Resources and the life course: Patterns through the demographic transition

In most mammals, and in the majority of traditional human societies for which data exist, status, power, or resource control correlates with lifetime reproductive success; male and female patterns differ. Because such correlations are often argued to have disappeared in human societies during the demographic transition of the nineteenth century, we analyzed wealth and lifetime reproductive success in a nineteenth-century Swedish population in four economically diverse parishes, subsuming geographic and temporal variation. Children of both sexes born to poorer parents were more likely than richer children to die or emigrate before reaching maturity. Poorer men, and women whose fathers were poorer, were less likely to marry in the parish than others, largely as a result of differential mortality and migration. Of all adults of both sexes who remained in their home parish and thus generated complete lifetime records, richer individuals had greater lifetime fertility and more children alive at age ten, than others. The age-specific fertility of richer women rises slightly sooner, and reaches a higher peak, than that of poorer women. These patterns persisted throughout the period of the sample (1824–1896). Thus, wealth appears, even during the demographic transition in an egalitarian society, to have influenced lifetime reproductive success positively.     

Natural selection on male wealth in humans

Although genomic studies suggest that natural selection in humans is ongoing, the strength of selection acting on particular characteristics in human populations has rarely been measured. Positive selection on male wealth appears to be a recurrent feature of human agrarian and pastoralist societies, and there is some evidence of it in industrial populations, too. Here we investigate the strength of selection on male wealth, first in contemporary Britain using data from the National Child Development Study and then across seven other varied human societies. The British data show positive selection on male income driven by increased childlessness among low-income men but a negative association between personal income and reproductive success for women. Across cultures, selection gradients for male wealth are weakest in industrial countries and strongest in subsistence societies with extensive polygyny. Even the weakest selection gradients observed for male wealth in humans are as strong as or stronger than selection gradients reported from field studies of other species. Thus, selection on male wealth in contemporary humans appears to be ubiquitous and substantial in strength.     

The coevolution of human fertility and wealth inheritance strategies.

Life history theory concerns the scheduling of births and the level of parental investment in each offspring. In most human societies the inheritance of wealth is an important part of parental investment. Patterns of wealth inheritance and other reproductive decisions, such as family size, would be expected to influence each other. Here I present an adaptive model of human reproductive decision-making, using a state-dependent dynamic model. Two decisions made by parents are considered: when to have another baby, and thus the pattern of reproduction through life; and how to allocate resources between children at the end of the parents'' life. Optimal decision rules are those that maximize the number of grandchildren. Decisions are assumed to depend on the state of the parent, which is described at any time by two variables: number of living sons, and wealth. The dynamics of the model are based on a traditional African pastoralist system, but it is general enough to approximate to any means of subsistence where an increase in the amount of wealth owned increases the capacity for future production of resources. The model is used to show that, in the unpredictable environment of a traditional pastoralist society, high fertility and a biasing of wealth inheritance to a small number of children are frequently optimal. Most such societies are now undergoing a transition to lower fertility, known as the demographic transition. The effects on fertility and wealth inheritance strategies of reducing mortality risks, reducing the unpredictability of the environment and increasing the costs of raising children are explored. Reducing mortality has little effect on completed family sizes of living children or on the wealth they inherit. Increasing the costs of raising children decreases optimal fertility and increases the inheritance left to each child at each level of wealth, and has the potential to reduce fertility to very low levels. The results offer an explanation for why wealthy families are frequently also those with the smallest number of children in heterogeneous, post-transition societies.     

Resources and reproductive success in women with an example from the Kipsigis of Kenya

Contradictory results regarding the relationship between resources and reproductive success of women have led some social scientists to conclude that evolutionary biological models are inappropriate to the study of human social behavior. This paper suggests instead that the variability across societies in this relationship reflects an inadequate specification of the nature and availability of the resources critical to reproduction as well as a failure to understand the mechanisms whereby resources confer reproductive success in traditional, developing, preindustrial, and modern societies. These methodological and conceptual issues are illustrated through use of data on the association between wealth and reproductive success from the Kipsigis, a polygynous agropastoralist population in southwestern Kenya. In this society, land is owned by men, and women gain access to land through marriage. In 3 of the 5 marriage cohorts studied, women with access to larger land plots had higher lifelong reproductive success than their poorer counterparts both in terms of enhanced fertility and survivorship of offspring. This association was independent of confounding factors such as education, age at menarche, husband's age, or occupation. Moreover, wealthy women were found not to make greater use of modern medical child health services when their children were sick than poor women. The Kipsigis data indicate that wealthy women had more nutritional resources than poor women and were able to introduce more suitable weaning foods, leading to a lower incidence of episodes of illness in offspring. Overall, the findings suggest that wealth-related differences in the nutrition and health of mothers and children are important factors in reproductive differentials in Kipsigis society.     

The Reproductive Ecology of Industrial Societies,Part II

Studies of the association between wealth and fertility in industrial populations have a rich history in the evolutionary literature, and they have been used to argue both for and against a behavioral ecological approach to explaining human variability. We consider that there are strong arguments in favor of measuring fertility (and proxies thereof) in industrial populations, not least because of the wide availability of large-scale secondary databases. Such data sources bring challenges as well as advantages, however. The purpose of this article is to illustrate these by examining the association between wealth and reproductive success in the United States, using the National Longitudinal Study of Youth 1979. We conduct a broad-based exploratory analysis of the relationship between wealth and fertility, employing both cross-sectional and longitudinal approaches, and multiple measures of both wealth (income and net worth) and fertility (lifetime reproductive success and transitions to first, second and third births). We highlight the kinds of decisions that have to be made regarding sample selection, along with the selection and construction of explanatory variables and control measures. Based on our analyses, we find a positive effect of both income and net worth on fertility for men, which is more pronounced for white men and for transitions to first and second births. Income tends to have a negative effect on fertility for women, while net worth is more likely to positively predict fertility. Different reproductive strategies among different groups within the same population highlight the complexity of the reproductive ecology of industrial societies. These results differ in a number of respects from other analyses using the same database. We suggest this reflects the impossibility of producing a definitive analysis, rather than a failure to identify the “correct” analytical strategy. Finally, we discuss how these findings inform us about (mal)adaptive decision-making.     

Resources and reproductive success in women with an example from the Kipsigis of Kenya

Contrary to findings in studies of many mammalian species, the relationship between resources and reproductive success of women is highly variable across societies. This has led some social scientists to suggest that evolutionary biological models are inappropriate to the study of human social behaviour. This paper proposes that variability in the relationship between resources and reproductive success arises from an inadequate specification of the nature and availability of resources critical to reproduction and a failure to understand the mechanisms whereby resources confer reproductive differentials in different environments. Data are presented from the Kipsigis of Kenya showing how land ownership affects female reproductive success; reasons why the effects of landholding are changing over time are examined.     

Costly reproductive competition between females in a monogamous cooperatively breeding bird

In many cooperatively breeding societies, only a few socially dominant individuals in a group breed, reproductive skew is high, and reproductive conflict is common. Surprisingly, the effects of this conflict on dominant reproductive success in vertebrate societies have rarely been investigated, especially in high-skew societies. We examine how subordinate female competition for breeding opportunities affects the reproductive success of dominant females in a monogamous cooperatively breeding bird, the Southern pied babbler (Turdoides bicolor). In this species, successful subordinate reproduction is very rare, despite the fact that groups commonly contain sexually mature female subordinates that could mate with unrelated group males. However, we show that subordinate females compete with dominant females to breed, and do so far more often than expected, based on the infrequency of their success. Attempts by subordinates to obtain a share of breeding impose significant costs on dominant females: chicks fledge from fewer nests, more nests are abandoned before incubation begins, and more eggs are lost. Dominant females appear to attempt to reduce these costs by aggressively suppressing potentially competitive subordinate females. This empirical evidence provides rare insight into the nature of the conflicts between females and the resultant costs to reproductive success in cooperatively breeding societies.     

Behavioural ecology in traditional societies

Can models from behavioural ecology explain cultural diversity in human populations? Studies of variation in reproductive and productive behaviour, both within and between traditional societies, are beginning to show that specific predictions from sexual selection and optimal foraging theory can be developed and tested with human data. Greatest success has been in the study of foraging; whereas attempts to understand patterns of marriage and parental investment have been most convincing in those cases where behaviour is related to specific ecological and social conditions. The aim of human behavioural ecologists in the future will be to determine the constraints that the dual goals of reproduction and production place on individuals.     

Matriliny as daughter-biased investment

From an evolutionary perspective, matriliny presents a puzzle because men in matrilineal societies transmit wealth to their sisters' sons, to whom they are only half as related as to their own sons. It has been argued that such systems would only maximise fitness under unrealistically high levels of paternity uncertainty. In this paper, we propose that matriliny can arise from daughter-biased investment by parents and/or grandparents. We show that daughter-biased investment is adaptive if the marginal benefit of wealth to sons (compared to daughters) does not outweigh the risk of nonpaternity in sons' offspring. We argue that such conditions will be rare where resource-holding polygyny is prevalent but could otherwise be widespread under even moderate levels of paternity uncertainty. The daughter-biased investment model explains two well-known characteristics of matrilineal descent: (a) matriliny's association with high levels of paternity uncertainty and (b) matriliny's ecological correlates, including its association with horticulture, its rarity in pastoralist and agro-pastoralist societies, and the tendency for matriliny to be replaced by son-biased inheritance during economic development. We present data on wealth, sex, and reproductive success (RS) in two African societies, the matrilineal Chewa in Malawi and patrilineal Gabbra in Kenya, which support the daughter-biased investment theory.     

Optimizing Modern Family Size

Modern industrialized populations lack the strong positive correlations between wealth and reproductive success that characterize most traditional societies. While modernization has brought about substantial increases in personal wealth, fertility in many developed countries has plummeted to the lowest levels in recorded human history. These phenomena contradict evolutionary and economic models of the family that assume increasing wealth reduces resource competition between offspring, favoring high fertility norms. Here, we review the hypothesis that cultural modernization may in fact establish unusually intense reproductive trade-offs in wealthy relative to impoverished strata, favoring low fertility. We test this premise with British longitudinal data (the Avon Longitudinal Study of Parents and Children), exploring maternal self-perceptions of economic hardship in relation to increasing family size and actual socioeconomic status. Low-income and low-education-level mothers perceived the greatest economic costs associated with raising two versus one offspring. However, for all further increases to family size, reproduction appears most expensive for relatively wealthy and well-educated mothers. We discuss our results and review current literature on the long-term consequences of resource dilution in modern families.     

Understanding the nature of wealth and its effects on human fitness

Studying fitness consequences of variable behavioural, physiological and cognitive traits in contemporary populations constitutes the specific contribution of human behavioural ecology to the study of human diversity. Yet, despite 30 years of evolutionary anthropological interest in the determinants of fitness, there exist few principled investigations of the diverse sources of wealth that might reveal selective forces during recent human history. To develop a more holistic understanding of how selection shapes human phenotypic traits, be these transmitted by genetic or cultural means, we expand the conventional focus on associations between socioeconomic status and fitness to three distinct types of wealth-embodied, material and relational. Using a model selection approach to the study of women's success in raising offspring in an African horticultural population (the Tanzanian Pimbwe), we find that the top performing models consistently include relational and material wealth, with embodied wealth as a less reliable predictor. Specifically, child mortality risk is increased with few household assets, parent nonresidency, child legitimacy, and one or more parents having been accused of witchcraft. The use of multiple models to test various hypotheses greatly facilitates systematic comparative analyses of human behavioural diversity in wealth accrual and investment across different kinds of societies.     

How to succeed in science

Adherence to these principles will not guarantee success, but the testimony of many famous scientists supports the hypothesis that these guidelines can significantly (p less than .05, Wilcoxon unpaired X-test run at pH 5.6) increase your chances of achieving recognition, acquiring wealth, and ultimately being known as a successful scientist. At the very least, they should prevent you from falling too far outside the boundaries of "normal" science where you could easily be branded for life as a troublemaker or heretic.     

Wealth, Status, and Reproductive Success among the Mukogodo of Kenya

The evolutionary biological hypothesis that culturally defined values and goals are proximate means of enhancing reproductive success is tested on data from the Mukogodo, a small group of Maa-speaking pastoralists in north-central Kenya who value the accumulation of livestock. The results support the prediction that, at least among males, livestock wealth should correlate with reproductive success. This correlation appears to be due mainly to greater polygyny among wealthier men. Lower age at first marriage among wealthier men may also contribute to the correlation between livestock wealth and reproductive success. The association between livestock wealth and reproductive success does not appear to be due to the productivity of wives and children, to bridewealths obtained when daughters marry, or to the effects of wealth on the reproductive success of men's wives.     

Evolution of monogamous marriage by maximization of inclusive fitness

The majority of human societies allow polygynous marriage, and the prevalence of this practice is readily understood in evolutionary terms. Why some societies prescribe monogamous marriage is however not clear: current evolutionary explanations—that social monogamy increases within‐group co‐operation, giving societies an advantage in competition with other groups—conflict with the historical and ethnographic evidence. We show that, within the framework of inclusive fitness theory, monogamous marriage can be viewed as the outcome of the strategic behaviour of males and females in the allocation of resources to the next generation. Where resources are transferred across generations, social monogamy can be advantageous if partitioning of resources among the offspring of multiple wives causes a depletion of their fitness value, and/or if females grant husbands higher fidelity in exchange for exclusive investment of resources in their offspring. This may explain why monogamous marriage prevailed among the historical societies of Eurasia: here, intensive agriculture led to scarcity of land, with depletion in the value of estates through partitioning among multiple heirs. Norms promoting high paternity were common among ancient societies in the region, and may have further facilitated the establishment of social monogamy. In line with the historical and ethnographic evidence, this suggests that monogamous marriage emerged in Eurasia following the adoption of intensive agriculture, as ownership of land became critical to productive and reproductive success.     

Cooperation and conflict: The behavioral ecology of the sexual division of labor

When it comes to subsistence, men and women in almost all societies do it differently. One long-standing explanation for this sexual division of labor is that men and women pair up to provision offspring and specialize in subsistence activities in order to maximize household productivity. This model of cooperative parental provisioning has generally been supported by the proposal that both male and female reproductive success is maximized by provisioning current offspring rather than deserting them in order to seek new mating opportunities. But recent analyses of bird behavior have often failed to support this premise. We now know that among many species conflicting reproductive strategies between males and females often result in less than optimal compromises with regard to mating and parenting. This new focus on the role of sexual selection in creating compromise and conflict between the sexes has the potential to illuminate many puzzling aspects of human partnerships between men and women. To demonstrate its potential, I compare the explanatory power of a cooperative provisioning model of sex difference in human foraging and food sharing with a model incorporating conflicting reproductive goals.     

An evolutionary account of status, power, and career in modern societies

We hypothesize that in modern societies the striving for high positions in the hierarchy of organizations is equivalent to the striving for status and power in historical and traditional societies. Analyzing a sample of 4,491 US men and 5,326 US women, we find that holding a supervisory position or being in charge of hiring and firing is positively associated with offspring count in men but not in women. The positive effect in men is attributable mainly to the higher proportion of childlessness among men in non-supervisory positions and those without the power to hire and fire. This effect is in accordance with the positive relationship between other status indicators and reproductive success found in men from traditional, historical, and modern societies. In women, we further find a curvilinear relationship between income percentile and offspring number by analyzing US census data, indicating that women may strive for resources associated with advancement rather than for status per se.     

An evolutionary perspective on the patterning of maternal investment in pregnancy

Pregnancy is thought to be a metabolically very expensive endeavor, yet investigations have produced inconsistent results concerning the responsiveness of human birth weight to maternal nutritional stress or nutritional intervention. These findings have led some researchers to conclude that fetal growth is strongly buffered against fluctuations in maternal energy balance, making the fetus in effect a “nearly perfect parasite.” This buffering would appear to be a reasonable adaptive response given the high risk of morbidity and mortality associated with low birth weight. However, a life-history approach leads to the prediction that maternal investment strategies in pregnancy should be geared toward maximizing lifetime reproductive success rather than simply the success of the current pregnancy, and by extension that maternal investment strategies should vary with reproductive value. The physiology of human pregnancy in fact appears to include a number of mechanisms that protect maternal energy resources from diversion to the fetus and preserve them for future reproductive events. These mechanisms include adjustment of blood flow to the uterus and perhaps minor adjustments in gestation length, although evidence for the latter is scant. Suggestions are made for ways of investigating these maternal options.     

Correlates and Consequences of Dominance in a Social Rodent

In harem‐polygynous societies, body condition is often correlated with dominance rank. However, the consequences of dominance are less clear. High‐ranking males do not inevitably have the highest reproductive success, especially in systems where females mate with multiple males. In such societies, we expect male reproductive success to be more highly skewed than female reproductive success, but reproductive skew in females can still arise from rankings established within matrilineal societies. Dominance can also impact life‐history decisions by influencing dispersal patterns in yearlings. To better understand the function of dominance in harem‐polygynous societies, we studied the causes and consequences of dominance in yellow‐bellied marmots (Marmota flaviventris), a social rodent with skewed male reproductive success and female reproductive suppression. Specifically, we examined body condition as a predictor and the probability of breeding, number of offspring, and dispersal as outcomes of dominance. Additionally, we looked at variation in dominance between males and females and adults and yearlings, because marmots can engage in distinct interactions depending on the type of individuals involved. We found that marmots in better body condition have higher dominance rank than those in poorer condition. In addition, adults are dominant over yearlings. Within yearlings, dominance does not influence dispersal, but those in better body condition are less likely to disperse. Within all adults, individuals in better condition produce more offspring per year. Within adult males, more dominant males have greater reproductive success. Despite previous evidence of reproductive suppression in females, we found no effects of dominance rank on female reproductive success in the current study. The function of dominance in female marmots remains enigmatic.     

Resource allocation between reproductive phases: the importance of thermal conditions in determining the cost of incubation

Changes in the resources allocated to particular stages of reproduction are expected to influence allocation to, and performance in, subsequent reproductive stages. Experimental manipulation of individual investment patterns provides important evidence that such physiological trade-offs occur, and can highlight the key environmental variables that influence reproductive costs. By temporarily altering the thermal properties of starling nests, we reduced the energetic demand of first-clutch incubation, and examined the effect of this manipulation on performance during the same and the subsequent reproductive attempts. Compared with controls, starlings investing less in incubation were more successful in fledging young, and were more likely to hatch all their eggs if a subsequent reproductive attempt was made. Our results show that incubation demands can limit reproductive success, and that resources saved during incubation can be reallocated to later stages of the same reproductive attempt and to future reproductive attempts. This study also shows that small changes in thermal environment can affect breeding success by altering the energetic demands imposed on incubating parents, independently of the effect of temperature on other environmental variables such as food supply.     

Effects of gender inequality and wealth inequality on within-sex mating competition under hypergyny

Resources are often central to the formation and persistence of human consortships, and to the evolutionary fitness consequences of those consortships. As a result, the distribution of resources within a society should influence the number and quality of mating opportunities an individual of given status/wealth experiences. In particular, in a wide variety of societies, both contemporary and historic, women have been shown to prefer mates of higher rather than lower status and wealth, a pattern known as ‘hypergyny’. Such status-dependent within-sex competition is influenced not only by the preferences individuals express but also by the distribution of resources within and between sexes. Empirical studies show that economic inequality within a sex can amplify mating competition, and that inequalities between women and men also influence behaviours related to mating competition, but the links between resource distribution and mating competition have attracted limited systematic attention. We present simulation models of hypergynous preferences and the effects on mating competition among men and among women within a heterosexual mating market. Our modelling shows that the lower mating success of poorer men and richer women (when compared with richer men and poorer women) is worsened when resource gender gaps are relatively small or when women out-earn men on average. Likewise, high economic inequality, especially among men, amplifies the competition experienced by these groups. We consider the political implications in terms of sex- and status-dependent attitudes to gender equity, wealth inequality, and hypergynous mating norms.