Wagner's Law would imply an exponential growth process of the ratio between public expenditures and national income (G/Y). However the law may be rejected both on theoretical and empirical reasons, because it disregards the role of ever increasing distortionary taxation. But, under some conditions, the combination of Wagner's Law and the Pigou's conjecture that the excess burden of taxation constraints the growth of public expenditures, may be captured by a non-linear first order differential equation. The equation is the Verhulst's logistic, originally invented to model Malthusian predictions on population growth. The integration of a Verhulst equation generates a S-shaped curve and preliminary econometric estimates on very long run trends (around 1870-2000) of G/Y in US, UK, France, Germany, Italy, Sweden seem to confirm similar logistic trajectories, in spite of different individual parameters.