Party politics and electoral research generally assume that party members are loyal voters. This article first assesses the empirical basis for this assumption before providing individual-level explanations for defection. It combines prominent theories from party politics and electoral behaviour research and argues that internal disagreement and external pressure can each bring about disloyal voting. The hypotheses are motivated with multi-country European survey data and tested on two sets of party-level national surveys. The results show, first, that, on average, 8 per cent of European party members cast a defecting vote in the last election, and second, that dissatisfaction with the leadership is the strongest predictor of defection. Additionally, internal ideological disagreement is associated with higher probabilities of defection, whereas the effects of pull factors in the form of contentious policies are rather limited. These findings emphasise the importance of testing scientific assumptions and the potential significance of party leadership contests.