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.     

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.     

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.     

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%.     

Predictors of Grandparental Investment Decisions in Contemporary Europe: Biological Relatedness and Beyond

Across human cultures, grandparents make a valued contribution to the health of their families and communities. Moreover, evidence is gathering that grandparents have a positive impact on the development of grandchildren in contemporary industrialized societies. A broad range of factors that influence the likelihood grandparents will invest in their grandchildren has been explored by disciplines as diverse as sociology, economics, psychology and evolutionary biology. To progress toward an encompassing framework, this study will include biological relatedness between grandparents and grandchildren, a factor central to some discipline''s theoretical frameworks (e.g., evolutionary biology), next to a wide range of other factors in an analysis of grandparental investment in contemporary Europe. This study draws on data collected in the Survey of Health, Ageing and Retirement in Europe from 11 European countries that included 22,967 grandparent–child dyads. Grandparents reported biological relatedness, and grandparental investment was measured as the frequency of informal childcare. Biological and non-biological grandparents differed significantly in a variety of individual, familial and area-level characteristics. Furthermore, many other economic, sociological, and psychological factors also influenced grandparental investment. When they were controlled, biological grandparents, relative to non-biological grandparents, were more likely to invest heavily, looking after their grandchildren almost daily or weekly. Paradoxically, however, they were also more likely to invest nothing at all. We discuss the methodological and theoretical implications of these findings across disciplines.     

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.     

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.     

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.     

Extra-pair young in house wren broods are more likely to be male than female

Sex-allocation theory predicts that females should preferentially produce offspring of the sex with greater fitness potential. In socially monogamous animal species, extra-pair mating often increases the variance in fitness of sons relative to daughters. Thus, in situations where offspring sired by a female''s extra-pair mate(s) will typically have greater fitness potential than offspring sired by the within-pair mate, sex-allocation theory predicts that females will bias the sex of offspring sired by extra-pair mates towards male. We examined the relationship between offspring sex and paternity over six breeding seasons in an Illinois population of the house wren (Troglodytes aedon), a cavity-nesting songbird. Out of the 2345 nestlings that had both sex and paternity assigned, 350 (15%) were sired by extra-pair males. The sex ratio of extra-pair offspring, 0.534, was significantly greater than the sex ratio of within-pair offspring, 0.492, representing an increase of 8.5 per cent in the proportion of sons produced. To our knowledge, this is the first confirmed report of female birds increasing their production of sons in association with extra-pair fertilization. Our results are consistent with the oft-mentioned hypothesis that females engage in extra-pair mating to increase offspring quality.     

Coevolution of female fidelity and male help in populations with alternative reproductive tactics

In socially monogamous species, pair-bonded males often continue to provide care to all offspring in their nests despite some degree of paternity loss due to female extra-pair copulation. Previous theoretical models suggested that females can use their within-pair offspring as ‘hostages'' to blackmail their social mates, so that they continue to provide care to the brood at low levels of cuckoldry. These models, however, rely on the assumption of sufficiently accurate male detection of cuckoldry and the reduction of parental effort in case of suspicion. Therefore, they cannot explain the abundant cases where cuckolded males continue to provide extensive care to the brood. Here we use an analytical population genetics model and an individual-based simulation model to explore the coevolution of female fidelity and male help in populations with two genetically determined alternative reproductive tactics (ARTs): sneakers that achieve paternity solely via extra-pair copulations and bourgeois that form a mating pair and spend some efforts in brood care. We show that when the efficiency of mate guarding is intermediate, the bourgeois males can evolve to ‘specialize'' in providing care by spending more than 90% of time in helping their females while guarding them as much as possible, despite frequent cuckoldry by the sneakers. We also show that when sneakers have tactic-specific adaptations and thus are more competitive than the bourgeois in gaining extra-pair fertilizations, the frequency of sneakers and the degrees of female fidelity and male help can fluctuate in evolutionary cycles. Our theoretical predictions highlight the need for further empirical tests in species with ARTs.     

Hereditary recombined three-allele variant of the Gc system

A three-allele variant with Gc 2, Gc 1F and Gc 1A2 alleles was detected in both a baby and his mother during paternity testing by isoelectric focusing. His father had a normal Gc phenotype, Gc 2-1F. Further examination of his mother's relatives revealed that his grandfather also had the same three-allele variant, while his grandmother and his aunt had normal Gc 2-1F and Gc 2-2. From these results, it was considered that the Gc 1F and Gc 1A2 alleles were on the same single chromosome. It was suggested that recombination had occurred between two chromosomes that had the Gc 1F and Gc 1A2 allele, respectively, forming the variant allele Gc 1F1A2 on a single chromosome.     

Matriliny as daughter-biased investment

From an evolutionary perspective, matriliny presents a puzzle because men in matrilineal societies transmit wealth to their sisters' sons, to whom they are only half as related as to their own sons. It has been argued that such systems would only maximise fitness under unrealistically high levels of paternity uncertainty. In this paper, we propose that matriliny can arise from daughter-biased investment by parents and/or grandparents. We show that daughter-biased investment is adaptive if the marginal benefit of wealth to sons (compared to daughters) does not outweigh the risk of nonpaternity in sons' offspring. We argue that such conditions will be rare where resource-holding polygyny is prevalent but could otherwise be widespread under even moderate levels of paternity uncertainty. The daughter-biased investment model explains two well-known characteristics of matrilineal descent: (a) matriliny's association with high levels of paternity uncertainty and (b) matriliny's ecological correlates, including its association with horticulture, its rarity in pastoralist and agro-pastoralist societies, and the tendency for matriliny to be replaced by son-biased inheritance during economic development. We present data on wealth, sex, and reproductive success (RS) in two African societies, the matrilineal Chewa in Malawi and patrilineal Gabbra in Kenya, which support the daughter-biased investment theory.     

Female extra-pair behaviour and environmental quality in the serin (Serinus serinus): a test of the 'constrained female hypothesis'

Recent behavioural and molecular studies have shown that in most monogamous bird species extra-pair copulations and fertilizations outside the pair bond occur routinely. The consequences of female extra-pair behaviour might comprise effects on important life-history traits, such as the extent of male parental care. In this study we test the assumption that, within a species, females'' options for extra-pair mating depend on female quality and the environments that females occupy. This ''constrained female hypothesis'' predicts that females in good environments or high-quality females are able to resist males'' control efforts better than females in poor environments or low-quality females. We test the idea in the socially monogamous serin. We found that the likelihood of extra-pair paternity is significantly higher in territories with high availability of food. There was a negative relationship between environmental quality (food availability) and paternity both in natural and in experimentally manipulated habitats. Male feeding rates were negatively related to food availability and positively related to paternity. These data and the additional result that in better environments all of a females'' offspring were sired by one extra-pair male provide support for Gowaty''s ''constrained female hypothesis''.     

Breaking the rules: sex roles and genetic mating system of the pheasant coucal

Generally in birds, the classic sex roles of male competition and female choice result in females providing most offspring care while males face uncertain parentage. In less than 5% of species, however, reversed courtship sex roles lead to predominantly male care and low extra-pair paternity. These role-reversed species usually have reversed sexual size dimorphism and polyandry, confirming that sexual selection acts most strongly on the sex with the smaller parental investment and accordingly higher potential reproductive rate. We used parentage analyses and observations from three field seasons to establish the social and genetic mating system of pheasant coucals, Centropus phasianinus, a tropical nesting cuckoo, where males are much smaller than females and provide most parental care. Pheasant coucals are socially monogamous and in this study males produced about 80% of calls in the dawn chorus, implying greater male sexual competition. Despite the substantial male investments, extra-pair paternity was unusually high for a socially monogamous, duetting species. Using two or more mismatches to determine extra-pair parentage, we found that 11 of 59 young (18.6%) in 10 of 21 broods (47.6%) were not sired by their putative father. Male incubation, starting early in the laying sequence, may give the female opportunity and reason to seek these extra-pair copulations. Monogamy, rather than the polyandry and sex-role reversal typical of its congener, C. grillii, may be the result of the large territory size, which could prevent females from monopolising multiple males. The pheasant coucal’s exceptional combination of classic sex-roles and male-biased care for extra-pair young is hard to reconcile with current sexual selection theory, but may represent an intermediate stage in the evolution of polyandry or an evolutionary remnant of polyandry.     

Do male paternity guards ensure female fidelity in a duetting fairy-wren?

Most socially monogamous bird species engage in extra-pair mating,and consequently males may adopt various behavioral strategiesto guard paternity. However, the relationship between mate guardingand extra-pair paternity is unclear: low levels of extra-pairpaternity can be associated either with no mate guarding orwith intense mate guarding. We investigate paternity guardsin the purple-crowned fairy-wren (Malurus coronatus), a duettingspecies where extra-pair paternity is rare. This species isunusual in a genus known for extremely high levels of extra-pairmating. We examine the behavioral interactions between the sexesunderlying these low rates of extra-pair paternity and showthat male purple-crowned fairy-wrens do not use frequent copulationor courtship feeding to assure paternity or guard females acousticallyby duetting. Males maintain close proximity to females bothwhen they are fertile and when they are not breeding and donot invest in courtship displays to extra-pair females. Consistentwith predictions of theoretical models, low extra-pair paternityin this species may be related to female fidelity rather thanmale paternity assurance strategies, but short-term removalof males would be necessary to test this experimentally.     

Direct and indirect estimation of the sex ratio of mutation frequencies in hemophilia A.

Carrier detection tests were carried out in 119 families with hemophilia A by using the data obtained with current DNA techniques (e.g., RFLP analysis and direct identification of mutations), conventional carrier detection tests (e.g., factor VIII:C and von Willebrand factor antigen), and pedigree information. On the basis of this data, we estimated the sex ratio of mutation frequencies with three completely different methods and compared the results. Since the classical indirect method derived from Haldane is substantially influenced by reproductive fitness (f), the sex ratio of mutation frequencies was estimated for both f = .3 and f = .5, resulting in a male:female mutation ratio of 5.37 (95% confidence interval 2.16-13.02) and 3.26 (95% confidence interval 0.97-8.73), respectively. According to the equilibrium-independent indirect method formulated by Rosendaal et al., the male:female ratio was estimated to be 3.4 (95% confidence interval 1.18-8.81). Since current DNA techniques provide information on the grandparental origin of the patient''s X chromosome, we were twice able to estimate directly the male:female mutation ratio as 15:1, by using the quotients of mutation origin (maternal grandfather/maternal grandmother and maternal grandfather/patient''s mother, respectively). Application of the Fisher test shows that the male mutation rate is higher than the female rate (P = 8.55 x 10(-7). Since all three completely different approaches unequivocally showed a higher male than female mutation frequency, this should be considered to be an established fact in calculating the risk in hemophilia A families.     

Extra-pair paternity in monogamous and polygynous Savannah sparrows,Passerculus sandwichensis

Extra-pair paternity can influence mating systems by affecting the fitness costs associated with polygyny. Polygyny is disadvantageous to males when the time and energetic demands of multiple pairings decrease either a male's success at gaining extra-pair fertilizations or his ability to ensure paternity among harem members. In Savannah sparrows,Passerculus sandwichensison Kent Island, New Brunswick, Canada, multilocus DNA fingerprinting of 136 adults and young revealed substantial female infidelity: overall, 31 of 92 young (33.7%) in 15 of 24 nests (62.5%) were the product of extra-pair fertilizations. Male mating status was a strong predictor of paternity. Each of seven monogamous females produced at least one extra-pair offspring, but only six of 11 primary females (54.5%) and two of six secondary females (33.3%) were unfaithful. As a result, nearly 80% of the young in nests of polygynous males resulted from within-pair fertilizations, compared with only 40% of the young in nests of monogamous males. Kent Island Savannah sparrows are simultaneously polygynous, and the absence of paternity costs associated with polygyny is surprising. The observed pattern of paternity suggests the operation of female choice, although male control of parentage cannot be excluded.     

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.     

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.