Only a small percentage of citizens in advanced industrial democracies are members of a political party; the numbers have largely been declining over the last five decades. Current knowledge tells us that the minority who do join have five dimensions of incentives for doing so, some being more popular than others. Yet, when a party experiences an acute period of expansion in its memberbase, do peoples’ incentives for joining change? Are different kinds of people attracted? How does this affect the party and wider society? Using advanced statistical methods, supplemented with qualitative interviews and content analysis, the project aims to answers these questions.