Individuals tend to explain the characteristics of others with reference to an underlying essence, a tendency that has been termed psychological essentialism. Drawing on current conceptualizations of essentialism as a fundamental mode of social thinking, and on prior studies investigating belief in genetic determinism (BGD) as a component of essentialism, we argue that BGD cannot constitute the sole basis of individuals’ essentialist reasoning. Accordingly, we propose belief in social determinism (BSD) as a complementary component of essentialism, which relies on the belief that a person’s essential character is shaped by social factors (e.g., upbringing, social background). We developed a scale to measure this social component of essentialism. Results of five correlational studies indicate that (a) BGD and BSD are largely independent, (b) BGD and BSD are related to important correlates of essentialist thinking (e.g., dispositionism, perceived group homogeneity), (c) BGD and BSD are associated with indicators of fundamental epistemic and ideological motives, and (d) the endorsement of each lay theory is associated with vital social-cognitive consequences (particularly stereotyping and prejudice). Two experimental studies examined the idea that the relationship between BSD and prejudice is bidirectional in nature. Study 6 reveals that rendering social-deterministic explanations salient results in increased levels of ingroup favoritism in individuals who chronically endorse BSD. Results of Study 7 show that priming of prejudice enhances endorsement of social-deterministic explanations particularly in persons habitually endorsing prejudiced attitudes. (PsycINFO Database Record (c) 2011 APA, all rights reserved)