1Acknowledgements: I acknowledge financial support from Orkestra- Basque Institute of

21643/ECON). I am grateful to Mikel Navarro for very valuable comments and suggestions on previous drafts of this paper. I also want to thank Benita Aybar, José Manuel Feito, and particularly Cristina Casaseca and María Cristina González from the Spanish Statistical Institute (INE) for their constant support and advice during the analysis of the database