Received: 14 April 2000/Accepted: 29 December 2000
All correspondence to Pedro Mira. Namkee Ahn is grateful for financial support received from the Bank of Spain and from Spain's Ministerio de Educación y Cultura, grant SEC97-1249. We benefited from comments by two anonymous referees and by seminar participants at FEDEA, CEMFI and ESPE-98. All remaining errors are our own. Responsible editor: John F. Ermisch.