1. Centre for Economic Policy Research, London, EC1V 0DX, United Kingdom
2. KOF Swiss Economic Institute, ETH Zurich, 8092 Zurich, Switzerland
3. Tinbergen Institute and Department of Spatial Economics, Vrije Universiteit (VU) Amsterdam, 1081 HV Amsterdam, the Netherlands
4. Department of Economics, University of Michigan, Ann Arbor, MI 48109
5. National Bureau of Economic Research, Cambridge, MA 02138-5398
6. Centre for Networks and Collective Behaviour, Department of Mathematical Sciences, University of Bath, BA2 7AY Bath, United Kingdom
7. Department of Economics, Yale University, New Haven, CT 06520-8268