Despite the emerging signs of deterioration of human reproductive performance in high developed countries, the general incidence figures of birth defects do not change. An acceptable explanation has been found in the specific feature of dose-response relationships in teratology characterized by transformations of embryotoxicity end points (i.e. altered growth, structural abnormality, functional deficiency, and death of the developing organism) along the dose scale. Considering the probable nature of individual exposures under different environmental conditions we coin a hypothesis that the incidence of inborn structural and functional defects acquires a character of a dose-dependent quasi-continuous variable which invades more and more the functional sphere and ultimately concerns even the highest brain functions. In this way the frontiers of developmental toxicology and developmental psychology may be fuzzy and future investigations may bring both disciplines into an intimate and fruitful contact.