• 1 Introduction
  • 2 The Model
  • 2.1 Unobservable Contributions
  • 2.2 Observable Contributions
  • 3 Experimental Evidence
  • 3.1 Experimental Design and Procedures
  • 3.2 Hypotheses
  • 3.3 Results
  • 4 Discussion
  • 5 Appendix
  • References