The concept of nurture kinship in the anthropological study of human social relationships (kinship) highlights the extent to which such relationships are brought into being through the performance of various acts of nurture between individuals. The concept stands in contrast to the earlier anthropological concepts of human kinship relations being fundamentally based on "blood ties", some other form of shared substance, or a proxy for these (as in fictive kinship). This conception of the ontology of social ties has become stronger in the wake of David M. Schneider's influential Critique of the Study of Kinship and Holland's subsequent Social Bonding and Nurture Kinship, demonstrating that as well as the ethnographic record, biological theory and evidence also more strongly support the nurture perspective than the blood perspective. Both argue that the earlier blood theory of kinship derived from an unwarranted extension of symbols and values from anthropologists' own cultures (see ethnocentrism).