In a recent post on Demography Matters Claus offered a nice summary of the latest revision of the low fertility trap hypothesis as advanced by Wolfgang Lutz et al. As summarized below, there are three basic components to this hypothesis:
As Lutz says the key idea is that once fertility falls below a certain level (and even in the event that the hypothesis proved to be well founded this level could only be determined empirically, on the basis of actual experience) a self-reinforcing demographic regime may be established from which it is hard to escape, in the sense of raising fertility back up towards replacement levels. The cut-off point which Lutz et al start from is 1.5 (and in this they take their lead from a proposal by Peter Macdonald in this paper ). This figure does seem to have some coherence in terms of actual experience to date, since with the exception of Denmark - which did briefly fall under 1.5 tfr in the 1990s - no country seems to have gone below it and come back up again.