People express essentialist beliefs about social categories from an early age, but essentialist beliefs about specific social categories vary over development and in different contexts. Adapting two paradigms used with Western samples to measure social essentialism, we examined the development of essentialist beliefs about seven social categories (gender, race, nationality, religion, socioeconomic status, ethnicity, and team fan bases) among 5- to 10-year-old children (N = 88) and adults (N = 273) in Iran, a population that is underrepresented in psychology research. Focusing on natural-kind reasoning, we investigated the relative contribution of biological perception of social categories as well as cultural and motivational factors in the development of essentialist beliefs about these categories. Our findings suggest that biological perception of social categories plays a key role and that cultural and motivational factors become more relevant in essentialist reasoning about social categories that are not perceived as biologically marked. The developmental patterns of essentialist reasoning in our study also closely parallel those found in other cultures, namely the United States and Turkey, further suggesting the primary role of biological perception of social categories in natural-kind reasoning about the social world. (PsycInfo Database Record (c) 2024 APA, all rights reserved).