[1] Univ Alabama, Dept Econ Finance & Legal Studies, Tuscaloosa, AL 35487 USA
[2] Nankai Univ, Sch Math Sci, Tianjin 300071, Peoples R China
[3] Inst Study Labor IZA, D-53113 Bonn, Germany
[4] Univ Pittsburgh, Dept Econ, Pittsburgh, PA 15260 USA
[5] Univ Oklahoma, Dept Econ, Norman, OK 73019 USA