Previous research indicates that birth order is a strong predictor of familial sentiments, with middleborns less family-oriented than first- or last-borns. In this research, effects of sex and birth order on the actual frequency of contact with maternal and paternal kin were examined in two studies. In Study 1, one hundred and forty undergraduates completed a questionnaire relating to the amount of time they spent in contact with specific relatives, while in Study 2, one hundred and twelve undergraduates completed the (...) same questionnaire with the addition of two questions relating to the subjects’ parents’ birth orders. Subjects were more likely to have frequent contact with maternal, as opposed to paternal, kin and women experienced more frequent contact than men with relatives in general. The birth order of subjects did not appear to have a significant influence on contact but the birth order of the subjects’ parents did, with the offspring of middleborn mothers having relatively little contact with maternal grandparents and the offspring of middleborn fathers having relatively little contact with paternal grandparents. These sex and birth order differences are discussed in relation to possible differences in how women and men use kinship ties and in terms of how birth order may influence parental solicitude. (shrink)

Young men are more distressed by a partner’s sexual infidelity, whereas young women are more distressed by a partner’s emotional infidelity. The present research investigated (a) whether the sex difference in jealousy replicates in an older sample, and (b) whether younger people differ from older people in their selection of the more distressing infidelity scenario. We presented forced-choice dilemmas to 202 older people (mean age = 67 years) and to 234 younger people (mean age = 20 years). The sex difference (...) replicated in the older sample. In addition, older women were less likely than younger women to select a partner’s emotional infidelity as more distressing than a partner’s sexual infidelity. Discussion offers directions for future work on sex differences and age differences in jealousy. (shrink)

Menopause is an evolutionary mystery: how could living longer with no capacity to reproduce possibly be advantageous? Several explanations have been offered for why female humans, unlike our closest primate relatives, have such an extensive post-reproductive lifespan. Proponents of the so-called “grandmother hypothesis” suggest that older women are able to increase their fitness by helping to care for their grandchildren as allomothers. This paper first distinguishes the grandmother hypothesis from several other hypotheses that attempt to explain menopause, and then develops (...) a formal model by which these hypotheses can be compared and tested by empirical researchers. The model is then modified and used to respond to a common objection to the grandmother hypothesis: that human fathers, rather than grandmothers, are better suited to be allomothers due to their physical strength and a high incentive to invest in their own children. However, fathers—unlike maternal grandmothers—can never be sure that the children they are caring for are their own. Incorporating paternity uncertainty into the model demonstrates the conditions under which the grandmother hypothesis is more plausible than a hypothesis that focuses on the contributions of men. (shrink)

Many studies in Western societies show a pattern of discriminative grandparental investment as follows: maternal grandmothers (MGMs) > maternal grandfathers (MGFs) > paternal grandmothers (PGMs) > paternal grandfathers (PGFs). This pattern is in line with the expectation from evolutionary reasoning. Yet whether or not this pattern applies in China is in question. The present study was based on a questionnaire survey at a university in Central China (N = 1,195). Results show that (1) when grandparent–grandchild residential distance during grandchildren’s childhood (...) is controlled, in the case of grandsons and granddaughters as a whole and granddaughters only, both grandparental caregiving and grandchildren’s emotional closeness to grandparents display a rank order of MGM > MGF > PGM > PGF, but in the case of grandsons only, this order is not statistically significant. (2) There are stable relationships between grandparental caregiving/grandchildren’s emotional closeness and residential distance/similarity in appearance. (3) The effects of residential distance on either PGFs’ or PGMs’ caregiving exceed those on either MGFs’ or MGMs’. (4) The PGF and PGM prefer grandsons to granddaughters in their caregiving, whereas the MGF and MGM do not have a sex preference, and (5) the fact that the PGF and PGM invest more in grandsons than in granddaughters does not depend on grandsons’ duration of living in a rural area. Our results suggest that (1) in general, the Chinese display a pattern of differential grandparental investment predicted by an evolutionary perspective, (2) the evolutionary perspective that combines the two factors of paternal uncertainty and sex-specific reproductive strategies is applicable to grandparental investment in China, and (3) the traditional son-preference culture also plays some role in affecting grandparental investment in China, though the roles of culture and urban–rural cultural difference should not be exaggerated. (shrink)

Relatives play an important role in human reproduction according to evolutionary theories of reproductive behavior, but previous empirical studies show large differences in the effects of kin on fertility outcomes. In our paper we examine the effect of co-resident kin and non-kin on the length of birth intervals over the reproductive life course of Dutch women born between 1842 and 1920. We estimate Cox proportional hazard models for parity progression based on the presence of kin and non-kin in the household (...) while controlling for a large number of individual and community-level characteristics. We find that couples living with their brothers experienced shorter birth intervals whereas couples residing with a widowed father had relatively longer birth intervals. The effects of these types of kin on reproduction were most pronounced up to the birth of the fifth child, but not thereafter. We found no effect for mothers or other types of kin. (shrink)

In a study of the kin investment of aunts and uncles we show that the laterality effect expected as a result of paternity uncertainty is statistically reliable but somewhat smaller than the sex effect. Matrilateral aunts invest significantly more than patrilateral aunts, and the same is true for uncles. Regardless of laterality, however, aunts invest significantly more than uncles. Multivariate controls show that the matrilateral bias is fully independent of any age or distance confounds that might result from sex differences (...) in age at marriage or dispersal. We discuss our results in relation to recent findings on the kin investment of grandparents (Euler and Weitzel 1996). In addition, we propose a simple method for estimating the level of paternity uncertainty from kin investment data; application of this method to our data on aunts and uncles suggests that between 13% and 20% of children are not the offspring of their putative father. Our parallel analyses of Euler and Weitzel’s (1996) data on grandparental investment suggest a similar estimate, that paternity uncertainty lies between 9% and 17%. (shrink)

The presence of a kinship link between nuclear families is the strongest predictor of interhousehold sharing in an indigenous, predominantly Dolgan food-sharing network in northern Russia. Attributes such as the summed number of hunters in paired households also account for much of the variation in sharing between nuclear families. Differences in the number of hunters in partner households, as well as proximity and producer/consumer ratios of households, were investigated with regard to cost-benefit models. The subset of households involved in reciprocal (...) meal sharing is 26 of 84 household host-guest pairs. The frequency of reciprocal meal sharing between families in this subset is positively correlated with average household relatedness. The evolution of cooperation through clustering may illuminate the relationship between kinship and reciprocity at this most intimate level of food sharing. (shrink)

Paternity certainty and matrilineal family ties have been used to explain the asymmetric caregiving of grandparents and aunts and uncles. The proximate mechanisms underlying biased kin investment, however, remain unclear. A central question of the study presented here was whether the parent-kin relationship is an important link in the caregiving. In a two-generational questionnaire study, we asked subjects to estimate the intensity of their relationships to parents, grandparents, aunts, and uncles (emotional closeness, investment received in childhood). In addition, the subjects’ (...) parents rated their emotional closeness to their parents and siblings. We found that the parent-kin relationship was closely linked to the relatives’ child care and could partly explain asymmetric caregiving. Maternal aunts played a special role as caregivers. Especially the mother’s younger or last-born sister cared intensively for nieces and nephews, regardless of her closeness to the subjects’ mother. (shrink)

Low birth rates in developed societies reflect women’s difficulties in combining work and motherhood. While demographic research has focused on the role of formal childcare in easing this dilemma, evolutionary theory points to the importance of kin. The cooperative breeding hypothesis states that the wider kin group has facilitated women’s reproduction during our evolutionary history. This mechanism has been demonstrated in pre-industrial societies, but there is no direct evidence of beneficial effects of kin’s support on parents’ reproduction in modern societies. (...) Using three-generation longitudinal data anchored in a sample of grandparents aged 55 and over in 1992 in the Netherlands, we show that childcare support from grandparents increases the probability that parents have additional children in the next 8 to 10 years. Grandparental childcare provided to a nephew or niece of childless children did not significantly increase the probability that those children started a family. These results suggest that childcare support by grandparents can enhance their children’s reproductive success in modern societies and is an important factor in people’s fertility decisions, along with the availability of formal childcare. (shrink)

This response outlines more reasons why we need the integrative framework of grandparental investments and intergenerational transfers that we advocated in the target article. We discusses obstacles that stand in the way of such a framework and of a better understanding of the effects of grandparenting in the developed world. We highlight new research directions that have emerged from the commentaries, and we end by discussing some of the things in our target article about which we may have been wrong.

Supporting Hamilton’s inclusive fitness theory, archival analyses of inheritance patterns in wills have revealed that people invest more of their estates in kin of closer genetic relatedness. Recent classroom experiments have shown that this genetic relatedness effect is stronger for relatives of direct lineage (children, grandchildren) than for relatives of collateral lineage (siblings, nieces, nephews). In the present research, multilevel modeling of more than 1,000 British Columbian wills revealed a positive effect of genetic relatedness on proportions of estates allocated to (...) relatives. This effect was qualified by an interaction with lineage, such that it was stronger for direct than for collateral relatives. Exploratory analyses of the moderating role of benefactors’ sex and estate values showed the genetic relatedness effect was stronger among female and wealthier benefactors. The importance of these moderators to understanding kin investment in modern humans is discussed. (shrink)

The prevalence of nonpaternity in human societies is difficult to establish. To obtain a current and fairly unbiased estimate of the nonpaternity rate in Germany, we analysed a dataset consisting of 971 children and their parents in whom human leukocyte antigen (HLA) typing had been carried out in the context of bone marrow transplantation. In this sample, nine exclusions (0.93%) could be identified on the basis of more than 300 HLA-haplotypes defined by four HLA genes. Given this number of exclusions, (...) a maximum likelihood estimate of the nonpaternity rate in the population of 0.94% was obtained with asymptotic 95% confidence limits of 0.33% and 1.55%, respectively. This result is in accordance with recent surveys as well as findings from Switzerland for a comparable sample, and it suggests that earlier estimates of the nonpaternity rate which were often in excess of 10% may have been largely exaggerated. (shrink)

Dispersal of individuals from their natal communities at sexual maturity is an important determinant of kin association. In this paper we compare postmarital residence patterns among Pumé foragers of Venezuela to investigate the prevalence of sex-biased vs. bilateral residence. This study complements cross-cultural overviews by examining postmarital kin association in relation to individual, longitudinal data on residence within a forager society. Based on cultural norms, the Pumé have been characterized as matrilocal. Analysis of Pumé marriages over a 25-year period finds (...) a predominant pattern of natalocal residence. We emphasize that natalocality, bilocality, and multilocality accomplish similar ends in maximizing bilateral kin affiliations in contrast to sex-biased residential patterns. Bilateral kin association may be especially important in foraging economies where subsistence activities change throughout the year and large kin networks permit greater potential flexibility in residential mobility. (shrink)

One of the central tenets of Christian theology is the denial of self for the benefit of another. However, many views on the evolution of altruism presume that natural selection inevitably leads to a self-seeking human nature and that altruism is merely a façade to cover underlying selfish motives. I argue that human altruism is an emergent characteristic that cannot be reduced to any one particular evolutionary explanation. The evolutionary processes at work in the formation of human nature are not (...) necessarily in conflict with the possibility of altruism; rather, aspects of human nature are uniquely directed toward the care and concern of others. The relationship between altruism, human nature, and evolution can be reimagined by adopting an emergent view of the hierarchy of science and a theological worldview that emphasizes self-renunciation. The investigation of altruism necessitates an approach that analyzes several aspects of altruistic behavior at different levels in the hierarchy of sciences. This research includes the study of evolutionary adaptations, neurological systems, cognitive functions, behavioral traits, and cultural influences. No one level is able to offer a full explanation, but each piece adds a unique dimension to a much larger puzzle. (shrink)

Cooperative child care among humans, where individuals other than the biological mother (allomothers) provide care, may increase a mother’s fertility and the survivorship of her children. Although the potential benefits to the mother are clear, the motivations for allomothers to provide care are less clear. Here, we evaluate the kin selection allomothering hypothesis using observations on Hadza hunter-gatherers collected in ten camps over 17 months. Our results indicate that related allomothers spend the largest percentage of time holding children. The higher (...) the degree of relatedness among kin, the more time they spend holding, supporting the hypothesis of nepotism as the strongest motivation for providing allomaternal care. Unrelated helpers of all ages also provide a substantial amount of investment, which may be motivated by learning to mother, reciprocity, or coercion. (shrink)

What motivates grandparents to their altruism? We review answers from evolutionary theory, sociology, and economics. Sometimes in direct conflict with each other, these accounts of grandparental investment exist side-by-side, with little or no theoretical integration. They all account for some of the data, and none account for all of it. We call for a more comprehensive theoretical framework of grandparental investment that addresses its proximate and ultimate causes, and its variability due to lineage, values, norms, institutions (e.g., inheritance laws), and (...) social welfare regimes. This framework needs to take into account that the demographic shift to low fecundity and mortality in economically developed countries has profoundly altered basic parameters of grandparental investment. We then turn to the possible impact of grandparental acts of altruism, and examine whether benefits of grandparental care in industrialized societies may manifest in terms of less tangible dimensions, such as the grandchildren's cognitive and verbal ability, mental health, and well-being. Although grandparents in industrialized societies continue to invest substantial amounts of time and money in their grandchildren, we find a paucity of studies investigating the influence that this investment has on grandchildren in low-risk family contexts. Under circumstances of duress there is converging evidence that grandparents can provide support that helps to safeguard their children and grandchildren against adverse risks. We conclude by discussing the role that grandparents could play in what has been referred to as Europe's demographic suicide. (shrink)

Integration of different lines of research concerning grandparental investment appears to be both promising and necessary. However, it must stop short when confronted with incommensurate arguments and hypotheses, either within or between disciplines. Further, some hypotheses have less plausibility and veridicality than others. This point is illustrated with results that conflict previous conclusions from evolutionary psychology about differential grandparental investment.

Grandparental presence is known to correlate with the number of grandchildren born, and this effect may vary according to grandparental sex and lineage. However, existing studies of grandparental effects on fertility mostly concern traditional subsistence societies, while evidence from contemporary developed societies is both scarce and mixed. Here, we explore how grandparents affect the transition to second and subsequent children in the contemporary United Kingdom. The longitudinal Millennium Cohort Study (n = 10,295 families) was used to study the association between (...) grandparental investment and parents’ probability of having a new child within 4.5 years. Results show that contact with paternal grandparents is associated with higher probability of parents having a second child. In contrast, contact with maternal grandparents is associated with lower probability of having a third or subsequent child. Kin may have opposite effects on fertility even in contemporary societies, which may explain the lack of consistent effects of grandparental investment on fertility in previous studies. (shrink)

We conducted a meta-analysis of 17 studies that tested for an association between grandparental survival and grandchild survival in patrilineal populations. Using two different methodologies, we found that the survival of the maternal grandmother and grandfather, but not the paternal grandmother and grandfather, was associated with decreased grandoffspring mortality. These results are consistent with the findings of psychological studies in developed countries (Coall and Hertwig Behavioral and Brain Sciences 33:1-59, 2010). When tested against the predictions of five hypotheses (confidence of (...) paternity; grandmothering, kin proximity, grandparental senescence, and local resource competition), our meta-analysis results are most in line with the local resource competition hypothesis. In patrilineal and predominantly patrilocal societies, the grandparents who are most likely to live with the grandchildren have a less beneficial association than those who do not. We consider the extent to which these results may be influenced by the methodological limitations of the source studies, including the use of retrospective designs and inadequate controls for confounding variables such as wealth. (shrink)

Several theoretical perspectives have generated research on grandparental investment, notably socialization and evolutionary psychological perspectives. Using data collected from more than 200 older adults (mean age 67 years), we test three hypotheses derived from socialization and evolutionary perspectives about grandparents’ relationships with and investment in grandchildren. Results indicate that (1) emotional closeness with both children and children-in-law is positively related to reports of emotional closeness with grandchildren; (2) maternal grandmothers invest more in grandchildren than do other grandparents; and (3) grandparents (...) invest more in daughters’ children than in sons’ children. Discussion addresses limitations of self-report methodology in this context and situates the current findings within the body of research indicating the existence of psychological mechanisms designed to guide differential nepotism. (shrink)

In this paper I examine the intracultural variability of parental and alloparental caregiving among the Aka foragers of the Central African Republic. It has been suggested that maternal kin offer higher frequencies of allocare than paternal kin and that maternal investment in infants will decrease when alloparental assistance is provided. Behavioral observations were conducted on 15 eight- to twelve-monthold infants. The practice of brideservice and the flexibility of Aka residence patterns offered a means to test the effect of maternal residence (...) on parental and alloparental investment. There was significant variation in the frequency of investment and who supplied care to infants depending on whether mothers resided with their kin or their husbands’ kin. However, in spite of the variation in allocare, when all categories of caregivers were examined collectively, infants received similar overall levels of care. (shrink)

Several studies have suggested a matrilateral bias in allomaternal (non-maternal) infant and child caregiving. The bias has been associated with the allomother’s certainty of genetic relatedness, where allomothers with high certainty of genetic relatedness will invest more in children because of potential fitness benefits. Using quantitative behavioral observations collected on Ngandu 8- to 12-month-old infants from the Central African Republic, I examine who is caring for infants and test whether certainty of genetic relatedness may influence investment by allomothers. Results indicate (...) a matrilateral bias in caregiving by extended kin members, but this does not affect the total level of care infants receive when fathers and siblings are included in the analysis. These results replicate a previous study done among an adjacent foraging population and emphasize the importance of examining children’s complete social environments when addressing caregiving and child development. (shrink)

Gaulin, McBurney, and Brakeman-Wartell (1997) found that college students reported both matrilateral and sex biases in the investment of aunts and uncles (aunts invested more than uncles). They interpreted the matrilateral bias as a consequence of paternity uncertainty. We replicated that study with Orthodox Jewish college students, selected because they come from a population we presume to have higher paternity certainty than the general population. The Orthodox sample also showed matrilateral and sex biases. Comparing the two data sets, the Orthodox (...) sample reported more investment, and slightly less matrilateral and sex biases, but the differences were not statistically significant. We did find an interaction between sex of relative and group membership, resulting from greater investment by Orthodox uncles. We interpret the results as reflecting the operation of a facultative investment mechanism whose upper limit is tuned to the maximum levels of paternity certainty found in ancestral environments. Lack of a difference in matrilateral bias between groups may result from levels of paternity certainty near to, or above, that maximum in both groups. (shrink)