JESPER RYBERG is Professor of Practical Philosophy, University of Roskilde, Denmark. His recent books include The Ethics of Proportionate Punishment (Kluwer, 2004) and, as editor, The Repugnant Conclusion: Essays on Population Ethics (Kluwer, 2005). He has published in philosophical journals such as Philosophical Papers, Theoria, Philosophical Quarterly, Res Publica, Ethical Theory and Moral Practice and the Journal of Applied Philosophy. THOMAS S. PETERSEN is Assistant Professor of Practical Philosophy, University of Roskilde, Denmark. He has published in jo