SIBLING CONFLICTS IN FULL- AND HALF-SIBLING HOUSEHOLDS IN THE UK.

Sibling relations are by nature ambivalent with high levels of both altruistic helping and competition. Higher relatedness is often assumed to reduce the occurrence of conflicts between siblings, but evidence of this has been scarce and mixed. Siblings typically compete over resources and parental attention, and parental constellations vary with sibship types. Since full-siblings compete over the same two biological parents, while half-siblings have only one shared biological parent and often a higher number of parents overall, it is hypothesized that conflicts are more common between full- than half-siblings. This study tested this assumption using the British Millennium Cohort Study (n=7527 children at age 11). Conflicts were measured as children's reports of how much siblings picked on and hurt each other. Households with full-siblings only, maternal half-siblings only, and both full- and maternal half-siblings were compared. The results show that children who were living with only their full-siblings were more likely to experience sibling conflicts compared with children living with their maternal half-siblings only. This was the case also after controlling for several potentially confounding variables. The results suggest that differential access to parental resources of available biological and step-parents may explain the higher amount of sibling conflict between full- compared with maternal half-siblings.

Short- and long-term health consequences and current satisfaction levels for altruistic anonymous, identity-release and known oocyte donors.

STUDY QUESTION: What are the short- and long-term health consequences and current satisfaction of altruistic oocyte donors? SUMMARY ANSWER: Donating oocytes in the majority (>90%) of donation cycles is not associated with harmful long-term general or reproductive health effects. WHAT IS KNOWN ALREADY: Knowledge of long-term health effects of oocyte donation on donors is sparse and follow-up studies have usually been carried out on commercial donors. Thus far, no major long-term harmful effects have been demonstrated. Most studies have reported a high level of donor satisfaction, but also less favorable experiences have been published. STUDY DESIGN, SIZE AND DURATION: A retrospective cross-sectional survey of all women who had donated oocytes between 1990 and 2012 at three fertility clinics was carried out in spring 2013. A self-administered questionnaire was sent out to a total of 569 former oocyte donors. PARTICIPANTS, SETTING, METHODS: In all, 428 past donors answered the questionnaire assessing donor's demographic characteristics, short- and long-term medical and psychological experiences and satisfaction related to donations. Of the donors, 87% (371/428) were unknown and 13% (57/428) were known to the recipient. The mean follow-up time after the donation was 11.2 years (range from 0.5 to 23 years) and the mean age of the respondents was 42 years at the time of the study. To learn whether the demographic profile of donors was affected by the Finnish Assisted Reproduction Technology (ART) Act of 2007, we divided the 428 respondents into two groups: (i) women whose first donation took place between 1990 and 2007 (79% of the respondents) and (ii) women whose first donation took place between 2008 and 2012 (21% of the respondents). Before 2008, donors were non-identifiable (anonymous) but after 2008 persons born as a result of gamete donation could, from the age of 18, receive on request information identifying the donor. MAIN RESULTS AND THE ROLE OF CHANCE: The response rate was 75% (428/569). The mean age of the donors did not differ between the two time periods, but there was a higher proportion of donors in the youngest age group (20-24 years) and more childless donors (P < 0.05) after 2008 than between 1990 and 2007. Immediate complications occurred in 7.2% (42/582) of the donation cycles and the most common complication was ovarian hyperstimulation syndrome (OHSS) in 5.0% (29/582) of the treatments. There were no reports of ovarian or uterine cancer and only one case of breast cancer. After the donation, 11.5% of the donors experienced unsuccessful attempts to become pregnant. Almost all donors (99%) were satisfied or very satisfied with their decision to donate and 95% would warmly recommend it to other women. There were no differences between the known and unknown donors in this respect, or between the two time periods (before or after the ART Act in 2008). Four donors (1%) had regretted donation, and 7% would have wanted to have more support before and 14% after the donation. LIMITATIONS, REASON FOR CAUTION: Although the response rate was high, 25% of all former donors in the three participating clinics could not be included due to lack of response. The results are based on self-reported assessment of the experiences of former donors, and it is not possible to estimate the influence of recall bias. WIDER IMPLICATIONS OF THE FINDINGS: To our knowledge, this is the largest study of health consequences and satisfaction levels on oocyte donors. Data from this study can be used to inform donor candidates about the medical aspects involved in the treatment and it provides information on how to support these women during and after the donation. STUDY FUNDING/COMPETING INTERESTS: This study was supported by grants from the Medical Society Life and Health, and from the Otto A. Malm Foundation. There are no competing interests to report.

Natural and sexual selection in a monogamous historical human population.

Whether and how human populations exposed to the agricultural revolution are still affected by Darwinian selection remains controversial among social scientists, biologists, and the general public. Although methods of studying selection in natural populations are well established, our understanding of selection in humans has been limited by the availability of suitable datasets. Here, we present a study comparing the maximum strengths of natural and sexual selection in humans that includes the effects of sex and wealth on different episodes of selection. Our dataset was compiled from church records of preindustrial Finnish populations characterized by socially imposed monogamy, and it contains a complete distribution of survival, mating, and reproductive success for 5,923 individuals born 1760-1849. Individual differences in early survival and fertility (natural selection) were responsible for most variation in fitness, even among wealthier individuals. Variance in mating success explained most of the higher variance in reproductive success in males compared with females, but mating success also influenced reproductive success in females, allowing for sexual selection to operate in both sexes. The detected opportunity for selection is in line with measurements for other species but higher than most previous reports for human samples. This disparity results from biological, demographic, economic, and social differences across populations as well as from failures by most previous studies to account for variation in fitness introduced by nonreproductive individuals. Our results emphasize that the demographic, cultural, and technological changes of the last 10,000 y did not preclude the potential for natural and sexual selection in our species.

Face masks to prevent transmission of respiratory infections: Systematic review and meta-analysis of randomized controlled trials on face mask use.

OBJECTIVES: To examine the use of face mask intervention in mitigating the risk of spreading respiratory infections and whether the effect of face mask intervention differs in different exposure settings and age groups. DESIGN: Systematic review and meta-analysis. We evaluated the risk of bias using the Cochrane Risk of Bias 2 tool (ROB2). DATA SOURCES: We searched PubMed, Embase, Cochrane Central Register of Controlled Trials, and Web of Science were searched for randomized controlled trials investigating the effect of face masks on respiratory infections published between 1981 and February 9, 2022. We followed the PRISMA 2020 guidelines. ELIGIBILITY CRITERIA FOR SELECTING STUDIES: We included randomized controlled trials investigating the use of face mask intervention in mitigating the risk of spreading respiratory infections across different exposure settings. RESULTS: We identified 2,400 articles for screening. 18 articles passed the inclusion criteria for both evidence synthesis and meta-analysis. There were N = 189,145 individuals in the face mask intervention arm and N = 173,536 in the control arm, and the follow-up times ranged from 4 days to 19 months. Our results showed between-study heterogeneity (p < 0.0001). While there was no statistically significant association over all studies when the covariate unadjusted intervention effect estimates were used (RR = 0.977 [0.858-1.113], p = 0.728), our subgroup analyses revealed that a face mask intervention reduced respiratory infections in the adult subgroup (RR = 0.8795 [0.7861-0.9839], p = 0.0249) and in a community setting (RR = 0.890 [0.812-0.975], p = 0.0125). Furthermore, our leave-one-out analysis found that one study biased the results towards a null effect. Consequently, when using covariate adjusted odds ratio estimates to have a more precise effect estimates of the intervention effect to account for differences at the baseline, the results showed that a face mask intervention did reduce respiratory infections when the biasing study was excluded from the analysis (OR = 0.8892 [0.8061-0.9810], p = 0.0192). CONCLUSION: Our findings support the use of face masks particularly in a community setting and for adults. We also observed substantial between-study heterogeneity and varying adherence to protocol. Notably, many studies were subject to contamination bias thus affecting the efficacy of the intervention, that is when also some controls used masks or when the intervention group did not comply with mask use leading to a downward biased effect of treatment receipt and efficacy. TRIAL REGISTRATION: PROSPERO registration number CRD42020205523.

Grandparental Childcare for Biological, Adopted, and Step-Offspring: Findings From Cross-National Surveys.

Based on kin selection theory, amounts of grandparental investment should reflect the probability to share common genes with offspring. Adoption may represent a special case, however, yet grandparental investment in adopted children has previously been both theoretically misconstrued and little investigated. Here, we study for the first time how grandparental childcare provision is distributed between biological, adopted, and step-offspring. Using Generations and Gender Surveys (n = 15,168 adult child-grandmother and 12,193 adult child-grandfather dyads) and the Survey of Health, Ageing, and Retirement in Europe (n = 17,233 grandmother-adult child and 13,000 grandfather-adult child dyads), we find that grandparents were less likely to provide care to stepchildren than to adopted and biological children, but no difference between adopted and biological children. These findings were present in both data sets and for both grandmothers and grandfathers, after several potentially confounding factors were taken into account. The stepchild disadvantage is in line with kin selection theory. The congruent amounts of care provided to adopted and biological children may reflect similar levels of adult-child attachment, selection effects, and greater need in adoptive families, as well as some degree of genetical relatedness in the case of kin adoption. The study provides new evidence of biased kin investments in contemporary societies and stresses the importance of psychological motivation and attachment in evolutionary studies of kin investment.

Shorter birth intervals between siblings are associated with increased risk of parental divorce.

Birth intervals are a crucial component of fertility behaviour and family planning. Short birth intervals are associated-although not necessarily causally-with negative health-related outcomes, but less is known about their associations with family functioning. Here, the associations between birth intervals and marital stability were investigated by Cox regression using a nationally representative, register-based sample of individuals with two (N = 42,481) or three (N = 22,514) children from contemporary Finland (observation period 1972-2009). Shorter interbirth intervals were associated with an increased risk of parental divorce over a ten-year follow-up. Individuals with birth intervals of up to 1.5 years had 24-49 per cent higher divorce risk compared to individuals whose children were born more than 4 years apart. The pattern was similar in all socioeconomic groups and among individuals with earlier and later entry to parenthood. Our results add to the growing body of research showing associations between short birth intervals and negative outcomes in health and family functioning.

Homophily in Personality Enhances Group Success Among Real-Life Friends.

Personality affects dyadic relations and teamwork, yet its role among groups of friends has been little explored. We examine for the first time whether similarity in personality enhances the effectiveness of real-life friendship groups. Using data from a longitudinal study of a European fraternity (10 male and 15 female groups), we investigate how individual Big Five personality traits were associated with group formation and whether personality homophily related to how successful the groups were over 1 year (N = 147-196). Group success was measured as group performance/identification (adoption of group markers) and as group bonding (using the inclusion-of-other-in-self scale). Results show that individuals' similarity in neuroticism and conscientiousness predicted group formation. Furthermore, personality similarity was associated with group success, even after controlling for individual's own personality. Especially higher group-level similarity in conscientiousness was associated with group performance, and with bonding in male groups.

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.

Communication with Family and Friends across the Life Course.

Each stage of the human life course is characterised by a distinctive pattern of social relations. We study how the intensity and importance of the closest social contacts vary across the life course, using a large database of mobile communication from a European country. We first determine the most likely social relationship type from these mobile phone records by relating the age and gender of the caller and recipient to the frequency, length, and direction of calls. We then show how communication patterns between parents and children, romantic partner, and friends vary across the six main stages of the adult family life course. Young adulthood is dominated by a gradual shift of call activity from parents to close friends, and then to a romantic partner, culminating in the period of early family formation during which the focus is on the romantic partner. During middle adulthood call patterns suggest a high dependence on the parents of the ego, who, presumably often provide alloparental care, while at this stage female same-gender friendship also peaks. During post-reproductive adulthood, individuals and especially women balance close social contacts among three generations. The age of grandparenthood brings the children entering adulthood and family formation into the focus, and is associated with a realignment of close social contacts especially among women, while the old age is dominated by dependence on their children.

Singing together or apart: The effect of competitive and cooperative singing on social bonding within and between sub-groups of a university Fraternity.

Singing together seems to facilitate social bonding, but it is unclear whether this is true in all contexts. Here we examine the social bonding outcomes of naturalistic singing behaviour in a European university Fraternity composed of exclusive 'Cliques': recognised sub-groups of 5-20 friends who adopt a special name and identity. Singing occurs frequently in this Fraternity, both 'competitively' (contests between Cliques) and 'cooperatively' (multiple Cliques singing together). Both situations were re-created experimentally in order to explore how competitive and cooperative singing affects feelings of closeness towards others. Participants were assigned to teams of four and were asked to sing together with another team either from the same Clique or from a different Clique. Participants (N = 88) felt significantly closer to teams from different Cliques after singing with them compared to before, regardless of whether they cooperated with (singing loudly together) or competed against (trying to singing louder than) the other team. In contrast, participants reported reduced closeness with other teams from their own Clique after competing with them. These results indicate that group singing can increase closeness to less familiar individuals regardless of whether they share a common motivation, but that singing competitively may reduce closeness within a very tight-knit group.

Women favour dyadic relationships, but men prefer clubs: cross-cultural evidence from social networking.

The ability to create lasting, trust-based friendships makes it possible for humans to form large and coherent groups. The recent literature on the evolution of sociality and on the network dynamics of human societies suggests that large human groups have a layered structure generated by emotionally supported social relationships. There are also gender differences in adult social style which may involve different trade-offs between the quantity and quality of friendships. Although many have suggested that females tend to focus on intimate relations with a few other females, while males build larger, more hierarchical coalitions, the existence of such gender differences is disputed and data from adults is scarce. Here, we present cross-cultural evidence for gender differences in the preference for close friendships. We use a sample of ∼112,000 profile pictures from nine world regions posted on a popular social networking site to show that, in self-selected displays of social relationships, women favour dyadic relations, whereas men favour larger, all-male cliques. These apparently different solutions to quality-quantity trade-offs suggest a universal and fundamental difference in the function of close friendships for the two sexes.

Multi-partner fertility is associated with lower grandparental investment from in-laws in Finland.

Divorce and remarriage influence family relations, yet few studies explore changes in grandparenting due to family recomposition. We study variations in grandparental investment when the parents have children from several unions. Using nationally representative data of younger adults from the Generational Transmissions in Finland survey conducted in 2012 (sample n = 760 parents), we compare the grandchild care that parents report having received from their parents and parents-in-law. Results show that multipartner fertility is not associated with the amount of grandparental investment a parent receives from his or her own parents, but is associated with the investment received from mother's parents-in-law. Mother's parents-in-law are less likely to invest in grandchild sets which include step-grandchildren, compared to grandchildren living with their original parents. Fully biological grandchildren are 31% more likely to receive grandparental care compared to grandchild sets including step grandchildren. Thus the reduction in grandparental investment associated with step-grandchildren may also affect children from the new union.

Gratitude for help among adult friends and siblings.

Although gratitude is a key prosocial emotion reinforcing reciprocal altruism, it has been largely ignored in the empirical literature. We examined feelings of gratitude and the importance of reciprocity in same-sex peer relations. Participants were 772 individuals (189 men; mean age = 28.80) who completed an online survey using a vignette design. We investigated (i) differences in reported gratitude and the importance of reciprocity among same-sex siblings and same-sex friends, and (ii) how relationship closeness moderates these associations. Based on the theory of kin altruism, we expect that people would feel more grateful towards friends than towards their siblings, and that lack of gratitude or failure to pay back a loan would bother more with friends than with siblings, irrespective of emotional closeness. Results showed that levels of gratitude and expectations of reciprocity were higher towards friends compared to siblings. This was the case also after controlling for emotional closeness. Being close generally made participants feel more grateful and expect lower displays of gratitude in the other. Closeness was also strongly associated with emotional gratitude among siblings compared to friends. We conclude that feelings and displays of gratitude have a special role in friendships. Although a close sibling may elicit as much gratitude as a friend does, even a very close friend is not exempt from the logic of reciprocity in the same way that a sibling is.

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.

Maternal risk of breeding failure remained low throughout the demographic transitions in fertility and age at first reproduction in Finland.

Radical declines in fertility and postponement of first reproduction during the recent human demographic transitions have posed a challenge to interpreting human behaviour in evolutionary terms. This challenge has stemmed from insufficient evolutionary insight into individual reproductive decision-making and the rarity of datasets recording individual long-term reproductive success throughout the transitions. We use such data from about 2,000 Finnish mothers (first births: 1880s to 1970s) to show that changes in the maternal risk of breeding failure (no offspring raised to adulthood) underlay shifts in both fertility and first reproduction. With steady improvements in offspring survival, the expected fertility required to satisfy a low risk of breeding failure became lower and observed maternal fertility subsequently declined through an earlier age at last reproduction. Postponement of the age at first reproduction began when this risk approximated zero-even for mothers starting reproduction late. Interestingly, despite vastly differing fertility rates at different stages of the transitions, the number of offspring successfully raised to breeding per mother remained relatively constant over the period. Our results stress the importance of assessing the long-term success of reproductive strategies by including measures of offspring quality and suggest that avoidance of breeding failure may explain several key features of recent life-history shifts in industrialized societies.

Grandparental child care in Europe: evidence for preferential investment in more certain kin.

Theories of kin selection and parental investment predict stronger investment in children and grandchildren by women and maternal kin. Due to paternity uncertainty, parental and grandparental investments along paternal lineages are based on less certain genetic relatedness with the children and grandchildren. Additionally, the hypothesis of preferential investment (Laham, Gonsalkorale, and von Hippel, 2005) predicts investment to vary according to available investment options. Two previous studies have tested this hypothesis with small samples and conflicting results. Using the second wave of the large and multinational Survey of Health, Ageing and Retirement in Europe (SHARE), collected in 2006-07, we study the preferential investment hypothesis in contemporary Europe based on self-reported grandparental provision of child care. We predict that 1) maternal grandmothers provide most care for their grandchildren, followed by maternal grandfathers, paternal grandmothers and last by paternal grandfathers; 2) maternal grandfathers and paternal grandmothers provide equal amounts of care when the latter do not have grandchildren via a daughter; 3) women who have grandchildren via both a daughter and a son will look after the children of the daughter more; and 4) men who have grandchildren via both a daughter and a son will look after the children of the daughter more. Results support all four hypotheses and provide evidence for the continuing effects of paternity uncertainty in contemporary kin behavior.

Maternal guilt.

The recent emphasis on humans as cooperative breeders invites new research on human family dynamics. In this paper we look at maternal guilt as a consequence of conditional maternal investment. Solicited texts written by Finnish mothers with under school-aged children in 2007 (n = 63) described maternal emotions perceived as difficult and forbidden. Content analysis of guilt-inducing situations showed that guilt arose from diverging interest and negotiations between the mother and child (i.e., classic parent- offspring conflict). Also cultural expectations of extensive and perpetual high-quality maternal investment or the "motherhood myth" induced guilt in mothers. We argue that guilt plays an important role in maternal-investment regulation. Maternal guilt is predicted to vary with social and cultural context but also to show universal characteristics due to parent-offspring conflict and allomaternal manipulation. Results are preliminary and intended to stimulate research into the mechanisms, gender differences and cultural variations of guilt and other social emotions in human parenting.