High levels of party voting cohesion are common in modern legislatures. Current explanations divide into sociological (based on norms and roles) and rational choice (based on systems of punishment and reward). The latter approach dominates, but cannot explain cohesion in systems with weak disciplinary sanctions, such as the British House of Lords. Social psychology has provided a great deal of insight into conformity in groups, but this has rarely been deployed in studying parties. Neuroscience now also allows us better to understand the physiological mechanisms underlying responses such as need to belong and fear of ostracism. This article outlines key theories and findings from psychology relevant to parliamentary party cohesion, and then explores these using survey data from the relatively ‘discipline free’ House of Lords. It is suggested that psychological factors such as social identity are important to the operation of party groups, and stronger interdisciplinary links are proposed between political science and psychology.