Why do some workers join trade unions and others do not? This article attempts to shed new light on this question by considering how certain behavioural and attitudinal differences among workers shape their propensities to join trade unions. In particular, it is hypothesized that variations in civic and political engagement, as well political and cultural values, are important but overlooked determinants of trade union membership. This hypothesis is tested with data on 12 affluent democracies from the World Value Survey. Results from binary logistic regression models indicate that individual-level variation in trade union participation positively correlates with civic and political participation, with left-leaning political views, but not with post-materialist values. The article concludes by discussing the research’s limitations and its implications for issues surrounding organized labour’s changing fortunes.