1. Department of Industrial Engineering and Management, Politecnico di Milano, 20133 Milan, Italy.
2. Department of Production Systems and Business Economics, Politecnico di Torino, 10129 Turin, Italy.
3. Bureau of Research on Innovation, Complexity and Knowledge, Collegio Carlo Alberto, 10024 Moncalieri, Italy.
4. Andrew Young School of Policy Studies, Georgia State University, Atlanta, GA 30302, USA.
5. Department of Economics Cognetti de Martiis, University of Torino, 10124 Turin, Italy.
6. National Bureau of Economic Research, Cambridge, MA 02138, USA.