1. University of Maryland, 3114 Tydings Hall, College Park, MD 20742 and Resources for the Future (e-mail: )
2. Loughborough University, Leicestershire LE11 3TU, United Kingdom (e-mail: )
3. London School of Economics, London WC2A 2AE, United Kingdom (e-mail: )
4. Duke University, Durham, NC 27708 ()