How does living in an environment with many or few family relatives shape our psychology? Here, we draw upon ideas from behavioral ecology to explore the psychological effects of ecological relatedness-the prevalence of family relatives in one's environment. We present six studies, both correlational and experimental, that examine this. In general, people and populations that live in ecologies with more family relatives (Studies 1-4b), or who imagine themselves to be living in such ecologies (Studies 2/3a/3b/4b), engage in more extreme pro-group behavior (e.g., being willing to go to war for their country), hold more interdependent self-concepts, are more punishing of antisocial behaviors (e.g., support the death penalty for murder), identify themselves as more connected to and trust nearby groups (e.g., their community and neighbors) but less so distant groups (e.g., foreigners, the world), and also judge sibling incest as more morally wrong. These effects are examined across three countries (the United States, Ghana, the Philippines) and are robust to a range of controls and alternative explanations (e.g., ingroup preferences, familiarity effects, kinship intensity). The current work highlights the psychological effects of an underexamined dimension of our social ecology, provides a set of methods for studying it, and holds implications for understanding the ecological origins of a range of social behaviors and cultural differences. (PsycInfo Database Record (c) 2024 APA, all rights reserved).