Why do only some people who support parties actually join them? Evidence from Britain

What makes people join a political party is one of the most commonly studied questions in research on party members. Nearly all this research, however, is based on talking to people who have actually joined parties. This article simultaneously analyses surveys of members of political parties in Britain and surveys of non-member supporters of those same parties. This uniquely enables us to model the decision to join parties. The results suggest that most of the elements that constitute the influential ‘General Incentives Model’ are significant. But it also reveals that, while party supporters imagine that selective benefits, social norms and opposing rival parties’ policies are key factors in members’ decisions to join a party, those who actually do so are more likely to say they are motivated by attachments to their party’s values, policies and leaders, as well as by an altruistic desire to support democracy more generally