What do we know about the timing of fertility transitions in Europe?

The Princeton project on the decline of fertility in Europe (the European Fertility Project) suggested that this historical fertility transition occurred virtually simultaneously in a wide variety of economic and social environments. This finding has been cited widely as evidence for an innovation/diffusion view of fertility transitions. We demonstrate that the demographic methods used to date the fertility transition in Europe--primarily Ig, and (to a lesser extent) the Coale-Trussell M&m indices--may fail to detect the initial stages of a fertility transition and therefore cannot be used as the basis for strong statements about the timing of transitions. We review these measurement problems and their implications for the current understanding of the European fertility transition.