This paper develops a non-linear theoretical relationship between public spending and economic growth. The model identifies the “optimal” size of government and the “optimal” composition of government spending. Given the size of the government, different allocations of public resources lead to different growth rates in the transition dynamics, depending on their elasticity. We argue that neglecting the hypothesis of non-linearity and the different impact different kinds of public spending have on economic performance results in models which suffer from mis-specification. Traditional linear regression analysis may thus be biased