Grandparental investment as a function of relational uncertainty and emotional closeness with parents

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.     

Grandparental Effects on Fertility Vary by Lineage in the United Kingdom

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.     

Do grandparents favor granddaughters? Biased grandparental investment in UK

Differential grandparental investment in grandchildren is often explained with paternity uncertainty. The asymmetric inheritance of the sex chromosomes, especially of the X chromosomes, may also bias grandparental investment. Recent studies show that ignoring the sex of the grandchild can mask important differences in the investment patterns of the same grandparent category, but this has not been tested in contemporary societies with nationally representative data. With 17 variables from the Involved Grandparenting and Child Well-Being 2007 survey, we tested differential grandparental investment as reported by British and Welsh adolescents and compared predictions based on X-chromosomal relatedness with predictions based on paternity uncertainty. The theories are expected to differ with regard to grandmaternal investment in grandsons and granddaughters. We test whether paternal grandmothers invest (H1) more in granddaughters than in grandsons, (H2) more in granddaughters than maternal grandmothers do and (H3) less in grandsons than maternal grandmothers do. In addition, following the suggestion that paternal grandmothers may reduce sibling competition between girls and boys by harming grandsons, we study whether (H4) paternal grandmothers channel more noninvestment into grandsons than into granddaughters. The results show no convincing support for the type of sex discrimination of grandchildren that is predicted by X-chromosomal relatedness theories, but do provide support for the paternity uncertainty theory. X-chromosomal relatedness does not appear to shape grandparental behavior in developed societies.     

Paternity uncertainty overrides sex chromosome selection for preferential grandparenting

With respect to autosomal genes, a grandparent is equally related to male and female grandchildren. Because males are heterozygous for sex chromosomes, however, grandparents are asymmetrically related to male and female grandchildren via the sex chromosomes. For example, the Y chromosome from the paternal grandfather passes directly down to grandsons. This asymmetry leads to a prediction that genes on the sex chromosomes could drive differential grandparental care. Alternatively, the paternity uncertainty hypothesis for differential grandparent care brings about a different set of predictions. A grandfather, for example, has two degrees of uncertainty to his son's children but only one to his daughter's children. Thus, under high extra-pair paternity rates, paternity uncertainty predicts that a grandfather will favor his daughter's children over his son's children. A paternity uncertainty vs. a genetic relatedness hypothesis was tested using data from questionnaires asking adult grandchildren to rate the amount and quality of care of their various grandparents. We found no support for preferential care based on expected sex chromosome similarities. Instead, our data were in general accord with the predictions of the paternity uncertainty hypothesis of grandparental care. A model is presented to predict the rates of extra-pair paternity required in a population to have the effects of paternity uncertainty outweigh sex chromosome effects.     

Caregiving within and beyond the family is associated with lower mortality for the caregiver: A prospective study

Grandparenting has been proposed as an ultimate evolutionary mechanism that has contributed to the increase in human life expectancy (see the grandmother hypothesis). The neural and hormonal system – originally rooted in parenting and thus grandparenting – that is activated in the process of caregiving has been suggested as a potential proximate mechanism that promotes engagement in prosocial behavior towards kin and non-kin alike. Evidence and theory suggest that activating this caregiving system positively impacts health and may reduce the mortality of the helper. Although some studies have found grandparental care to have beneficial effects on grandparents' health outcomes, most studies have focused on the detrimental health consequences of providing custodial care for grandchildren. Little is known about how non-custodial grandparental and other forms of caregiving relate to mortality hazards for the care provider. Using an evolutionary framework, we examined whether caregiving within and beyond the family is related to mortality in older adults. Survival analyses based on data from the Berlin Aging Study revealed that mortality hazards for grandparents who provided non-custodial childcare were 37% lower than for grandparents who did not provide childcare and for non-grandparents. These associations held after controlling for physical health, age, socioeconomic status and various characteristics of the children and grandchildren. Furthermore, the effect of caregiving extended to non-grandparents and to childless older adults who helped beyond their families. Potential ultimate and proximate mechanisms underlying these effects are discussed.     

The evolution of sex-specific grandparental harm

Recent empirical studies indicate that grandparents favour some categories of grandchildren over others. Here, we expand the previous theoretical foundation for this finding and show that grandchild-harming phenotypes are predicted to evolve by ‘sexually antagonistic zygotic drive (SA-zygotic drive) of the sex chromosomes’. We use the logic of Hamilton''s rule to develop a new ‘no-cost-to-self nepotism rule’ that greatly simplifies the determination of the invasion criteria for mutations that cause grandparents to harm grandchildren. We use this theory to generate predictions that distinguish SA-zygotic drive from theory based solely on paternity assurance. The major diagnostic prediction is that grandmothers, and to a lesser degree grandfathers, will evolve grandson-harming phenotypes that reduce the level of sib competition experienced by their more closely related granddaughters, especially in their sons'' families. This prediction is supported by data from recent studies showing (i) grandmothers invest more in granddaughters than grandsons, and counterintuitively, (ii) paternal grandmothers reduce the survival of their grandsons. We conclude that SA-zygotic drive is plausibly operating in humans via sexually antagonistic grandparental care.     

Matrilateral biases in the investment of aunts and uncles

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%.     

More on how and why: cause and effect in biology revisited

In 1961, Ernst Mayr published a highly influential article on the nature of causation in biology, in which he distinguished between proximate and ultimate causes. Mayr argued that proximate causes (e.g. physiological factors) and ultimate causes (e.g. natural selection) addressed distinct ‘how’ and ‘why’ questions and were not competing alternatives. That distinction retains explanatory value today. However, the adoption of Mayr’s heuristic led to the widespread belief that ontogenetic processes are irrelevant to evolutionary questions, a belief that has (1) hindered progress within evolutionary biology, (2) forged divisions between evolutionary biology and adjacent disciplines and (3) obstructed several contemporary debates in biology. Here we expand on our earlier (Laland et al. in Science 334:1512–1516, 2011) argument that Mayr’s dichotomous formulation has now run its useful course, and that evolutionary biology would be better served by a concept of reciprocal causation, in which causation is perceived to cycle through biological systems recursively. We further suggest that a newer evolutionary synthesis is unlikely to emerge without this change in thinking about causation.     

Lineage, Sex, and Wealth as Moderators of Kin Investment

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.     

Studying parents and grandparents to assess genetic contributions to early-onset disease

Suppose DNA is available from affected individuals, their parents, and their grandparents. Particularly for early-onset diseases, maternally mediated genetic effects can play a role, because the mother determines the prenatal environment. The proposed maximum-likelihood approach for the detection of apparent transmission distortion treats the triad consisting of the affected individual and his or her two parents as the outcome, conditioning on grandparental mating types. Under a null model in which the allele under study does not confer susceptibility, either through linkage or directly, and when there are no maternally mediated genetic effects, conditional probabilities for specific triads are easily derived. A log-linear model permits a likelihood-ratio test (LRT) and allows the estimation of relative penetrances. The proposed approach is robust against genetic population stratification. Missing-data methods permit the inclusion of incomplete families, even if the missing person is the affected grandchild, as is the case when an induced abortion has followed the detection of a malformation. When screening multiple markers, one can begin by genotyping only the grandparents and the affected grandchildren. LRTs based on conditioning on grandparental mating types (i.e., ignoring the parents) have asymptotic relative efficiencies that are typically >150% (per family), compared with tests based on parents. A test for asymmetry in the number of copies carried by maternal versus paternal grandparents yields an LRT specific to maternal effects. One can then genotype the parents for only the genes that passed the initial screen. Conditioning on both the grandparents' and the affected grandchild's genotypes, a third log-linear model captures the remaining information, in an independent LRT for maternal effects.     

Evolutionary Novelty and the Evo-Devo Synthesis: Field Notes

Accounting for the evolutionary origins of morphological novelty is one of the core challenges of contemporary evolutionary biology. A successful explanatory framework requires the integration of different biological disciplines, but the relationships between developmental biology and standard evolutionary biology remain contested. There is also disagreement about how to define the concept of evolutionary novelty. These issues were the subjects of a workshop held in November 2009 at the University of Alberta. We report on the discussion and results of this workshop, addressing questions about (i) how to define evolutionary novelty and understand its significance, (ii) how to interpret evolutionary developmental biology as a synthesis and its relation to neo-Darwinian evolutionary theory, and (iii) how to integrate disparate biological approaches in general.     

Alternatives to the Grandmother Hypothesis

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.     

Discriminative grandparental solicitude as reproductive strategy

1,857 adults rated the grandparental solicitude they received in childhood. Through a simple model based on the evolutionary concepts of ontogenetically differentiated reproductive strategy and paternity confidence, an ordered discriminative pattern of grandparental caregiving was predicted and confirmed by solid main effects, based on 603 complete cases. The maternal grandmother was the most caring. Unlike prevalent gender stereotypes, she was followed by the maternal grandfather, the paternal grandmother, and the paternal grandfather. The preferential grandparental solicitude was not influenced by residential distance, grandparent age, and availability of other grandparents. A predicted higher correlation for male than for female progenitors between solicitude and phenotypic resemblance could be confirmed.     

The economics of managing evolution

Humans are altering biological systems at unprecedented rates, and these alterations often have longer-term evolutionary impacts. Most obvious is the spread of resistance to pesticides and antibiotics. There are a wide variety of management strategies available to slow this evolution, and there are many reasons for using them. In this paper, we focus on the economic aspects of evolution management and ask: When is it economically beneficial for an individual decision-maker to invest in evolution management? We derive a simple dimensionless inequality showing that it is cost-effective to manage evolution when the percentage increase in the effective life span of the biological resource that management generates is larger than the percentage increase in annual profit that could be obtained by not managing evolution. We show how this inequality can be used to determine optimal investment choices for single decision-makers, to determine Nash equilibrium investment choices for multiple interacting decision-makers, and to examine how these equilibrium choices respond to regulatory interventions aimed at stimulating investment in evolution management. Our results are illustrated with examples involving Bacillus thuringiensis (Bt) crops and antibiotic use in fish farming.

Humans are altering biological systems at unprecedented rates and these alterations often have longer-term evolutionary impacts, such as the spread of resistance to pesticides and antibiotics. In this study, a simple mathematical criterion is derived determining when it is economically beneficial to invest in strategies for controlling and managing evolutionary change.     

Homology in comparative, molecular, and evolutionary developmental biology: the radiation of a concept

The present paper analyzes the use and understanding of the homology concept across different biological disciplines. It is argued that in its history, the homology concept underwent a sort of adaptive radiation. Once it migrated from comparative anatomy into new biological fields, the homology concept changed in accordance with the theoretical aims and interests of these disciplines. The paper gives a case study of the theoretical role that homology plays in comparative and evolutionary biology, in molecular biology, and in evolutionary developmental biology. It is shown that the concept or variant of homology preferred by a particular biological field is used to bring about items of biological knowledge that are characteristic for this field. A particular branch of biology uses its homology concept to pursue its specific theoretical goals.     

Parenting Strategies in Modern and Emerging Economies

Independent of ecology, subsistence strategy, social complexity, or other aspects of socioecology, the altricial nature of young humans requires mothers to have help raising their offspring. What seems to be context-dependent, however, is who the helpers are, how they invest, and what the impacts of that investment are. In a series of papers that focus on parental and alloparental investment across five populations, this special issue of Human Nature uses evolutionary theory to examine how socioecological context influences modes of direct parental investment among the boat-dwelling Shodagor of Bangladesh (Starkweather), modes of indirect paternal investment in the modern United States (Anderson), and the biological outcome of paternal investment for men in Jamaica (Gray et al.), as well as direct alloparental investment among village Bangladeshis (Perry) and indirect alloparental investment in breastfeeding practices in the United States (Cisco).     

Grandma plays favourites: X-chromosome relatedness and sex-specific childhood mortality

Biologists use genetic relatedness between family members to explain the evolution of many behavioural and developmental traits in humans, including altruism, kin investment and longevity. Women''s post-menopausal longevity in particular is linked to genetic relatedness between family members. According to the ‘grandmother hypothesis’, post-menopausal women can increase their genetic contribution to future generations by increasing the survivorship of their grandchildren. While some demographic studies have found evidence for this, others have found little support for it. Here, we re-model the predictions of the grandmother hypothesis by examining the genetic relatedness between grandmothers and grandchildren. We use this new model to re-evaluate the grandmother effect in seven previously studied human populations. Boys and girls differ in the per cent of genes they share with maternal versus paternal grandmothers because of differences in X-chromosome inheritance. Here, we demonstrate a relationship between X-chromosome inheritance and grandchild mortality in the presence of a grandmother. With this sex-specific and X-chromosome approach to interpreting mortality rates, we provide a new perspective on the prevailing theory for the evolution of human female longevity. This approach yields more consistent support for the grandmother hypothesis, and has implications for the study of human evolution.     

Entomopathogenic Nematodes as a Model System for Advancing the Frontiers of Ecology

Entomopathogenic nematodes (EPNs) in the families Heterorhabditidae and Steinernematidae have a mutualistic–symbiotic association with enteric γ-Proteobacteria (Steinernema–Xenorhabdus and Heterorhabditis–Photorhabdus), which confer high virulence against insects. EPNs have been studied intensively because of their role as a natural mortality factor for soil-dwelling arthropods and their potential as biological control agents for belowground insect pests. For many decades, research on EPNs focused on the taxonomy, phylogeny, biogeography, genetics, physiology, biochemistry and ecology, as well as commercial production and application technologies. More recently, EPNs and their bacterial symbionts are being viewed as a model system for advancing research in other disciplines such as soil ecology, symbiosis and evolutionary biology. Integration of existing information, particularly the accumulating information on their biology, into increasingly detailed population models is critical to improving our ability to exploit and manage EPNs as a biological control agent and to understand ecological processes in a changing world. Here, we summarize some recent advances in phylogeny, systematics, biogeography, community ecology and population dynamics models of EPNs, and describe how this research is advancing frontiers in ecology.     

Genetic relatedness to sisters' children has been underestimated

Males of many species help in the care and provisioning of offspring, and these investments often correlate with genetic relatedness. For example, many human males invest in the children of sisters, and this is especially so where men are less likely to share genes with children of wives. Although this makes qualitative sense, it has been difficult to support quantitatively. The prevailing model predicts investment in children of sisters only when paternity confidence falls below 0.268. This value is often seen as too low to be credible; so investment in sisters'' children represents an unsolved problem. I show here that the prevailing model rests on a series of restrictive assumptions that underestimate relatedness to sisters'' children. For this reason, it understates the fitness payoff to men who invest in these children. This effect can be substantial, especially in societies with low confidence in paternity. But this effect cannot be estimated solely from confidence in paternity. One must also estimate the probability that two siblings share the same father.     

A Cinderella effect in the childcare assistance provided by European grandparents

Survey data from 36,771 European grandparents were analyzed with respect to childcare assistance that interviewees provided to their birth children versus stepchildren. Interviewees with current partners provided far more such assistance to adult children who were the progeny of both partners than to those who were stepchildren of either one's self or one's partner, but both husband and wife helped the wife's children from prior unions more than they helped the husband's children, supporting the interpretation of stepfathers' involvement as primarily an investment in the new partnership (“mating effort”) rather than in the grandchildren. Interviewees resided farther away from their adult stepchildren than from their birth children, on average, but discriminative care did not depend merely on residential proximity, being substantial regardless of whether proximity was statistically controlled. Grandmothers without current partners exhibited the greatest discrimination, with stepchild: birth child Odds Ratios of 0.10 or less in various analyses, whereas grandfathers without partners exhibited no significant discrimination. Interviewees of both sexes listed far more stepchildren, proportionately, if they had a current partner than if they were either widowed or divorced, however, which suggests that single respondents may have omitted former stepchildren with whom they had ceased to interact from their list of children, and hence that the degree to which single grandparents divest from them may be even larger than estimated.