Social contagion in primates: Moderating factors and significance for individuals and the group

Social contagion, which involves behavioural and emotional state matching, is a crucial process for healthy social functioning. In humans as well as other species, it underpins key elements of social interaction, including empathy and social learning, thereby allowing individuals to effectively engage with others and their environment. Given their highly social nature and complex societies, studying social contagion in primates sheds light on the evolution of these processes and their significance in individuals' daily lives. Here, we review this research, and identify factors that are thought to moderate the presence of social contagion; namely age, sex, dominance rank and social closeness. Although there are few direct comparisons in the literature, patterns of contagion and the factors influencing them vary across species and behaviour, appearing to differ especially between emotional contexts. We therefore seek to categorise contagious behaviour along an affective dimension, highlighting when there are ambiguities, and then considering the distinct evolutionary benefits of positive and negative social contagion. We further consider the significance of social contagion within social learning, using this as a unifying framework to understand contagion.