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

Asymmetric grandparental caregiving is usually explained by the paternity certainty hypothesis. Accordingly, the lower investment by grandfathers (GFs) and paternal grandparents, as compared to grandmothers (GMs) and maternal grandparents, is based on differential kinship certainty to grandchildren. Hence, differential caregiving by grandparents is equated with their on-average different genetic relatedness to a grandchild. But what about nonbiological grandparents? All else being equal, step-grandparents should not invest highly in step-grandchildren and their investment should not be asymmetric because no differences in kinship certainty exist. However, coresidence with a biological grandparent might enhance step-grandparents' investment. From a total of 508 respondents from Germany and the USA, 108 were step-grandchildren who reported kin caregiving from 151 stepgrandparents. Further, we analyzed data of 45 stepparents, 1,005 biological parents, and 1,585 biological grandparents. We identified different types of step-grandparents. Subjects reported step-grandparents who were spouses of biological grandparents (Type I) much more often than step-grandparents who were parents of stepparents (Type II). Investment and emotional closeness ratings for step-grandparents were relatively high, however, on average somewhat lower than that of biological grandparents.Step-GFs provided more caregiving than step-GMs for step-grandchildren. More detailed analyses, however, revealed that this applied only for later partners of biological GMs (Type Ib) who were not already stepparents of the parents (Type Ia). Type Ib step-grandparents generally invested less in step-grandchildren than Type Ia; however, Type Ib maternal step-GFs, by contrast, invested more. Similar to step-GFs, stepfathers also invested more than stepmothers. However, this could be explained by the stepfathers' household connection with their stepchildren. We conclude that mating effort best describes the differential stepGFs' step-grandchild investment.

Evolutionary theories explain altruism between related individuals, not only for nonhuman animals but also humans themselves. In sociobiology and evolutionary psychology, the supposedly universally found stronger matrilineal kin caregiving by grandparents, aunts, and uncles is often explained by paternity uncertainty in the male descent line. The present article provides an overview of theories and results of the evolutionary research. I will focus, in particular, on the universal caregiving pattern as well as on cultural variety in kin caregiving, the role of actual paternity certainty in the society, theoretical inconsistencies, and nonconsanguineous step relationships. From the analysis of the empirical data, I will conclude that the paternity certainty hypothesis is in fact not a very suitable explanation for the asymmetric kin caregiving found in humans. I will discuss how human behavior toward relatives, in particular grandchildren, can be alternatively explained from an evolutionary perspective.

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