1. Department of Economics, London School of Economics, Houghton Street, London WC2A 2AE, UK and CIFAR (e-mail: )
2. Department of Economics and Centre for Macroeconomics, London School of Economics, Houghton Street, London WC2A 2AE, UK (e-mail: )
3. IIES, S-106 91 Stockholm, Sweden and Stockholm University and CIFAR (e-mail: )