Monique Borgerhoff Mulder

The demographic transition: are we any closer to an evolutionary explanation?

The radical shift in human reproduction in the late 19th century, known as the demographic transition, constitutes a major challenge to evolutionary approaches to human behaviour. Why would people ever choose to limit their reproduction voluntarily when, at the peak of the Industrial Revolution, resources were apparently so plentiful? Can the transition be attributed to standard life history tradeoffs, is it a consequence of cultural evolutionary processes, or is it simply a maladaptive outcome of novel and environmental social conditions? Empirical analyses and new models suggest that reproductive decision making might be driven by a human psychology designed by natural selection to maximize material wealth. If this is the case, the mechanisms governing fertility and parental investment are likely to respond to modern conditions with a fertility level much lower than that that would maximize fitness.

Kipsigis women's preferences for wealthy men: evidence for female choice in mammals?

SummaryIn contrast to studies of birds, in mammals there is no clear evidence that polygyny evolved through female choice for males with high quality resources. Among the Kipsigis people of Kenya, polygyny may be a consequence of womens preferences for wealthy men, because strong correlations exist between land ownership and the number of a mans wives (Borgerhoff Mulder 1987 a), and the resources men hold are primary determinants of womens reproductive success (Borgerhoff Mulder 1987b). This paper has two aims: first, to test whether Kipsigis women prefer wealthy men by examining the sequence of marriages among a group of pioneers (Table 1) who established a settlement in the territory of their enemies (1930–1949); second, to determine whether women suffer reproductively as a result of polygynous marriage. Data show that Kipsigis women, or their parents on their behalf, preferentially chose men offering high quality breeding opportunities, with respect to the number of acres available on which to settle (Fig. 2) ; controlling for quality of breeding opportunity there is a preference for bachelors over monogamists over polygynists. Analyses of the full demographic sample show that there are reproductive costs associated with having a large number of cowives (Table 2), costs which women attempt to minimize through judicous marital choices. These results are discussed in relation to resource defence polgyny, female choice and, specifically, the polygyny threshold model.

Bateman's principles and human sex roles

In 1948, Angus J. Bateman reported a stronger relationship between mating and reproductive success in male fruit flies compared with females, and concluded that selection should universally favour ‘an undiscriminating eagerness in the males and a discriminating passivity in the females’ to obtain mates. The conventional view of promiscuous, undiscriminating males and coy, choosy females has also been applied to our own species. Here, we challenge the view that evolutionary theory prescribes stereotyped sex roles in human beings, firstly by reviewing Batemans principles and recent sexual selection theory and, secondly, by examining data on mating behaviour and reproductive success in current and historic human populations. We argue that human mating strategies are unlikely to conform to a single universal pattern.

Intergenerational Wealth Transmission and the Dynamics of Inequality in Small-Scale Societies

Origins of Egalitarianism Wealthy contemporary societies exhibit varying extents of economic inequality, with the Nordic countries being relatively egalitarian, whereas there is a much larger gap between top and bottom in the United States. Borgerhoff Mulder et al. (p. 682; see the Perspective by Acemoglu and Robinson) build a bare-bones model describing the intergenerational transmission of three different types of wealth—based on social networks, land and livestock, and physical and cognitive capacity—in four types of small-scale societies in which livelihoods depended primarily on hunting, herding, farming, or horticulture. Parameter estimates from a large-scale analysis of historical and ethnographic data were added to the model to reveal that the four types of societies display distinctive patterns of wealth transmission and that these patterns are associated with different extents of inequality. Some types of wealth are strongly inherited and, hence, contribute to long-term economic inequality. Small-scale human societies range from foraging bands with a strong egalitarian ethos to more economically stratified agrarian and pastoral societies. We explain this variation in inequality using a dynamic model in which a population’s long-run steady-state level of inequality depends on the extent to which its most important forms of wealth are transmitted within families across generations. We estimate the degree of intergenerational transmission of three different types of wealth (material, embodied, and relational), as well as the extent of wealth inequality in 21 historical and contemporary populations. We show that intergenerational transmission of wealth and wealth inequality are substantial among pastoral and small-scale agricultural societies (on a par with or even exceeding the most unequal modern industrial economies) but are limited among horticultural and foraging peoples (equivalent to the most egalitarian of modern industrial populations). Differences in the technology by which a people derive their livelihood and in the institutions and norms making up the economic system jointly contribute to this pattern.

Human reproductive behaviour : a Darwinian perspective

Human mating and parenting behaviors are examined from a Darwinian perspective in this collection of 20 papers by anthropologists biologists and psychologists. The emphasis is on studies using quantitative data. The first part includes 9 studies on mating and marriage patterns in different societies. The second part has 10 studies on parenting and reproduction and the factors affecting them. There is no specific geographic or time period focus. A detailed subject index to the entire volume is included. (ANNOTATION)

Rethinking Polygyny: Co-Wives, Codes, and Cultural Systems [and Comments and Reply]

A new set of codes is offered to begin to unpack the dimensions of polygyny. Included are measures of frequency and statistical distributions of multiple wives, cultural rules, residential arrangements and kin relations among co-wives, male stratification, and marriage of captured women. Problems of coding and measurement are extensively illustrated. A series of hypotheses is supported regarding two types of polygyny: wealth-increasing and sororal. In the first, womens labor generates wealth and (if warfare allows) female captives are taken as secondary wives. Here polygyny stratifies males by wealth and most men are able to become polygynists with age. Residential autonomy of wives is an elaboration of this pattem. The second is marked by coresidence of husband and wives and dependence of the family mostly on resources generated by the husband. Here polygyny is usually dependent on the exceptional productivity of particular men such as hunters or shamans. The regional-historical adaptations of these types differ markedly. Neither fits the model of resource-defense polygyny found in other species. Explanations of polygyny, particularly of the first ype, require close attention to resource and demographic flows within regional ecologies. The second type requires further functional and historical analysis. Both require more consideration of the way polygyny operates from the female point of view, a task only partially begun here.

Brothers and sisters : How sibling interactions affect optimal parental allocations.

Data from the Kipsigis of Kenya are used to test two models for how parents invest in offspring, the Trivers-Willard and local resource competition/enhancement hypotheses. Investment is measured as age-specific survival, educational success, marital arrangements, and some components of property inheritance, permitting an evaluation of how biases persist or alter over the period of dependence. Changes through time in such biases are also examined. Despite stronger effects of wealth on the reproductive success of men than women, the survival of sons and daughters is not related to parental wealth. However, a Trivers-Willard effect characterizes educational investment: poor families show a greater concern for daughters’ (vis-à-vis sons’) schooling than do rich families, a trend that has increased over time. In regard to models of local resource competition and enhancement, men’s reproductive success decreases with number of brothers and increases with number of sisters; this pattern of competition with same-sex sibs and cooperation with opposite-sex sibs is not found among women. As predicted from these observations, parents show reduced investment in sons with a large number of brothers, and increased investment in sons with a large number of sisters. By contrast, investment in daughters is entirely unaffected by number of sisters and is influenced only in subtle ways by number of brothers. Levels of investment in relation to sibship size (irrespective of siblings’ sex) are highest for younger children of large sib sets.Discussion of the results in relation to those from other studies leads to three conclusions. First, predictive models for how investment biases vary across societies must consider a broad range of socioecological factors constraining parental options and payoffs. Second, the timing of investment biases within societies will be affected by the value of children and the costs of parental investment. Third, measures of investment appropriate for between-sex and between-class comparisons need careful attention. Each of these issues is brought to bear on the question of why, in contrast to so many other parts of the world, sex preferences are so muted in Africa.

Are East African Pastoralists Truly Conservationists

Controversy exists among anthropologists, conservation biologists, and development workers as to whether the concept of the “ecologically noble savage” is a myth. Central to this debate are the problem of how to identify conservationist behavior and the issue of whether sound management of common property is likely to evolve. While social scientists have documented instances of restraint in the use of resources, those who adopt an evolutionary perspective are challenged to identify the selective mechanisms whereby such altruistic conservation acts might be maintained in a population. Here a game‐theoretical approach is used to analyze the case of pastoralist grazing reserves. We demonstrate that under some conditions conservation can be the result of narrow self‐interest and there is no collective‐action problem. However, the range of these conditions is much broader for wealthy individuals, and thus the wealthy may also find it advantageous to coerce others into conserving. In conclusion, we propose an extension of the definition of conservation that is of greater generality for use in nonforaging populations and incorporates the essential political element of how conflicts over resource use are resolved.

Optimizing offspring: the quantity–quality tradeoff in agropastoral Kipsigis

The tradeoff between offspring quantity and offspring quality is at the heart of most evolutionary approaches to the fertility transition, as it is for demographers oriented towards economic explanations for this transition. To date, however, there have been few empirical tests of the key idea that humans trade offspring quantity for quality, and no strictly comparative work designed to identify the specific environmental conditions that favor such a tradeoff. This study suggests that in an East African community where the principal form of intergenerational inheritance is land, intermediate levels of offspring production are favored for women but not men. Women produce approximately the optimal number of surviving children, whereas men produce far fewer than the optimal number. The result highlights the significance of inheritable extrasomatic capital, in conjunction with evolved psychological mechanisms, in shaping fertility strategies that emphasize quality over quantity.

How national context, project design, and local community characteristics influence success in community-based conservation projects

Community-based conservation (CBC) promotes the idea that conservation success requires engaging with, and providing benefits for, local communities. However, CBC projects are neither consistently successful nor free of controversy. Innovative recent studies evaluating the factors associated with success and failure typically examine only a single resource domain, have limited geographic scope, consider only one outcome, or ignore the nested nature of socioecological systems. To remedy these issues, we use a global comparative database of CBC projects identified by systematic review to evaluate success in four outcome domains (attitudes, behaviors, ecological, economic) and explore synergies and trade-offs among these outcomes. We test hypotheses about how features of the national context, project design, and local community characteristics affect these measures of success. Using bivariate analyses and multivariate proportional odds logistic regressions within a multilevel analysis and model-fitting framework, we show that project design, particularly capacity-building in local communities, is associated with success across all outcomes. In addition, some characteristics of the local community in which projects are conducted, such as tenure regimes and supportive cultural beliefs and institutions, are important for project success. Surprisingly, there is little evidence that national context systematically influences project outcomes. We also find evidence of synergies between pairs of outcomes, particularly between ecological and economic success. We suggest that well-designed and implemented projects can overcome many of the obstacles imposed by local and national conditions to succeed in multiple domains.