First Author:   Riccardo Tilli Department of Public Economics - Sapienza - University of Rome riccardo.tilli@uniroma1.it   via del Castro Laurenziano, 9 Rome, 00161, Italy   +390649766376(Phone) +390649766376(Fax) http://dep.eco.uniroma1.it/~tilli/ Second Author: Enrico Saltari Department of Public Economics - Sapienza - University of Rome enrico.saltari@uniroma1.it   via del Castro Laurenziano, 9 Rome, 00161, Italy   +390649766213(Phone) +39064462040(Fax) http://www.webalice.it/esaltari/Personal%20Web%20Page.htm