Book excerpt: The interaction of open source and proprietary software and the implications for economic development. Discussions of the economic impact of open source software often generate more heat than light. Advocates passionately assert the benefits of open source while critics decry its effects. Missing from the debate is rigorous economic analysis and systematic economic evidence of the impact of open source on consumers, firms, and economic development in general. This book fills that gap. In The Comingled Code, Josh Lerner and Mark Schankerman, drawing on a new, large-scale database, show that open source and proprietary software interact in sometimes unexpected ways, and discuss the policy implications of these findings. The new data (from a range of countries in varying stages of development) documents the mixing of open source and proprietary software: firms sell proprietary software while contributing to open source, and users extensively mix and match the two. Lerner and Schankerman examine the ways in which software differs from other technologies in promoting economic development, what motivates individuals and firms to contribute to open source projects, how developers and users view the trade-offs between the two kinds of software, and how government policies can ensure that open source competes effectively with proprietary software and contributes to economic development.