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.