Kinship is a fundamental component of human sociality influencing a range of behaviors including altruism, aggression, and mating. Though a central focus in psychology's neighboring disciplines, kinship has been largely neglected within psychology. An illustrative example – and the focus here – is research on social categorization. Researchers investigating the categories into which our mind carves the social world have focused primarily on sex, age, and race. Here we present evidence that kinship belongs in the family of fundamental social categories. In a series of experiments using a memory confusion paradigm, we show that participants implicitly encode the kinship relations of social targets and do so to the same extent as sex and age, two previously established robust dimensions of social categorization. The functional framework applied here provides useful guide-rails for investigating how the human mind naturally parses the social world, and, more broadly, helps unite psychology with its neighboring disciplines in which kinship is treated as an important conceptual tool. Copyright © 2008 John Wiley & Sons, Ltd.