Dennis W. Carlton<BR>Randal C. Picker<BR>Working Paper 12902<BR><a href="http://www.nber.org/papers/w12902" target="_blank" >http://www.nber.org/papers/w12902</A><BR>NATIONAL BUREAU OF ECONOMIC RESEARCH<BR>1050 Massachusetts Avenue<BR>Cambridge, MA 02138<BR>February 2007 <BR>