This article provides an overview of previous studies on social mood and its implications for social behaviour. While economics and game theory conflate the idea of rationality with rational expectations and objective information, social psychologists and sociologists suggest that individuals are often subject to mood under many different circumstances, from social relationships to voting and consumer behaviour. Evolutionary and behavioural studies suggest that this is far from being "irrational", although it can lead to unexpected socially undesirable outcomes. By considering research in economics, behavioural sciences, sociology and psychology, our article suggests that social mood should be disentangled from emotions, and we need to discuss the need for a more sophisticated notion of social rationality. Our findings suggest that integrating (big, behavioural, population) data and (psycho-sociological) theory is key to understanding why social mood is increasingly influencing social relationships, economic exchanges and political preferences in complex, globally hyperlinked societies.