How much does family matter? Cooperative breeding and the demographic transition

Church attendance and alloparenting: an analysis of fertility, social support and child development among English mothers

Many aspects of religious rituals suggest they provide adaptive benefits. Studies across societies consistently find that investments in ritual behaviour return high levels of cooperation. Another line of research finds that alloparental support to mothers increases maternal fertility and improves child outcomes. Although plausible, whether religious cooperation extends to alloparenting and/or affects child development remains unclear. Using 10 years of data collected from the Avon Longitudinal Study of Parents and Children (ALSPAC), we test the predictions that church attendance is positively associated with social support and fertility (n = 8207 to n = 8209), and that social support is positively associated with fertility and child development (n = 1766 to n = 6561). Results show that: (i) relative to not attending, church attendance is positively related to a woman's social network support and aid from co-religionists, (ii) aid from co-religionists is associated with increased family size, while (iii) fertility declines with extra-religious social network support. Moreover, while extra-religious social network support decreased over time, co-religionist aid remained constant. These findings suggest that religious and secular networks differ in their longevity and have divergent influences on a woman's fertility. We find some suggestive evidence that support to mothers and aid from co-religionists is positively associated with a child's cognitive ability at later stages of development. Findings provide mixed support for the premise that ritual, such as church attendance, is part of a strategy that returns high levels of support, fertility and improved child outcomes. Identifying the diversity and scope of cooperative breeding strategies across global religions presents an intriguing new horizon in the evolutionary study of religious systems. This article is part of the theme issue 'Ritual renaissance: new insights into the most human of behaviours'.

The developmental support hypothesis: adaptive plasticity in neural development in response to cues of social support

Across mammals, cues of developmental support, such as touching, licking or attentiveness, stimulate neural development, behavioural exploration and even overall body growth. Why should such fitness-related traits be so sensitive to developmental conditions? Here, we review what we term the 'developmental support hypothesis', a potential adaptive explanation of this plasticity. Neural development can be a costly process, in terms of time, energy and exposure. However, environmental variability may sometimes compromise parental care during this costly developmental period. We propose this environmental variation has led to the evolution of adaptive plasticity of neural and behavioural development in response to cues of developmental support, where neural development is stimulated in conditions that support associated costs. When parental care is compromised, offspring grow less and adopt a more resilient and stress-responsive strategy, improving their chances of survival in difficult conditions, similar to existing ideas on the adaptive value of early-life programming of stress. The developmental support hypothesis suggests new research directions, such as testing the adaptive value of reduced neural growth and metabolism in stressful conditions, and expanding the range of potential cues animals may attend to as indicators of developmental support. Considering evolutionary and ecologically appropriate cues of social support also has implications for promoting healthy neural development in humans. This article is part of the theme issue 'Life history and learning: how childhood, caregiving and old age shape cognition and culture in humans and other animals'.

Counting on Potential Grandparents? Adult Children's Entry Into Parenthood Across European Countries

As populations age and longevity rises, the structure of the extended family is changing. Parents of young children are increasingly turning to the children’s grandparents to provide childcare and help them reconcile work and family. This study is the first to investigate whether would-be grandparents’ propensity to care for their grandchildren influences the adult children’s transition to parenthood. Because grandparental childcare provision is not observable at the time of the transition to the first birth, I built a measure based on the characteristics of both actual grandparents and adult children to act as a proxy for the childcare that prospective grandparents are expected to provide in the future. Using data from the first two waves of the Survey of Health, Aging, and Retirement in Europe, I examine changes in the likelihood of having a first birth by different levels of expected future childcare provision. Given that the role grandparents play varies depending on the national context, I estimate distinct models for different groups of countries. Furthermore, I analyze different intensities of grandparental childcare: regular, occasional, and any other type of positive childcare. The comparison across 11 countries reveals that grandparental propensity to provide occasional childcare has a positive effect on the transition to parenthood in all country clusters and that grandparental propensity to provide regular childcare has a positive and significant association with having a first child in both pro-natalist (Belgium and France) and pro-traditional countries (Austria, Germany, Greece, Italy, Spain, and Switzerland).

A mathematical model for the effects of grandmothering on human longevity

Although females in human and the great ape populations reach the end of fertility at similar ages (approximately 45 years), female humans often live well beyond their post-fertile years, while female primates typically die before or shortly after the end of fertility. The grandmother hypothesis proposes that the care-giving role provided by post-fertile females contributed to the evolution of longevity in human populations. When post-fertile females provide care for weaned infants, mothers are able to have their next baby sooner without compromising the chances of survival of their previous offspring. Thus, the post-menopausal longevity that is unique to human populations may be an evolutionary adaptation. In this work, we construct, simulate, and analyze an ordinary differential equations mathematical model to study the grandmother hypothesis. Our model describes the passage of the individuals of a population through five life stages in the cases with and without grandmothering. We demonstrate via numerical simulation of the mathematical model that grandmothering care is sufficient to significantly increase adult life expectancy. We also investigate the relationship between the number of weaned infants that a post-fertile female can care for at a given time and the steady-state age distributions of a population.

Hired helpers at the nest: The association between life-cycle servants and net fertility in North Orkney, 1851-1911

OBJECTIVES

The presence of kin is often, but not always, associated with higher fertility in historical populations. However, the effect of other household members on fertility is less frequently studied. While not genetically related, life-cycle servants lived and worked alongside household members and may have provided assistance to reproducing families. Female servants in particular may have helped mothers with small children through direct help with childcare activities or by replacing the economic effort of mothers whose work was not compatible with childcare. This study examines the presence of servants in the households of married women of reproductive age to assess whether households with young children are more likely to also have servants.

MATERIALS AND METHODS

This study uses individual-level census data from North Orkney, Scotland (1851-1911) to investigate the relationship between the presence of servants in households and a measure of recent net marital fertility, the number of women's own-children under age 5, using logistic regression models.

RESULTS

Households with young children were more likely to have a female, but not male, servant in the household after controlling for the effects of other possible helpers, including older children, mothers, and mothers-in-law.

DISCUSSION

These findings are consistent with prior research that indicates the importance of female labor to smallholder agricultural households and suggests that female servants may have provided support to reproducing families. Life-cycle servants should be considered one component of biocultural reproduction in historical Northwest Europe. The use of hired help is not restricted to contemporary or elite groups.

Market integration reduces kin density in women's ego-networks in rural Poland

It is widely assumed that as populations become more market integrated the 'inner circles' of people's social networks become less densely connected and family-oriented. This 'loosening' of kin networks may fundamentally alter the social dynamics of reproduction, facilitating demographic transitions to low fertility. Few data exist to test this hypothesis. Previous research in urbanized populations has not explicitly measured kin density in ego-networks, nor assessed how market integration influences network structure at different levels of aggregation. Here I analyze the ego-networks of ~2000 women in 22 rural Polish communities transitioning from subsistence farming to market-dependence. I compare how ego-network size, density and kin density co-vary with household and community-level market integration. Market integration is associated with less kin-dense networks, but not necessarily less dense ones, and is unrelated to network size. Declining kin density during economic transitions may be a critical mechanism for the broader cultural transmission of low fertility values.

Linked Lives and Childhood Experience of Family Death on Educational Attainment

Sociological theory and research suggest that experiencing family members' deaths during childhood and adolescence is an important event subject to significant disparities. Previous research links immediate family members' deaths to poor life outcomes, but it considers a limited set of family members and has not tested the association of family member death with educational attainment. This study estimates the rates and educational impacts of experiencing the deaths of immediate (siblings, parents) and extended family members (aunts and uncles, cousins, and grandparents) during childhood and adolescence for Black and White Americans. We find that family death is associated with educational attainment, but the associations differ by family member type and gender, and child's race. Experiences of family death are unequally distributed by race and demonstrate complex associations with educational attainment. This research broadens life course and family systems theory by incorporating childhood family experiences of death on adult educational attainment and stratification.

Bone spoons for prehistoric babies: Detection of human teeth marks on the Neolithic artefacts from the site Grad-Starčevo (Serbia)

Around 8000 years ago, throughout the Neolithic world a new type of artefact appeared, small spoons masterly made from cattle bone, usually interpreted as tools, due to their intensive traces of use. Contrary to those interpretations, the small dimensions of spoons and presence of intensive traces of use led us to the assumption that they were used for feeding babies. In order to test that assumption we compared 2230 marks on three spoons from the Neolithic site of Grad-Starčevo in Serbia (5800-5450 cal BC) with 3151 primary teeth marks produced experimentally. This study has shown that some of the marks on spoons were made by primary teeth, which indicate their usage in feeding babies. The production of a new type of artefact to feed babies is probably related to the appearance of a new type of weaning food, and the abundance of spoons indicates that new baby gruels became an important innovation in prehistoric baby-care.

Family Matters: Rethinking the Psychology of Human Social Motivation

What motives do people prioritize in their social lives? Historically, social psychologists, especially those adopting an evolutionary perspective, have devoted a great deal of research attention to sexual attraction and romantic-partner choice (mate seeking). Research on long-term familial bonds (mate retention and kin care) has been less thoroughly connected to relevant comparative and evolutionary work on other species, and in the case of kin care, these bonds have been less well researched. Examining varied sources of data from 27 societies around the world, we found that people generally view familial motives as primary in importance and mate-seeking motives as relatively low in importance. Compared with other groups, college students, single people, and men place relatively higher emphasis on mate seeking, but even those samples rated kin-care motives as more important. Furthermore, motives linked to long-term familial bonds are positively associated with psychological well-being, but mate-seeking motives are associated with anxiety and depression. We address theoretical and empirical reasons why there has been extensive research on mate seeking and why people prioritize goals related to long-term familial bonds over mating goals. Reallocating relatively greater research effort toward long-term familial relationships would likely yield many interesting new findings relevant to everyday people’s highest social priorities.

Testing adaptive hypotheses of alloparenting in Agta foragers

Human children are frequently cared for by non-parental caregivers (alloparents), yet few studies have conducted systematic alternative hypothesis tests of why alloparents help. Here we explore whether predictions from kin selection, reciprocity, learning-to-mother and costly signalling hypotheses explain non-parental childcare among Agta hunter-gatherers from the Philippines. To test these hypotheses, we used high-resolution proximity data from 1,701 child-alloparent dyads. Our results indicated that reciprocity and relatedness were positively associated with the number of interactions with a child (our proxy for childcare). Need appeared more influential in close kin, suggesting indirect benefits, while reciprocity proved to be a stronger influence in non-kin, pointing to direct benefits. However, despite shared genes, close and distant kin interactions were also contingent on reciprocity. Compared with other apes, humans are unique in rapidly producing energetically demanding offspring. Our results suggest that the support that mothers require is met through support based on kinship and reciprocity.

The Impact of Parental Death in Childhood on Sons' and Daughters' Status Attainment in Young Adulthood in the Netherlands, 1850-1952

Previous research on the impact of parental loss on labor market outcomes in adulthood has often suffered from low sample sizes. To generate further insights into the long-term consequences of parental death, I use the Historical Sample of the Netherlands (HSN). The HSN contains occupational information on life courses of a sample of more than 8,000 males and almost 7,000 females born between 1850 and 1922, a period of important labor market transformations. Roughly 20 % of the sample population experienced parental death before age 16. Linear regression models show that maternal loss is significantly associated with lower occupational position in adulthood for both men and women, which points to the crucial importance of maternal care in childhood for socioeconomic outcomes in later life. This interpretation is supported by the finding that a stepmother's entry into the family is positively related with sons' occupational position later in life. In contrast to expectations, the loss of economic resources related to the father's death is generally not associated with lower status attainment in adulthood for men or for women. The results indicate, however, that the negative consequences of paternal death on men's socioeconomic outcomes decreased over time, illustrating the complex interaction between individual life courses and surrounding labor market transformations.

A Cohort Perspective on the Demography of Grandparenthood: Past, Present, and Future Changes in Race and Sex Disparities in the United States

How has the demography of grandparenthood changed over the last century? How have racial inequalities in grandparenthood changed, and how are they expected to change in the future? Massive improvements in mortality, increasing childlessness, and fertility postponement have profoundly altered the likelihood that people become grandparents as well as the timing and length of grandparenthood for those that do. The demography of grandparenthood is important to understand for those taking a multigenerational perspective of stratification and racial inequality because these processes define the onset and duration of intergenerational relationships in ways that constrain the forms and levels of intergenerational transfers that can occur within them. In this article, we discuss four measures of the demography of grandparenthood and use simulated data to estimate the broad contours of historical changes in the demography of grandparenthood in the United States for the 1880-1960 birth cohorts. Then we examine race and sex differences in grandparenthood in the past and present, which reveal declining inequality in the demography of grandparenthood and a projection of increasing group convergence in the coming decades.

Grandparental childcare, health and well-being in Europe: A within-individual investigation of longitudinal data

Previous studies suggest grandparental childcare is associated with improved health and well-being of grandparents but limited information on the causal nature of this association exists. Here, we use the longitudinal Survey of Health, Ageing and Retirement in Europe (SHARE) of people aged 50 and above across 11 countries including follow-up waves between 2004 and 2015 (n = 41,713 person-observations from 24,787 unique persons of whom 11,102 had two or more measurement times). Between-person and within-person (or fixed-effect) regressions were applied, where between-person models show associations across participants and within-person models focus on each participant's variation over time. Health and well-being were measured according to self-rated health, difficulties with activities of daily living (ADLs), depressive symptoms, life satisfaction and meaning of life scores. Across all analyses, childcare assistance provided by older adults to their adult children, was associated with increased health and well-being of grandparents. However, these associations were almost completely due to between-person differences and did not hold in within-person analyses that compared the same participants over time. Fewer ADL limitations for grandparents who provided childcare assistance was the only association that remained in the within-individual analyses. These findings suggest that there might be only limited causal association between grandchild care and grandparental well-being and that it may be specific to physical rather than cognitive factors. The results are discussed with regard to evolutionary psychology assumptions of altruistic behavior and positive health outcomes for the helper.

Fertility decline in the United States, 1850-1930: New Evidence from Complete-Count Datasets

Between 1835 and 1935, total fertility in the United States fell from 7.0 to 2.1. New IPUMS complete-count microdata databases of the 1850, 1880, 1910, and 1930 U. S. censuses allow us to study the fertility decline in more detail than previously possible. We construct comprehensive models of couples' fertility incorporating a wide variety of economic, social, cultural and familial factors, including measures of parental religiosity and kin availability outside of the household. The results indicate that while shifts in the occupational structure and increasing urbanization of the population provide the most consistent and substantive contribution to fertility decline over the period, cultural and religious attitudes - as proxied by parents' nativities and child naming practices - played a major role in couples' childbearing decisions.

Cross-cultural evidence does not support universal acceleration of puberty in father-absent households

Father absence in early life has been shown to be associated with accelerated reproductive development in girls. Evolutionary social scientists have proposed several adaptive hypotheses for this finding. Though there is variation in the detail of these hypotheses, they all assume that family environment in early life influences the development of life-history strategy, and, broadly, that early reproductive development is an adaptive response to father absence. Empirical evidence to support these hypotheses, however, has been derived from WEIRD (Western, Educated, Industrialized, Rich and Democratic) populations. Data from a much broader range of human societies are necessary in order to properly test adaptive hypotheses. Here, we review the empirical literature on father absence and puberty in both sexes, focusing on recent studies that have tested this association beyond the WEIRD world. We find that relationships between father absence and age at puberty are more varied in contexts beyond WEIRD societies, and when relationships beyond the father-daughter dyad are considered. This has implications for our understanding of how early-life environment is linked to life-history strategies, and for our understanding of pathways to adult health outcomes, given that early reproductive development may be linked to negative health outcomes in later life This article is part of the theme issue 'Developing differences: early-life effects and evolutionary medicine'.

Integration involves a trade-off between fertility and status for World War II evacuees

Understanding how refugees integrate into host societies has broad implications for researchers interested in intergroup conflict and for governments concerned with promoting social cohesion. Using detailed records tracking the movements and life histories of Finnish evacuees during World War II, we find that evacuees who intermarry are more likely to be educated, work in professional occupations, marry someone higher in social status and remain in the host community. Evacuees who intermarry before the war have fewer children, whereas those who marry into their host community after the war have more children. These results indicate that life-history and assimilation outcomes depend on key differences between pre-war environments-when migrants are living in their own communities-and post-war environments-when migrants are living in the host community. Overall, this suggests that integration involves a trade-off between reproduction and status such that evacuees who integrate gain social status, whereas those who maintain stronger bonds with their natal communities have higher fertility. We discuss these results within the framework of social capital, intergroup conflict and life-history theory and suggest how they can inform our understanding of evolutionary adaptations that affect tribalism.

Limits to Fitness Benefits of Prolonged Post-reproductive Lifespan in Women

Recent advances in medicine and life-expectancy gains have fueled multidisciplinary research into the limits of human lifespan [1-3]. Ultimately, how long humans can live for may depend on selection favoring extended longevity in our evolutionary past [4]. Human females have an unusually extended post-reproductive lifespan, which has been explained by the fitness benefits provided from helping to raise grandchildren following menopause [5, 6]. However, formal tests of whether such grandmothering benefits wane with grandmother age and explain the observed length of post-reproductive lifespan are missing. This is critical for understanding prevailing selection pressures on longevity but to date has been overlooked as a possible mechanism driving the evolution of lifespan. Here, we use extensive data from pre-industrial humans to show that fitness gains from grandmothering are dependent on grandmother age, affecting selection on the length of post-reproductive lifespan. We find both opportunities and ability to help grandchildren declined with age, while the hazard of death of women increased greatly in their late 60s and 70s compared to menopausal ages, together implying waning selection on subsequent longevity. The presence of maternal grandmothers aged 50-75 increased grandchild survival after weaning, confirming the fitness advantage of post-reproductive lifespan. However, co-residence with paternal grandmothers aged 75+ was detrimental to grandchild survival, with those grandmothers close to death and presumably in poorer health particularly associated with lower grandchild survival. The age limitations of gaining inclusive fitness from grandmothering suggests that grandmothering can select for post-reproductive longevity only up to a certain point.

Using Geographic Distance as a Potential Proxy for Help in the Assessment of the Grandmother Hypothesis

Life-history theory predicts that selection could favor the decoupling of somatic and reproductive senescence if post-reproductive lifespan (PRLS) provides additional indirect fitness benefits [1, 2]. The grandmother hypothesis proposes that prolonged PRLS evolved because post-reproductive grandmothers gain inclusive fitness benefits by helping their daughters and grandchildren [3, 4]. Because most historical human data do not report direct evidence of help, we hypothesized that geographic distance between individuals may be inversely related to their capacity to help. Using an exceptionally detailed dataset of pre-industrial French settlers in the St. Lawrence Valley during the 17^th and 18^th centuries, we assessed the potential for grandmothers to improve their inclusive fitness by helping their descendants, and we evaluated how this effect varied with geographic distance, ranging between 0 and 325 km, while accounting for potential familial genetic and environmental effects [5-9]. Grandmothers (F0) who were alive allowed their daughters (F1) to increase their number of offspring (F2) born by 2.1 and to increase their number of offspring surviving to 15 years of age by 1.1 compared to when grandmothers were dead. However, the age at first reproduction was not influenced by the life status (alive or dead) of grandmothers. As geographic distance increased, the number of offspring born and lifetime reproductive success decreased, while the age at first reproduction increased, despite the grandmother being alive in these analyses. Our study suggests that geographic proximity has the potential to modulate inclusive fitness, supporting the grandmother hypothesis, and to contribute to our understanding of the evolution of PRLS.

Kin support and child health: Investigating two approaches in an African slum

Extensive research from sub-Saharan Africa shows that mothers frequently rely on help from other family members to ensure their children's health and well-being. Yet, there is considerable debate about the relative importance of support from grandmothers versus fathers. Using an innovative survey instrument to interview 462 unmarried mothers in a slum area of Nairobi, Kenya, we provide insight into this debate by showing that a status versus transfers approach to measuring kin support asks subtly different questions and yields different results. A status approach reflects an evolutionary perspective that argues that maternal grandmothers have a greater incentive than non-residential fathers to provide material and practical support for young children. In contrast, a transfers approach is consistent with social support theories whereby the social capital provided by fathers may be more beneficial to children's health than that afforded by grandmothers. Demonstrating that different approaches to measuring kin support matter highlights the need for kin research that crosses disciplinary boundaries and encourages the development of more nuanced family policies designed to protect children's health in Africa.

Do grandparents influence parents' decision to vaccinate their children? A systematic review

The global reduction in childhood infectious diseases since the 1960s is primarily due to the success of extensive worldwide immunisation campaigns. However, the universal vaccination coverage program appears to have lost momentum in the wake of negative, unfounded claims about the safety of vaccines. While parents of the 21st century have little first-hand knowledge of devastating childhood diseases, grandparents are more likely to remember family and community members who were afflicted. In the current age of vaccine hesitancy and science scepticism, where research-informed arguments are not always persuasive, grandparents, through their experience of the diseases, may positively influence paediatric vaccine uptake. This paper reviews the literature investigating potential direct or indirect influences of grandparents on parents' decisions to vaccinate their children. A database search using the keywords immunisation, vaccination, children and grandparents resulted in 1988 articles. Titles were screened for relevance and seventy-seven results were retained. After the abstracts were read, only five articles that either explored paediatric vaccines, factors promoting and/or inhibiting paediatric vaccine use and decision-making strategies were reviewed. One paper located through Google Scholar, which failed to show up on database searches, was also retained for a total of six papers. While none of the six papers set out to explore the impact of grandparents on vaccine uptake, they found that grandparents were involved to varying degrees in paediatric vaccine uptake within young families. The research clearly showing that grandparents, and older people more generally, promote vaccination uptake is not currently available. The dearth of literature shows the need for research exploring the perceived and real influences of grandparents on childhood vaccination. This will establish whether grandparents' memory and knowledge of preventable childhood infectious diseases could be harnessed as a public health measure to counteract the current, ill-informed, negative attention on paediatric vaccines.

Grandmotherhood across the demographic transition

Grandmothers provide key care to their grandchildren in both contemporary and historic human populations. The length of the grandmother-grandchild relationship provides a basis for such interactions, but its variation and determinants have rarely been studied in different contexts, despite changes in age-specific mortality and fertility rates likely having affected grandmotherhood patterns across the demographic transition. Understanding how often and long grandmothers have been available for their grandchildren in different conditions may help explain the large differences between grandmaternal effects found in different societies, and is vital for developing theories concerning the evolution of menopause, post-reproductive longevity, and family living. Using an extensive genealogical dataset from Finland spanning the demographic transition, we quantify the length of grandmotherhood and its determinants from 1790–1959. We found that shared time between grandmothers and grandchildren was consistently low before the demographic transition, only increasing greatly during the 20^th century. Whilst reduced childhood mortality and increasing adult longevity had a role in this change, grandmaternal age at birth remained consistent across the study period. Our findings further understanding of the temporal context of grandmother-grandchild relationships, and emphasise the need to consider the demography of grandmotherhood in a number of disciplines, including biology (e.g. evolution of the family), sociology (e.g. changing family structures), population health (e.g. changing age structures), and economics (e.g. workforce retention).

Early-life mortality clustering in families: A literature review

Research on early-life mortality in contemporary and historical populations has shown that infant and child mortality tend to cluster in a limited number of high-mortality families, a phenomenon known as 'mortality clustering'. This paper is the first to review the literature on the role of the family in early-life mortality. Contemporary results, methodological and theoretical shortfalls, recent developments, and opportunities for future research are all discussed in this review. Four methodological approaches are distinguished: those based on sibling deaths, mother heterogeneity, thresholds, and excess deaths in populations. It has become clear from research to date that the death of an older child harms the survival chances of younger children in that family, and that fertility behaviour, earlier stillbirths, remarriages, and socio-economic status all explain mortality clustering to some extent.

Evolutionary demography of age at last birth: integrating approaches from human behavioural ecology and cultural evolution

Cultural evolutionary theory and human behavioural ecology offer different, but compatible approaches to understanding human demographic behaviour. For much of their 30 history, these approaches have been deployed in parallel, with few explicit attempts to integrate them empirically. In this paper, we test hypotheses drawn from both approaches to explore how reproductive behaviour responds to cultural changes among Mosuo agriculturalists of China. Specifically, we focus on how age at last birth (ALB) varies in association with temporal shifts in fertility policies, spatial variation and kinship ecologies. We interpret temporal declines in ALB as plausibly consistent with demographic front-loading of reproduction in light of fertility constraints and later ages at last birth in matrilineal populations relative to patrilineal ones as consistent with greater household cooperation for reproductive purposes in the former. We find little evidence suggesting specific transmission pathways for the spread of norms regulating ALB, but emphasize that the rapid pace of change strongly suggests that learning processes were involved in the general decline in ALB over time. The different predictions of models we employ belie their considerable overlap and the potential for a synthetic approach to generate more refined tests of evolutionary hypotheses of demographic behaviour.This article is part of the theme issue 'Bridging cultural gaps: interdisciplinary studies in human cultural evolution'.

Who Supports Breastfeeding Mothers? : An Investigation of Kin Investment in the United States

Breastfeeding is one important form of maternal investment that is influenced by support from kin and non-kin. This paper investigates who provides support for breastfeeding mothers and their children, what type of support they provide, and how support impacts breastfeeding duration. The data were derived from a survey of 594 American mothers and were analyzed using quantitative methods, including Cox regression. Analyses indicate that mothers receive significant support, particularly from spouses and maternal grandmothers. More frequent breastfeeding discussions with La Leche League and maternal grandfathers were associated with longer duration, whereas discussions with physicians were associated with shorter breastfeeding duration. Results indicate that consulting others specifically about breastfeeding may influence breastfeeding decisions. The results are consistent with the idea that social support may influence breastfeeding duration and that some types of support are more influential than others. Furthermore, support persons should be educated about breastfeeding to prevent early weaning.

Males' Later-Life Mortality Consequences of Coresidence With Paternal Grandparents: Evidence From Northeast China, 1789-1909

In this study, we investigate the effect of early-life coresidence with paternal grandparents on male mortality risks in adulthood and older age in northeast China from 1789 to 1909. Despite growing interest in the influence of grandparents on child outcomes, few studies have examined the effect of coresidence with grandparents in early life on mortality in later life. We find that coresidence with paternal grandmothers in childhood is associated with higher mortality risks for males in adulthood. This may reflect the long-term effects of conflicts between mothers and their mothers-in-law. These results suggest that in extended families, patterns of coresidence in childhood may have long-term consequences for mortality, above and beyond the effects of common environmental and genetic factors, even when effects on childhood mortality are not readily apparent.

The impact of kin availability, parental religiosity, and nativity on fertility differentials in the late 19th-century United States

METHODS

Most quantitative research on fertility decline in the United States ignores the potential impact of cultural and familial factors. We rely on new complete-count data from the 1880 U.S. census to construct couple-level measures of nativity/ethnicity, religiosity, and kin availability. We include these measures with a comprehensive set of demographic, economic, and contextual variables in Poisson regression models of net marital fertility to assess their relative importance. We construct models with and without area fixed effects to control for unobserved heterogeneity.

CONTRIBUTION

All else being equal, we find a strong impact of nativity on recent net marital fertility. Fertility differentials among second generation couples relative to the native-born white population of native parentage were in most cases less than half of the differential observed among first generation immigrants, suggesting greater assimilation to native-born American childbearing norms. Our measures of parental religiosity and familial propinquity indicated a more modest impact on marital fertility. Couples who chose biblical names for their children had approximately 3% more children than couples relying on secular names while the presence of a potential mother-in-law in a nearby households was associated with 2% more children. Overall, our results demonstrate the need for more inclusive models of fertility behavior that include cultural and familial covariates.

Living with own or husband's mother in the household is associated with lower number of children: a cross-cultural analysis

It has been discussed in the literature that the presence of kin, particularly the presence of a women's own mother or her mother in law, may have fertility effects. We aimed to examine the effects of the presence of a woman's own or her husband's mother in the household on a woman's fertility in terms of number of children on a broad basis by analysing census data of over two million married women aged between 15 and 34 years from 14 countries worldwide. We find that with the exception of Iraq, across all countries, the majority of women live only with their spouse in the household. We further find that the presence of any mother in the household is invariably associated with a significantly lower number of children compared to women living only with their spouse. In addition, in most countries, a woman's number of children is lower if she lives with her own mother as compared to her husband's mother in the household. Number of children is nonlinearly associated with woman's age, the presence of any mother being related with an earlier start of childbearing but a shallower increase in number of children. We speculate that the presence of a mother in the household may slow down woman's reproduction, but also discuss alternative explanations.

Beyond Orphanhood: Parental Nonresidence and Child Well-being in Tanzania

This article used data from the Rufiji Health and Demographic Surveillance System in Tanzania to examine the influence of parental non-residence on child survival and school entry. Using survival analysis methods, the article tested variations by parent and by cause, examining parental death, non-residence due to parental relationship status, and migration. In general, maternal non-residence was more consequential for child survival, while paternal non-residence influences school entry. This is consistent with gendered parenting patterns in the setting. There was important variation by cause and by outcome, particularly for paternal non-residence. Paternal non-residence due to non-marital birth was associated with increased risk of child death, while paternal migration was associated with improved survival. Paternal death and migration were associated with lower odds of school entry. This article moves beyond orphanhood to consider multiple causes of parental non-residence simultaneously, demonstrating that parental non-residence is not uniformly deleterious for children.

Multigenerational Effects on Children's Cognitive and Socioemotional Outcomes: A Within-Child Investigation

Associations between grandparental investment and child outcomes were investigated using three waves of a longitudinal British Millennium Cohort Study that included children between the ages of 9 months and 5 years (n = 24,614 person-observations from 13,744 children). Grandparental investment was measured by parent-grandparent contact frequency and grandparental financial support. Child cognitive development was measured using the British Ability Scale and socioemotional outcomes using the Strength and Difficulties Questionnaire. Grandparental investment was associated with improved cognitive and socioemotional outcomes among children. However, these associations occurred because of between-person effects and did not exist in within-person analyses that compared the same children over time. The results are discussed in terms of their contribution to multigenerational relationships research.

HIGH PREVALENCE OF VOLUNTARY STERILIZATION AMONG AMERICAN WOMEN EXPLAINED BY TRADE-OFFS RESULTING FROM MALE PARENTAL COMMITMENT

Tubal ligation is the modal form of family planning among American women aged 30 and older. As the preference for tubal ligation over cheaper, lower risk and more reliable methods, such as vasectomy, has puzzled experts, a theoretical approach that explains this preference would be useful. The present study investigates the high prevalence of voluntary sterilization among American women from the perspective of life history theory, arguing that the trade-offs between investing in current and future offspring will favour tubal ligation when women cannot obtain reliable male commitment to future parental investment. Data came from the National Survey of Fertility Barriers (NSFB), a nationally representative survey of 4712 American women aged 25-45 conducted between 2004 and 2007. Four novel predictions of the prevalence of tubal ligation, drawn from life history theory, were developed and tested: 1) it is most common among unpartnered women with children, and least common among married women with children; 2) it is negatively correlated with age at first birth; 3) it is least common among highly educated women without children, and most common among less educated women with children; and 4) among women with two or more children, it is positively correlated with lifetime number of long-term partners. These predictions were tested using multivariate regression analysis. The first prediction was not supported: women with children were more likely to be sterilized, regardless of their marital status. The other three predictions were all supported by the data. The results suggest that trade-offs influence women's decisions to undergo voluntary sterilization. Women are most likely to opt for tubal ligation when the costs of an additional child will impinge on their ability to invest in existing offspring, especially in the context of reduced male commitment.

Intergenerational relations before and after offspring arrive: A within-person investigation

The birth of a child may re-orientate the relations between adult children and their parents; however, the previous studies on the topic are both scarce and methodologically limited. The current study investigates whether younger adults' entry into parenthood (i.e., the birth of the first child) is associated with increased contact frequency, emotional closeness, intimacy and conflict with their own parents. The participants are from the German Panel Analysis of Intimate Relationships and Family Dynamics, which is a prospective survey of younger adults with six annual follow-up waves between 2009 and 2014 (n = 17,662 person-observations from 4821 persons). Within-person regression with a focus on each individual's variation over time was used to detect changes in intergenerational relations. The contact frequency between daughters and mothers increased after the arrival of their first child. In contrast, the emotional closeness and intimacy between sons and mothers decreased after the birth of their first child. The results are discussed with reference to gender-based differences in intergenerational relations.

Support for new mothers and fertility in the United Kingdom: Not all support is equal in the decision to have a second child

Low fertility across Europe highlights the need to understand reproductive decisions in high-income countries better. Availability of support may be one factor influencing reproductive decisions, though within high-income countries availability varies between environments, including socio-economic environments. We test whether receiving higher levels of support, from different sources (informal and formal) and of different types (practical and emotional), is positively correlated with second births in the United Kingdom (UK) Millennium Cohort Study, and whether these relationships differ by socio-economic position (SEP). Our hypothesis is only partially supported: receiving emotional support correlates with higher likelihood of second birth, but the opposite is true for practical support. Availability of different types of support varies across SEP, but relationships between support and fertility are similar, with one exception: kin-provided childcare increases the likelihood of birth only among lower-SEP women. Our results highlight that not all support is equal in the decision to have a second child.

Human reproduction and health: an evolutionary perspective

According to life history theory, increased investment in reproductive function (physiology and behaviour) at different times throughout the life course affects the risk of many diseases and, ultimately, longevity. Although genetic factors contribute to interindividual and interpopulation variation in reproductive traits, the dominant source of variability is phenotypic plasticity during development and adult life. Reproductive traits in both sexes evolved sensitivity to ecological conditions, as reflected in contemporary associations of hormone concentrations with geographical setting, nutritional status, and physical activity level. Lifetime exposure to increased concentrations of sex hormones is associated with the risk of some cancers, hence decreasing fertility patterns contribute to secular increases in their incidence. Conversely, increased investment in reproductive function might compromise somatic investment in health, such that faster sexual maturation and higher parity increases risk of diabetes and cardiovascular disease. An evolutionary perspective on reproductive biology could improve the efficacy of public health efforts to reduce the risk of hormone-sensitive cancers and other non-communicable diseases.

Father absence and gendered traits in sons and daughters

Research has previously found a number of apparently contradictory patterns in the relationship between ‘father absence’ (having a non-resident father during childhood) and the expression of gender roles, as well as other sexually dimorphic traits such as aggression. In the current study we measured a battery of sexually differentiated traits in relation to family background. 133 men and 558 women from the United States and Australia completed the Bem Sex Role Inventory, the Barrett Impulsivity Scale, the Fear Survey Schedule and the Buss & Perry Aggression Questionnaire. Principal components analysis found two main axes of variation in these traits. Firstly, a general ‘reactivity’ factor, on which aggression, impulsivity, and fear all loaded positively, was weakly associated with father absence in women. Secondly, ‘masculinity’ (consisting of high scores on masculine traits, low fear, and physical and verbal aggression) was not associated with father absence. Participants (except American males) reporting a poor childhood relationship with their parents also had high ‘reactivity’ but not higher ‘masculinity’. We found some evidence of a link between father absence and earlier age of first coitus in American females (although not in Australia), but there was no link with age of menarche in either country. Overall, the current results suggest that previous findings linking gender development with father absence in girls may have arisen from a tendency towards greater externalising and reactive behaviour rather than a change in gender development per se.

A prospective study of associations among helping, health, and longevity

How does helping behavior contribute to the health and the longevity of older helpers? From an evolutionary perspective, the ultimate cause may be rooted in ancestral parenting and grandparenting. These activities may have generalized to a neural and hormonal caregiving system that also enabled prosocial behavior beyond the family. From a psychological perspective, helping others may be associated with healthy aging, which, in turn, contributes to longevity as a proximate cause. Yet little is known about the extent to which mediating factors such as the health benefits of helping behaviors translate into enhanced longevity, particularly in regard to grandparenting. To fill this gap, we conducted mediation analyses (structural equation models) to examine whether grandparenting and supporting others in the social network contributed directly or indirectly (through better health 5-6 years later) to the longevity of older helpers. We drew on longitudinal data from the Berlin Aging Study (N = 516), in which older adults in Berlin, Germany, were interviewed at baseline (1990-1993, mean age at entry = 85 years) and continuously followed up until 2009. Results suggest that the associations of both grandparenting and supporting others with enhanced longevity are mediated by better prospective health (indirect effect). The effect of helping was not fully mediated, however-helping was also directly associated with increased longevity independently of the health indicators measured. The results were robust against effects of the helper's preexisting health status and sociodemographic characteristics of participants, their children, and grandchildren. We conclude that better prospective health contributes to the link between helping and longevity, but does not fully account for it. Other potential contributing mechanisms remain to be identified. As populations age across the globe, identifying mechanisms that foster health in old age can help to highlight potential targets for public health interventions.

The Reproductive Ecology of Industrial Societies, Part I : Why Measuring Fertility Matters

Is fertility relevant to evolutionary analyses conducted in modern industrial societies? This question has been the subject of a highly contentious debate, beginning in the late 1980s and continuing to this day. Researchers in both evolutionary and social sciences have argued that the measurement of fitness-related traits (e.g., fertility) offers little insight into evolutionary processes, on the grounds that modern industrial environments differ so greatly from those of our ancestral past that our behavior can no longer be expected to be adaptive. In contrast, we argue that fertility measurements in industrial society are essential for a complete evolutionary analysis: in particular, such data can provide evidence for any putative adaptive mismatch between ancestral environments and those of the present day, and they can provide insight into the selection pressures currently operating on contemporary populations. Having made this positive case, we then go on to discuss some challenges of fertility-related analyses among industrialized populations, particularly those that involve large-scale databases. These include “researcher degrees of freedom” (i.e., the choices made about which variables to analyze and how) and the different biases that may exist in such data. Despite these concerns, large datasets from multiple populations represent an excellent opportunity to test evolutionary hypotheses in great detail, enriching the evolutionary understanding of human behavior.

Modernizing Evolutionary Anthropology : Introduction to the Special Issue

Evolutionary anthropology has traditionally focused on the study of small-scale, largely self-sufficient societies. The increasing rarity of these societies underscores the importance of such research yet also suggests the need to understand the processes by which such societies are being lost-what we call "modernization"-and the effects of these processes on human behavior and biology. In this article, we discuss recent efforts by evolutionary anthropologists to incorporate modernization into their research and the challenges and rewards that follow. Advantages include that these studies allow for explicit testing of hypotheses that explore how behavior and biology change in conjunction with changes in social, economic, and ecological factors. In addition, modernization often provides a source of "natural experiments" since it may proceed in a piecemeal fashion through a population. Challenges arise, however, in association with reduced variability in fitness proxies such as fertility, and with the increasing use of relatively novel methodologies in evolutionary anthropology, such as the analysis of secondary data. Confronting these challenges will require careful consideration but will lead to an improved understanding of humanity. We conclude that the study of modernization offers the prospect of developing a richer evolutionary anthropology, by encompassing ultimate and proximate explanations for behavior expressed across the full range of human societies.

Marital Dissolution and Child Educational Outcomes in San Borja, Bolivia

Serial monogamy is likely an adaptive mating strategy for women when the expected future fitness gains with a different partner are greater than expected future fitness with one's current partner. Using interview data from more than 400 women in San Borja, Bolivia, discrete-time event history analyses and random effects regression analyses were conducted to examine predictors of marital dissolution, separated by remarriage status, and child educational outcomes. Male income was found to be inversely associated with women's risk of "divorce and remarriage," whereas female income is positively associated with women's risk of "divorce, but not remarriage." Children of women who divorce and remarry tend to have significantly lower educational outcomes than children of married parents, but women with higher incomes are able to buffer their children from the negative educational outcomes of divorce and remarriage. Counter to predictions, there is no evidence that women with kin in the community have a significant difference in likelihood of divorce or a buffering effect of child outcomes. In conclusion, predictors of divorce differ depending on whether the woman goes on to remarry, suggesting that male income may be a better predictor of a serial monogamy strategy whereas female income predicts marital dissolution only. Thus, women who are relatively autonomous because of greater income may not benefit from remarriage.

Intergenerational relations and child development in England

Evolutionary studies have shown that in many traditional populations the beneficial effects of grandparental presence for grandchildren may vary according to the sex and lineage of the grandparents, as well as by the sex of the grandchild. However, few studies have investigated the relevance of these factors in modern developed societies. The present investigation uses the Millennium Cohort Study (n = 4,636 children) to analyse the association between grandparental investment and child development in contemporary England. Grandparental investment is measured by parent-grandparent contact frequencies at the child’s age of 3 and child development by “early learning goals” over the first year of primary school assessed with the Foundation Stage Profile (FSP). Children whose mothers reported contacts with maternal grandparents receive higher FSP scores compared to those with no contact at all. In addition, children whose fathers reported daily contacts with paternal grandfathers have lower FSP scores than other children. The study provides evidence of the relevance of grandparental investment on grandchild development also in developed societies. The results are discussed with reference to the grandmother hypothesis, sex-specific reproductive strategies and sex chromosome hypothesis.