No other species on Earth cooperates with non-relatives to the extent and over the range of contexts that humans do. Also, humans are everywhere – which makes them quite easy to find and experiment on. Our research asks how punishment and reputation-based partner choice stabilise cooperation. We also explore how culture shapes variation in social behaviour. Finally, a new strand of our research is exploring how mental disorders, such as psychosis, affect social cognition. We use online crowdsourcing websites, lab-based experiments, large online datasets and real-world methods to identify the mechanisms that promote cooperation – and the contexts and settings that undermine it.