Cross-cultural and cross-species comparisons of mutual gaze and infant emotion: Challenging Western, Educated, Industrialized, Rich, and Democratic (WEIRD) and Barren, Institutional, Zoo, And other Rare Rearing Environments (BIZARRE) assumptions.

In this study of mutual gaze (MG) and emotion, using multiple groups of chimpanzee infants to compare with multiple groups of human infants, we provide a methodology to assess two deeply rooted assumptions: (1) Psychological outcomes in chimpanzees are uninfluenced by socioecological conditions, and (2) samples from humans living in Western, middle-class family settings are adequate representatives of humankind. Even though MG is widely viewed as foundational for social cognition, and gaze becomes coordinated with emotion around 1 year of age, these topics have rarely been studied in 1-year-old chimpanzees. We studied four diverse chimpanzee groups-that is, 1-year-old infants living at Chester Zoo (n = 4), in Gombe National Park (n = 12), at the Primate Research Institute (n = 3), and in a human home (n = 2)-and compared them with three diverse groups of human 1-year-olds: urban-UK (n = 8), subsistence farming-Nso (n = 12), and foraging-Aka (n = 10). MG occurred 22% of the time and did not differ across groups or species. We found no species differences in any infant Emotion or in any MG-Emotion co-occurrence pattern. In three of five types of MG-Emotion co-occurrence, the Primate Research Institute chimpanzee infants differed from at least one of the other chimpanzee groups (we reject Assumption 1), and the UK infants differed from at least one of the other human groups (we reject Assumption 2). We conclude there are meaningful influences of socioecology on the functions of some social cognition outcomes, and, thus, it is necessary to sample multiple diverse groups to detect true species differences. (PsycInfo Database Record (c) 2026 APA, all rights reserved).