Preschoolers' thinking about kinship was explored by means of a simple induction task. A target animal was described as possessing a property, and children were asked whether each of 2 other animals shares the property or not. When no information about kinship was given, children in Experiment 1 based their inductions of biological properties on physical similarity. However, when kinship relations were specified, children judged that dissimilar-looking kin share more biological properties than similar-looking but unrelated members of the same species. In Experiment 2, describing the similar animals as socially related did not change the basic pattern of inductions obtained in the first experiment. Moreover, subjects in Experiment 3 did not induce more acquired physical and psychological properties among families than among unrelated animals. The results of Experiments 1 and 2 illustrate one case where young children favor a nonperceptible relation (kinship) over a perceptible one (similarity) as a basis for judg-ment. The overall pattern of data suggest that young children distinguish to some extent between the biological and social domains.