This article examines the dependence of unionization on labor market i
nstitutions and long-run trends in inflation, unemployment, and the se
ctoral composition of the labor force in 18 OECD countries. The large
number of contending explanations highlights the difficulty in compara
tive sociology of drawing confident conclusions when there are few cas
es to compare. This problem is addressed through diverse evidence-a re
view of the comparative historical record and a time-series analysis-t
o form a Bayesian prior distribution, which is combined with a cross-s
ectional data set to sharpen conventional estimates of economic and in
stitutional effects. This analysis indicates working-class organizatio
n in trade unions expanded in the postwar period where labor acquired
an institutional influence over capitalist labor markets.