It has often been argued theoretically that a ‘critical mass,’ ranging from 10 to 35 per cent women, is needed before major changes in legislative institutions, behaviour, policy priorities and policy voting occurs. This paper examines one of the less‐explored dimensions of the critical mass concept: Is there a process by which women reaching a critical mass of the legislature accelerates the election of further women? Using data from the Inter‐Parliamentary Union and International Institute for Democracy and Electoral Assistance, we analyze this question for 20 industrialized democracies over a period of half a century, longer than any other relevant research. Descriptive results indicate that gains in women's representation have been incremental rather than a critical mass accelerating the election of women to legislatures. In a multivariate analysis of the percentage of women in the lower house of the legislature, the critical mass is tested against established explanations of women's gains in seats: institutional rules, egalitarian political culture, political parties and economic development. Of two measures of the critical mass theory, one has no impact and the second results in only a small increase in women's gains. Far from being clearly demonstrated, critical mass theories need empirical testing.