ABSTRACTThis is an investigation into the reasons for young people's union membership decisions in the UK. It is carried out by an examination of a survey of London sixteen‐year‐old school‐leavers (1979–80). The data are used in two ways. First, there is a comparative examination of a similar study based on the Bain‐Elsheikh propensity‐opportunity model. Second, the data are re‐examined to identify different and distinct strands of motivation or action, intrinsically and externally determined. These are related to formal theories from different disciplines which are linked together by the establishment of a definition of a trade union as a group, institution and social movement. The theory is extended to develop hypotheses about membership behavio