1. University of Exeter, Exeter EX4 4PY, United Kingdom;
2. University of Innsbruck, 6020 Innsbruck, Austria;
3. Middlesex University, London NW4 4BT, United Kingdom;
4. Max Planck Institute for Research on Collective Goods, 53113 Bonn, Germany;
5. University of Cologne, 50923 Cologne, Germany;
6. Institute of Labor Economics, 53113 Bonn, Germany