Can progressive, statutory legal reform improve the lives of the poor in places where formal legal institutions have limited reach? Using new survey data on over 4,500 legal disputes in rural Liberia, we show that the vast majority of cases are taken to customary institutions, including by demographic groups who possess limited customary rights, such as women, minorities and the poor. We develop and test a simple model of forum choice in which the socially disadvantaged trade off the repressive aspects of customary law against the formal system's high costs and punitive approach to justice. Consistent with the model, plaintiffs facing a disadvantageous pairing under the custom -- e.g., women suing men -- are more likely to choose the formal system and are relatively happier when they do. We apply these insights to a randomized trial of a pro bono legal aid program designed to overcome the tradeoff between restorative customary remedies and more egalitarian formal law. On average, legal aid has strong and robust impacts on case outcomes, as well as significant downstream economic benefits -- including increases of 0.24 and 0.38 standard deviations, respectively, in household and child food security. Furthermore, in line with our model, both demand for and impacts of the program are greater for plaintiffs facing poor odds in the customary system. Our results suggest that there are large socioeconomic gains to be had from improving access to formal law, by making its institutions more competitive with the organizational forms of the custom.