This article investigates causes of the legislative choice to mobilize private litigants to enforce statutes. It specifies the statutory mechanism, grounded in economic incentives, that Congress uses to do so, and presents a theoretical framework for understanding how certain characteristics of separation of powers structures, particularly conflict between Congress and the president over control of the bureaucracy, drive legislative production of this mechanism. Using new and original historical data, the article presents the first empirical model of the legislative choice to mobilize private litigants, covering the years 1887 to 2004. The findings provide robust support for the proposition that interbranch conflict between Congress and the president is a powerful cause of congressional enactment of incentives to mobilize private litigants. Higher risk of electoral losses by the majority party, Democratic control of Congress, and demand by issue-oriented interest groups are also significant predictors of congressional enactment of such incentives.