life-history variation

Humans exhibit considerable diversity in timing and rate of reproduction. Life-history theory (LHT) suggests that ecological cues of resource richness and survival probabilities shape human phenotypes across populations. Populations experiencing high extrinsic mortality due to uncertainty in resources should exhibit faster life histories. Here we use a path analytic (PA) approach informed by LHT to model the multiple pathways between resources, mortality rates, and reproductive behavior in 191 countries. Resources that account for the most variance in population mortality rates are predicted to explain the most variance in total fertility rates. Results indicate that resources (e.g., calories, sanitation, education, and health-care expenditures) influence fertility rates in paths through communicable and noncommnunicable diseases. Paths acting through communicable disease are more strongly associated with fertility than are paths through noncommunicable diseases. These results suggest that a PA approach may help disaggregate extrinsic and intrinsic mortality factors in cross-cultural analyses. Such knowledge may be useful in developing targeted policies to decrease teenage pregnancy, total fertility rates, and thus issues associated with overpopulation.

Adolescent fertility and risky environments: a population-level perspective across the lifespan

Timing of first reproduction is a key life-history variable with important implications for global economic development and health. Life-history theory predicts that human reproductive strategies are shaped by mortality regimes. This study provides the first test of the relationship between population-level adolescent fertility (AF) and extrinsic risk at two time points. Data are from United Nations database and were analysed using mediation and moderation techniques. The goals were to determine whether (i) early risk has a stronger impact on fertility than current risk; (ii) current risk mediates the relationship between early risk and fertility outcomes; and (iii) different levels of early risk influence the relationship between current risk and fertility. Results indicated that current risk partially mediated the relationship between early risk and fertility, with early risk having the strongest impact on reproduction. Measures for early and current mortality did not show significant interaction effects. However, a series of separate regression analyses using a quantile split of early risk indicated that high levels of early risk strengthened the relationship between current risk and AF. Overall, these findings demonstrate that reproductive strategies are significantly influenced by fluctuations of early mortality as well as current environmental cues of harshness.

Maternal risk of breeding failure remained low throughout the demographic transitions in fertility and age at first reproduction in Finland

Radical declines in fertility and postponement of first reproduction during the recent human demographic transitions have posed a challenge to interpreting human behaviour in evolutionary terms. This challenge has stemmed from insufficient evolutionary insight into individual reproductive decision-making and the rarity of datasets recording individual long-term reproductive success throughout the transitions. We use such data from about 2,000 Finnish mothers (first births: 1880s to 1970s) to show that changes in the maternal risk of breeding failure (no offspring raised to adulthood) underlay shifts in both fertility and first reproduction. With steady improvements in offspring survival, the expected fertility required to satisfy a low risk of breeding failure became lower and observed maternal fertility subsequently declined through an earlier age at last reproduction. Postponement of the age at first reproduction began when this risk approximated zero-even for mothers starting reproduction late. Interestingly, despite vastly differing fertility rates at different stages of the transitions, the number of offspring successfully raised to breeding per mother remained relatively constant over the period. Our results stress the importance of assessing the long-term success of reproductive strategies by including measures of offspring quality and suggest that avoidance of breeding failure may explain several key features of recent life-history shifts in industrialized societies.