For chart 5.2A, the regression of the real per capita GDP growth rate over the entire period on the change in the top marginal income tax rate using robust standard errors gives a non-significant coefficient at the 10 per cent threshold (p-value = 0.126) and a very low R-squared (R-squared = 0.07). For chart 5.2B, the regression of the change in the top 1 per cent income share on the change in the top marginal income tax rate using robust standard errors gives a highly significant coefficient (p-value = 0.001) and a much higher R-squared (R-squared = 0.50). In addition, a century ago, United States tax revenues, measured as a share of GDP, were much smaller than they are at present (Hinrichs, 1966). The other main contribution to revenue growth derived from a new emphasis on progressive taxation (Comia, Gomez-Sabaini and Martorano, 2011). People contribute to social welfare projects in the form of both money and labour, in often complex arrangements that determine how much each household should pay and what penalties apply to free riders (Olken and Singhal, 2011). Given the nature of these arrangements, it is likely that the contributions are quite progressive.