I am grateful to the John D. and Catherine T. MacArthur Foundation and the John Simon Guggenheim Memorial Foundation for financial support, to Marla Lieberman for interesting and useful conversations, to Helen Tauchen for insightful comments, and especially to Shelly Lundberg for suggestions that have shaped and reshaped this paper. I am also grateful to Ted Bergstrom and to anonymous referees for their help. Of course I retain responsibility for the views expressed. Earlier versions of this paper were presented at Washington University in St. Louis, the University of Washington, Southern Illinois University in Carbondale, Princeton University, the Population Association of America, Northwestern University, the University of Chicago, the University of Pennsylvania, the University of Michigan, and Johns Hopkins University. Responsible editor: Alessandro Cigno.