Why dogs have puppies and cats have kittens: the role of birth in young children's understanding of biological origins.

Three studies examined young children's understanding of the biologically causal role of birth in determining animal properties and species kind identity. In Studies 1 and 2, 4- to 7-year-olds and adults were told stories in which a baby was born to an animal of one species (e.g., a horse) but was adopted and raised by an animal of another species (e.g., a cow). In Study 1, children were asked to judge which parent the baby would resemble on a set of physical properties and beliefs. The majority of children were unable to say that the baby would resemble the birth parent on physical properties but share the beliefs of the adoptive parent. These results indicate that children were not using domain-specific causal understandings to reason about the origins of these properties. In Study 2, however, when asked to explicitly predict the kind of the baby, even 5-year-olds were able to reliably judge that the baby would be of the same species kind as the birth parent rather than the adoptive parent. This result suggests that children do understand at some level that birth determines species kind. Study 3 examined further the extent to which knowledge about birth influenced children's inferences about properties. Five-year-olds were asked to judge whether a baby would share a set of physical and nonphysical properties with its mother or its father. The results showed that children who knew the factual information about where babies come from (i.e., inside mommies' tummies) were more likely to attribute the mother's properties to the baby than the father's, regardless of whether the properties were physical or nonphysical. But this finding was true only if the property of one of the parents was not inherently more desirable or true than that of the other parent. In sum, the results of these 3 studies indicate that knowledge of birth does play a role in children's inferential reasoning, even for 5-year-olds, but that that role is not domain-specific. The implications for children's understanding of biological inheritance are discussed.