Since its beginning in the early 1990s, the empirical growth literature has extended equation A3.8 to account for a variety of long run growth determinants (as public and social capital, trade openness, financial development, quality of institutions etc.). Early works focussing on the role of inequality include Persson and Tabellini (1994) and Alesina and Rodrik (1994). Moreover, equation A3.8 can be estimated for any time interval.