Predictors of Grandparental Investment
Grandparental investment can be basically defined as any contribution directed toward grandchildren by grandparents. Grandparental investment can include emotional support, physical care, and financial aid, and may also be indirect (e.g., through supporting the grandchildren’s parents). Typically, there are costs associated with grandparental investment (e.g., time or money), which are dependent on the nature of the investment. From an evolutionary perspective, grandparents are predicted to gain indirect fitness (i.e., ensuring the proliferation of their genes) from grandparental investment, either through increasing the number of grandchildren by investing in their adult children to increase fertility or by increasing the well-being and survival of their grandchildren.
Grandparental investment is determined by the length of time grandparents are simultaneously alive with their grandchildren, in the way that when shared lifetime between grandparents and grandchildren increases, so do the grandparents’ possibilities to invest. During recent centuries, the shared lifetime between grandparents and grandchildren has increased substantially (Chapman et al. 2017), and in present-day societies, grandparents and grandchildren have more shared lifetime than ever before (Coall and Hertwig 2010).
Although the length of time grandparents and grandchildren share has increased, it does not alone drive the patterns of grandparental investment. Fertility and mortality declines mean that more grandparents are available to grandchildren, while there are fewer grandchildren to invest in, so there is less competition between siblings and cousins for grandparental investment. Grandparents today are also healthier and wealthier than before. Finally, technological advances and rapid long-distant travel mean that even distant grandparents can easily maintain contact with their grandchildren. Thus, grandparents today have great opportunities to invest time and other resources in their descendants.
Although grandparents are known to invest a large amount of time and resources in their grandchildren, all grandparents tend not to invest equally (Tanskanen and Danielsbacka 2018). The inclusive fitness theory (Hamilton 1964) predicts that biological grandparents should invest more in grandchildren compared to non-biological grandparents. The relatively scarce amount of studies detecting differences between biological and non-biological grandparents has indicated that, in accordance with the theory, investments are more often made by biological than by non-biological grandparents. For instance, using data from 11 European countries, it was found that biological grandparents more likely provided childcare at least weekly to their grandchildren compared to non-biological grandparents (Coall et al. 2014). Moreover, two recent studies using data from the USA and Germany showed that biological grandparents invested more in grandchildren than step-grandparents (Gray and Brogdon 2017; Pashos et al. 2016). Finally, a US study detected the role of genetic relatedness in two-mother families and asked whether grandparents from the biological mothers’ side invest more than grandparents from the non-biological mothers’ side. More regular contact between children and grandparents from the biological mothers’ side than from the non-biological mothers’ side was detected (Patterson et al. 1998).
Sex and Lineage
In addition to genetic relatedness, grandparental investment in their grandchildren tends to vary by sex and lineage. One of the most common findings in grandparental investment studies conducted in contemporary Western societies is that maternal grandmothers invest the most, followed by maternal grandfathers, paternal grandmothers, and finally paternal grandfathers (Euler 2011). This biased grandparental investment pattern has been found in several societies and for many different grandparental investment variables, for instance, childcare, emotional support, and monetary transfers.
Perhaps the most important evolutionary theory explaining this biased investment pattern by grandparent types is based on paternity uncertainty (Daly and Perry 2017). Only maternal grandmothers can be 100% sure that they are related to their grandchildren, while maternal grandfathers (via themselves and their daughters) and paternal grandmothers (via their sons and sons’ children) have one uncertain link of paternity. Paternal grandfathers are in the most uncertain position and have two uncertain links of paternity (via themselves and their sons and via their sons and sons’ children).
Nonetheless, the cultural context may remarkably shape the biased grandparental investment pattern. In patrilocal societies, where women become part of their husbands’ family after marrying, it is observed that paternal grandparents invest more than maternal ones (Daly and Perry 2017; Pashos 2017). In patrilocal societies, grandchildren usually live nearer to the men’s than the women’s relatives, meaning that the grandchildren are likely to interact with their paternal grandparents constantly and may see their maternal grandparents only occasionally. This situation also means that paternal grandparents can become closer to the grandchildren than maternal grandparents. The opposite tends to be true in matrilocal societies, where children usually grow up with their maternal kin. It could also be argued that contemporary Western societies may provide a good base for the study of grandparental investment behavior because they lack clear patrilocal or matrilocal living arrangements (Tanskanen and Danielsbacka 2018). In these societies, individuals can commonly choose the relatives with whom they are willing to interact.
Sex of a Grandchild
During recent years, several evolutionary studies have asked, “Does the sex of grandchildren bias the investments by grandparents?” Genetic relatedness between biological grandparents and grandchildren is, on average, 25%. Because of the asymmetric inheritance of X and Y chromosome, the relatedness actually varies between 23% and 27% (Euler 2011). According to X chromosome relatedness, maternal grandmothers are 25% related to both grandsons and granddaughters, while paternal grandmothers are 0% related to grandsons and 50% related to granddaughters. Relative to Y chromosome relatedness, maternal grandfathers are 0% related to both granddaughters and grandsons, while paternal grandfathers are 0% related to granddaughters and 100% related to grandsons.
According to the inclusive fitness theory (Hamilton 1964), increased genetic relatedness between kin is associated increased investment. According to this logic, there should be no differences based on the sex of grandchildren in the case of maternal grandparents. In contrast, it could be predicted that paternal grandmothers invest more in their granddaughters than grandsons, while paternal grandfathers invest more in their grandsons than granddaughters. A handful of studies have tested the X and Y chromosome relatedness predictions using data from contemporary societies (Chrastil et al. 2006; Tanskanen et al. 2011). These studies have not, however, found convincing support for the hypotheses. Thus, it could be that paternity uncertainty overrides any X or Y chromosome effect in present-day societies.
Outcomes of Grandparental Investment
Quantification of grandparental investment is not always easy, making it difficult to assess how the importance of grandparental investment has changed through time. In traditional and historical societies (i.e., natural fertility and mortality populations), the importance of grandparental investment in grandchildren is often harder to quantify than it is for contemporary societies. While current surveys can directly ask those giving and receiving care about the nature of investment, this is notably absent from historical records. In traditional and historical populations, the presence of older parents in the same household or in the same village as adult children has been used as a proxy for grandparental investment because when older parents live closer to their descendants, they also have a higher probability to invest (Sear and Coall 2011).
Existing research indicates that in traditional and historical populations, grandparental presence has often been associated with increased female fertility (Sear and Coall 2011). According to an extensive review, the presence of mothers was associated with increased female fertility in 7/12 studies, the presence of fathers in 5/8 studies, the presence of mothers-in-law in 4/6 studies, and the presence of fathers-in-law in 3/5 studies. Additionally, mother presence was associated with decreased female fertility in two studies. Overall, in nearly 90% of populations, the attendance of at least one parent or in-law was associated with fertility.
During recent years, an increasing number of studies have detected whether grandparental investment (e.g., childcare or emotional support grandparents provide to their adult children) is associated with parental fertility in contemporary Western societies. Although several studies have found that grandparental investment is associated with improved fertility, others have not detected such an association (Tanskanen and Danielsbacka 2018). These mixed results indicate that in contemporary affluent societies with public support to families, grandparental investment could not be as important for fertility compared to traditional and historical populations.
There is also evidence showing that, in traditional and historical populations, grandparental presence has been associated with improved child survival. Based on a review of the existing literature, maternal grandmother presence was associated with increased child survival in 9/13 studies, paternal grandmother presence in 9/17 studies, maternal grandfather presence in 2/12 studies, and paternal grandfather presence in 3/12 studies (Sear and Mace 2008). Furthermore, paternal grandfather presence was correlated with decreased child survival in 3/12 studies (Sear and Mace 2008).
In contemporary Western societies with low rates of childhood mortality, grandparents are not needed to keep their grandchildren alive compared to traditional and historical populations. However, it is argued that grandparents can still benefit their descendants when measured by “softer” outcomes, for instance, child development and psychological well-being (Sear and Coall 2011). Several studies have shown that grandparental investment is associated with increased grandchild well-being in particular, during family crises, such as parental divorce, severe illness, or even death, although it is unclear whether grandparental investment causally improves child well-being in present-day societies (Tanskanen and Danielsbacka 2017). Hence, in traditional and historical populations, grandmaternal presence may have had a positive influence on child survival, while the presence of grandfathers may have no effect or may even have a negative effect.
Finally, it is assumed that the asymmetric inheritance of X chromosomes may bias the grandparental effect, meaning that granddaughters and grandsons survive differently with the presence of maternal and paternal grandmothers. According to a review of previous studies, in 7/7 studies, boys were found to survive more probably with maternal than paternal grandmothers, and in 6/7 studies, girls were detected to survive more likely than boys with paternal grandmothers (Fox et al. 2010). Nonetheless, in only 4/7 studies, girls were found to survive more probably with paternal grandmothers than maternal ones, which was in contrast with the theory (Fox et al. 2010). Additionally, a more recent study from historical Finland showed only limited support for the effect of X chromosome relatedness (Chapman et al. 2018). These findings indicate that an alternative explanation for the potential grandparental effect is needed.
Grandparental investment tends to vary according to genetic relatedness, sex, and lineage. Biological grandparents typically invest more than non-biological grandparents, and maternal grandmothers invest the most and paternal grandfathers the least. Studies concerning traditional and historical populations have used grandparental presence in the same household or village as a proxy for investment. Several studies have shown that in these populations, grandparental presence has been associated with both increased parental fertility and improved child survival. Studies from contemporary Western societies have shown that grandparental investment is often associated with child well-being, although it is unclear whether the association is causal. Studies detecting associations between grandparental investment and parental fertility have provided mixed results. Although grandparents tend to be highly involved in their grandchildren’s lives in different societies, the outcomes of grandparental investment may vary substantially.
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