The IPAT-model developed by Ehrlich and Holdren is widespread in ecological economics in order to quantify the impact of population growth on environmental deterioration. We comment on this model and extensions proposed by several authors from a theoretical and empirical point of view.

In this paper, we investigate the external effects of the parent's decisions on the number of newly born children and the firm's decisions on the amount of polluting emissions that occur in industrial production. We employ an optimal control model which comprises three stock variables...