1. Department of Economics, Finance and Legal Studies, University of Alabama, Tuscaloosa, AL, USA
2. School of Mathematical Sciences, Nankai University, Tianjin, People's Republic of China
3. Institute for the Study of Labor (IZA), Bonn, Germany
4. Department of Economics, Athens University of Economics and Business, Athens, Attica, Greece
5. Department of Economics, University of Miami, Coral Gables, FL, USA