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 step-grandparents. 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 step-GFs' step-grandchild investment.
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