Barro's model is an AK model, and there cannot be dynamic inefficiency since the social yield of the capital is higher than the growth rate. But it may be that the private yield and thus the interest rate are lower than the growth rate. One can thus have a Ponzi game and the government can allow...

Capital (physical and human) doesn't flow from rich to poor countries. We show that in order to solve these twin paradoxes, assumption of externality of physical capital is better than assumption of externality of human capital.