Why do political actors undertake reforms that constrain their own discretion? We argue that uncertainty generated by political competition is a major driver of such reforms, and test this argument using subnational data on Mexican states' adoption of state-level access to information (ATI) laws. Examining data from 31 Mexican states plus the Federal District, we find that more politically competitive states passed ATI laws more rapidly, even taking into account the party in power, levels of corruption, civil society, and other factors. The fine-grained nature of our data, reflecting the staggered timing of elections, inauguration dates, and dates of passage, allows us to distinguish between different theoretical mechanisms. We find the greatest evidence in favor of an insurance mechanism, by which incumbent parties who face uncertainty over future political control seek to ensure access to government information, and means of monitoring incumbents, in the future in case they lose power.