I suppose by-now every right thinking and reasonably well read adult knows what the ‘poverty-trap’ is, even if most of us aren’t too clear about what there is to do about it. Being stuck in one of these traps could be thought to be like being stuck in a (not necessarily very deep) well with a slimy surround wall. The more you struggle to get out, the harder it gets: your strength disippates, and the walls get to be even more slippery. This could also be called a negative feedback loop.
Well now there is the suggestion that something similar may exist in the world of fertility. As Wolfgang Lutz suggests in this power point presentation, the critical level may be 1.5. No society which has fallen below this level has -to date – returned above it. (Many thanks here to commenter CapTvK who sent me the link).