Top-down and bottom-up research in biodemography

The most efficient way to make scientific progress in biodemography is to encourage bi-directional exchange between ‘top-down’ and ‘bottom-up’ research. This will entail exchange along the continuum of research from microscopic intracellular processes to population-level consequences. In addition, our understanding of the biology of aging and its demographic consequences will be enriched by mutual influence between studies of mechanistic or ‘proximate’ causal processes and investigations of the evolutionary processes underlying the same phenomena. Researchers working at these different levels of explanation could be more productive if they were informed by research at other levels and interacted with scientists with complementary expertise. Such collaborations could be encouraged both through interdisciplinary workshops, research projects, program projects and training programs.aging, biodemography, evolution, life history

Father Death and Adult Success Among the Tsimane: Implications for Marriage and Divorce

Human fathers are heavily involved in the rearing of children around the world. While there is great cross-cultural variation, the father is a recognizable role in all populations. This deviates from the standard mammalian pattern of little paternal investment. A logical explanation offered early by evolutionary theorists is that human fathers evolved the capacity for paternal concern because human children are remarkably needy and impose a great encumbrance on the mother (Lancaster & Lancaster, 1983; Lovejoy, 1981). Thus, fathers have greater opportunity to enhance the wellbeing of child and mother, as there is a deeper well of need to fill. Marginal gains of family investment are thus steeper, leading to greater possibility for such returns to supersede those provided by the short-term mating strategies that are typical of most mammals. However, the numerous studies that have explored the cross-cultural impact of father presence on child survivorship report mixed results (Sear & Mace, 2008), indicating that father presence (and by assumption, investment) does not universally associate with better-off children. Fathers may also play an important role in enhancing the future competitiveness of their children by enhancing their physical condition, teaching them important skills, accumulating heritable wealth, or by building social alliances (Hewlett, 1992; Scelza, 2010). Previous studies have largely focused on the wellbeing of juvenile children, but a more complete test of the impact of paternal investment concerns its effect on the reproductive value of children, which must include adult fertility. Our goal in this paper is to fill this gap in the literature by reporting several measures of achieved success of adults based on the number of years their fathers were alive and present during their childhood. Specifically, we explore the impact of father presence on offspring height, body mass index (BMI), age of first reproduction, completed fertility for age, and number of surviving children for age. We report only one significant finding out of ten specific tests (five predictions for both men and women), thus failing to find any robust pattern of father death impacting the achieved success of adult children. Finally, we relate our findings to the nature of Tsimane marriage. Marriage in humans is often considered a means of facilitating the providing of bi-parental care (Hurtado & Hill, 1992; Lovejoy, 1981). Among the Tsimane, marriages are fairly stable, particularly after children have been born, strengthening the prediction that the presence of Tsimane fathers should be important to the success of children. We thus explore alternative explanations for the stability of Tsimane marriages by examining alternative fitness pathways and constraints experienced by Tsimane men

Culture Sometimes Matters: Intra-cultural Variation in Pro-social Behavior Among Tsimane Amerindians

Agent-centered models usually consider only individual-level variables in calculations of economic costs and benefits. There has been little consideration of social or cultural history on shaping payoffs in ways that impact decisions. To examine the role of local expectations on economic behavior, we explore whether village affiliation accounts for the variation in dictator game offers among the Tsimane of the Bolivian Amazon independently of other factors that could confound such an effect. Our analysis shows that significant differences in altruistic giving exist among villages, village patterns are recognized by residents, and offers likely reflect variation in social expectations rather than stable differences in norms of fairness

Sleep Variability and Nighttime Activity among Tsimane Forager‐Horticulturalists

Objectives A common presumption in sleep research is that “normal” human sleep should show high night‐to‐night consistency. Yet, intra‐individual sleep variation in small‐scale subsistence societies has never been studied to test this idea. In this study, we assessed the degree of nightly variation in sleep patterns among Tsimane forager‐horticulturalists in Bolivia, and explored possible drivers of the intra‐individual variability. Methods We actigraphically recorded sleep among 120 Tsimane adults (67 female), aged 18–91, for an average of 4.9 nights per person using the Actigraph GT3X and Philips Respironics Actiwatch 2. We assessed intra‐individual variation using intra‐class correlations and average deviation from each individual\u27s average sleep duration, onset, and offset times ( ). Results Only 31% of total variation in sleep duration was due to differences among different individuals, with the remaining 69% due to nightly differences within the same individuals. We found no statistically significant differences in Tsimane sleep duration by day‐of‐the‐week. Nightly variation in sleep duration was driven by highly variable sleep onset, especially for men. Nighttime activities associated with later sleep onset included hunting, fishing, housework, and watching TV. Conclusions In contrast to nightly sleep variation in the United States being driven primarily by “sleeping‐in” on weekends, Tsimane sleep variation, while comparable to that observed in the United States, was driven by changing “bedtimes,” independent of day‐of‐the‐week. We propose that this variation may reflect adaptive responses to changing opportunity costs to sleep/nighttime activity

The Multiple Dimensions of Male Social Status in An Amazonian Society

In all human societies, individuals differ in social status depending upon their age and personal ability (Sahlins, 1958; Service, 1971). In laboratory-based small group studies, status hierarchies emerge spontaneously (Bass, 1954; Campbell et al., 2002; Kalma, 1991). Even among “egalitarian” foragers, who are characterized by widespread resource sharing (Kaplan & Gurven, 2005; Winterhalder, 1986) and some degree of status-leveling (Cashdan, 1980), certain individuals consume more resources, get the best pick of mates, and take a more central role in group decision-making (Boehm, 1999; Trigger, 1985; Wiessner, 1996). Whether implicit or overt, classification by social status is a human universal. While women as well as men compete for status (Campbell, 2002; Hess & Hagen, 2006; Hrdy, 1999; Rucas et al., 2006), this paper focuses exclusively on male status hierarchies

Why household inefficiency? An experimental approach to assess spousal resource distribution preferences in a subsistence population undergoing socioeconomic change

Two disparate views of the sexual division of labour have dominated the 53 representation of intra-household resource allocations. These joint and separate interests 54 views differ in their interpretation of the relative roles of men and women, and make different 55 predictions about the extent to which marriage promotes economic efficiency (i.e. maximized 56 household production). Using an experimental “distribution task” stipulating a trade-off 57 between household efficiency and spousal equality in allocating surpluses of meat and 58 money, we examine factors influencing spousal distribution preferences among Tsimane 59 forager-horticulturalists of Bolivia (n=53 couples). Our primary goal is to understand whether 60 and how access to perfectly fungible and liquid resources – which increases with greater 61 participation in market economies – shifts intra-household distribution preferences. We 62 hypothesize that greater fungibility of money compared to meat results in greater squandering 63 of money for individual fitness gain at a cost to the family. Money therefore requires costly 64 strategies to insure against a partner’s claims for consumption. Whereas nearly all Tsimane 65 spouses prefer efficient meat distributions, we find a substantially reduced efficiency 66 preference for money compared to meat controlling for potential confounders (adjusted 67 OR=0.087, 95% CI: 0.02-0.38). Reported marital conflict over paternal disinvestment is 68 associated with a nearly 13-fold increase in odds of revealing a selfish money distribution 69 preference. Selfish husbands are significantly more likely than other husbands to be paired 70 with selfish wives. Lastly, Tsimane husbands and wives are more likely than Western 71 Europeans to prefer an efficient money distribution, but Tsimane wives are more likely than 72 Western European wives to exhibit a selfish preference. In sum, preferences for the 73 distribution of household production surplus support joint and separate interests views of 74 marriage; a hybrid approach best explains how ecological-, family-, and individual-level 75 factors influence spousal preferences through their effects on perceptions of marginal gains 76 within and outside the household

Infidelity, Jealousy, and Wife Abuse Among Tsimane Forager-Farmers: Testing Evolutionary Hypotheses of Marital Conflict

What causes marital conflict, and which marital conflicts are more likely to result in men’s violence against their wives? It has long been argued that men’s jealousy over women’s infidelity is the strongest impetus to men’s lethal and non-lethal violence against female partners. Less is known about the extent to which women’s jealousy over men’s infidelity precipitates men’s violence against female partners. Husbands are more likely than wives to commit infidelity, and men and women report a similar frequency and intensity of jealous emotions during recalls of potential infidelity. If men are likely to use time and resources for pursuit of extramarital sexual relationships, wives’ jealousy may play a critical role in mate retention, but at potential cost of instigating marital arguments and violence against wives. Given men’s greater size and strength, violence against wives may be used as a “bargaining” tool to strategically leverage a selfish outcome, despite potential costs to the victim, aggressor, and offspring. This is the first study to document content and prevalence of marital arguments, and prevalence of men’s violence against wives during such arguments in a small-scale society, the Tsimane of Bolivia. We show that men’s diversion of resources from the family is a major source of arguments between spouses and husbands’ violence against their wives. We argue that husbands employ violence to limit wives’ mate retention effort and maintain men’s opportunities to pursue extramarital sexual relationships. We define violence against wives as any physical contact initiated by a husband with intent to harm a wife (hereafter termed wife abuse). The research design minimizes response and sampling bias in two ways: 1) data are obtained independently from both spouses instead of only one spouse, permitting assessment of spousal consistency in reporting; and 2) couples are sampled randomly rather than being self-selected for a high degree of marital conflict

Low Perceived Control Over Health is Associated With Lower Treatment Uptake in a High Mortality Population of Bolivian Forager-Farmers

Indigenous people worldwide suffer from higher rates of morbidity and mortality than neighboring populations. In addition to having limited access to public health infrastructure, indigenous people may also have priorities and health perceptions that deter them from seeking adequate modern healthcare. Here we propose that living in a harsh and unpredictable environment reduces motivation to pursue deliberate, costly action to improve health outcomes. We assess whether variation in Health Locus of Control (HLC), a psychological construct designed to capture self-efficacy with respect to health, explains variation in treatment uptake behavior among Tsimane Amerindians (N=690; age range: 40–89 years; 55.8% female; data collection: 2008–2012), a high mortality and morbidity indigenous population in the Bolivian Amazon, Beni Department. Comparisons with two industrialized populations in Japan (Miyagi prefecture; e0=76.6 years) and the United Kingdom (Caerphilly county borough; e0=81.2 years) confirm that Tsimane (e0=54.1 years) have a more externalized HLC. Multilevel level models were used to investigate whether HLC predicts treatment uptake, and mediates the relationship between modernization and treatment uptake. External HLC scores were predictive of treatment outcomes: Powerful others scores were positively associated with probability of receiving modern treatment (adjusted odds ratio [OR] =1.33), while Chance scores were negatively associated with probability of receiving modern treatment (adjusted OR=0.76). We found no effects, however, of Internal HLC or educational capital on treatment uptake. Overall, our findings indicate that health-related decision-making is influenced more by a psychological orientation affecting self-efficacy, shaped in part by perceptions of environmental unpredictability and harshness, than by limited knowledge, education or other indicators of modernization

Why Do Women Have More Children Than They Want? Understanding Differences in Women\u27s Ideal and Actual Family Size in a Natural Fertility Population

Objectives—We develop and test a conceptual model of factors influencing women’s ideal family size (IFS) in a natural fertility population, the Tsimane of Bolivia. The model posits affects of socioecology, reproductive history, maternal condition, and men’s IFS. We test three hypotheses for why women may exceed their IFS despite experiencing socioeconomic development: (H1) limited autonomy; (H2) improved maternal condition; and (H3) low returns on investments in embodied capital. Methods—Women’s reproductive histories and prospective fertility data were collected from 2002 to 2008 (n = 305 women). Semistructured interviews were conducted with Tsimane women to study the perceived value of parental investment (n = 76). Multiple regression, t-tests, and analysis of variance (ANOVA) are used to test model predictions. Results—Women’s IFS is predicted by their socioecology, reproductive history, maternal condition, and husband’s IFS. Hypotheses 2 and 3 are supported. Couples residing near town have smaller IFS (women = 3.75 ± 1.64; men = 3.87 ± 2.64) and less variance in IFS. However, the degree fertility exceeds IFS is inversely correlated with distance to town (Partial r = −0.189, df = 156, P = 0.018). Women living near town have greater maternal condition but 64% value traditional skills over formal schooling and 88% believe living in town is unfeasible. Conclusions—While reduced IFS is evident with socioeconomic development, fertility decline may not immediately follow. When perceived benefits of investment in novel forms of embodied capital are low, and somatic wealth and large kin networks persist as important components of fitness, fertility may remain high and increase if maternal condition improves

Differences in Tsimane Children’s Growth Outcomes and Associated Determinants as Estimated by WHO Standards vs. Within-Population References

Anthropometric measures are commonly converted to age stratified z-scores to examine variation in growth outcomes in mixed-age and sex samples. For many study populations, z-scores will differ if calculated from World Health Organization (WHO) growth standards or within-population references. The specific growth reference used may influence statistical estimates of growth outcomes and their determinants, with implications for biological inference. We examined factors associated with growth outcomes in a sample of 152 Tsimane children aged 0–36 months. The Tsimane are a subsistence-scale population in the Bolivian Amazon with high rates of infectious disease and growth faltering. To examine the influence of growth reference on statistical inferences, we constructed multiple plausible models from available infant, maternal, and household attributes. We then ran identical models for height-for-age (HAZ), weight-for-age (WAZ), and weight-for-height (WHZ), with z-scores alternately calculated from WHO and robust Tsimane Lambda-Mu-Sigma growth curves. The distribution of WHO relative to Tsimane HAZ scores was negatively skewed, reflecting age-related increases in lower HAZ. Standardized coefficients and significance levels generally agreed across WHO and Tsimane models, although the strength and significance of specific terms varied in some models. Age was strongly, negatively associated with HAZ and WAZ in nearly all WHO, but not Tsimane models, resulting in consistently higher R2 estimates. Age and weaning effects were confounded in WHO models. Biased estimates of determinants associated with WHO HAZ may be more extreme in small samples and for variables that are strongly age-patterned. Additional methodological considerations may be warranted when applying WHO standards to within-population studies, particularly for populations with growth patterns known to systematically deviate from those of the WHO reference sample