NBER Working Paper No. 13934
---- Acknowledgements -----
This paper was presented at the ISI Conference in Seoul, Korea, on August 27, 2001. It has benefited from discussions with Jaap Abbring, Pedro Carneiro, Steve Durlauf, Seong Moon, Salvador Navarro, T.N. Srinivasan, John Trujillo, Ed Vytlacil, Jordan Weil and Yu Xie; and comments by the editor, Eugene Seneta, and two anonymous referees. This research was supported by NIH R01-HD043411, NSF SES-0241858 and the Geary Institute, University College Dublin, Ireland. The views expressed in this paper are those of the author and not necessarily those of the funders listed here nor of the National Bureau of Economic Research.