1. Department of Economics and Statistics, University of Salerno , Salerno, Italy
2. Department of Industrial and Information Engineering and Economics, University of L'Aquila , L’Aquila, Italy
3. Department of Social and Economic Sciences, University of Rome, Rome, Italy, and IPEG, SEF, University of the Witwatersrand , Johannesburg, South Africa
4. Department of Economics, Business, Mathematics and Statistics, University of Trieste, Trieste, Italy, and Department of Economics, University of Graz , Austria