Psychological adaptations for fitness interdependence underlie cooperation across human ecologies.

Humans evolved to solve adaptive problems with kin and nonkin across fitness-relevant domains, including childcare and resource sharing, among others. Therefore, there is a great diversity in the types of interdependences humans experience across activities, relationships, and ecologies. To identify human psychological adaptations for cooperation, we argue that researchers must accurately characterize human fitness interdependence (FI). We propose a theoretical framework for assessing variation in FI that applies to the social interactions humans would have experienced across situations, relationships, and ecologies in the ancestral past and continue to experience today. According to this model, FI is characterized along four dimensions: (a) corresponding versus conflicting interests (b) mutual dependence versus independence, (c) asymmetrical versus symmetrical dependence (i.e., power), and (d) coordination. Because humans evolved to be highly mutually dependent on others to solve myriad adaptive problems, even compared to our closest living relatives, there is immense variability in the types of interdependences humans experience in daily life. Here, we describe the kinds of variation in interdependence humans experience, paying particular attention to social life in small-scale societies. In demonstrating the diversity of conflicts and coordination problems humans manage, we contend that humans evolved psychological adaptations to infer from signals, cues, and properties of the environment the four dimensions of FI under degrees of uncertainty to reduce the costs of cooperation. We conclude by discussing the theoretical implications of FI theory and emphasize that when individuals understand that others depend on them, it gives way to a new means of leverage to influence how others behave toward them. (PsycInfo Database Record (c) 2025 APA, all rights reserved).