In this study, I examine the effectiveness of a national board gender diversity disclosure requirement, which is, arguably, on the “soft” end of the continuum of boardroom interventions. Using a panel dataset of 1847 hand-collected corporate disclosures from 2015 to 2018, I perform a post-event, historical trend analysis of the efficacy of Canada's 2014 intervention. I find evidence of real progress in the 4 years following intervention against several benchmarks. Specifically, improvements in critical mass are documented, with the proportion of (...) boards with three of more women reaching 18.8% in 2018. Tokenism remains an issue with very little change in the number of Canadian boards with only one woman. Further analyses using a sorting methodology and panel regression analysis reveal that progress toward board gender diversity is achieved by those with board gender targets, board seat renewal policies, and written board diversity policies. In addition, I find that larger companies achieve greater progress and that progress is clustered by industry. (shrink)