The author is grateful to Carl Bergstrom, Laura Betzig, Hillard Kaplan
, Jack Hirshleifer, David Lam, Bobbi Low, Robert Pollak, Alan Rogers,
Oded Stark, Robert Willis, and some anonymous referees for encourageme
nt, helpful advice, and useful comments. This research ions supported
by grants from the National Science Foundation and the National Instit
ute of Child Health and Development.