Theoretically, it is possible that a wealthy society might be in a Malthusian equilibrium because people would like to reproduce more but have very high minimum standards for per-capita family wealth.
This society would not be evolutionarily stable since the members with the lowest standards will reproduce more causing the minimum standard to decrease. This process will continue until it reaches the point where standards are so low that any additional children would simply starve to death.
I can't see anything obviously wrong in that reasoning, but in Italy that situation has more-or-less obtained for decades and I can't see any sign of such a process happening.
I guess what's happening is some process preventing the standards of some members to fall much below the standards of the rest of the society. (If you know your children will be ostracized by their peers, making their life much harder, unless they wear expensive clothes, have expensive toys, etc., then you might not want your children to wear cheap clothes and have cheap toys, even if in isolation they'd enjoy them just as much as expensive ones.)
I was browsing my RSS feed, as one does, and came across a New York Times article, "A Village With the Numbers, Not the Image, of the Poorest Place", about the Satmar Hasidic Jews of Kiryas Joel (NY).
Their interest lies in their extraordinarily high birthrate & population growth, and their poverty - which are connected. From the article:
From Wikipedia:
Robin Hanson has argued that uploaded/emulated minds will establish a new Malthusian/Darwinian equilibrium in "IF UPLOADS COME FIRST: The crack of a future dawn" - an equilibrium in comparison to which our own economy will look like a delusive dreamtime of impossibly unfit and libertine behavior. The demographic transition will not last forever. But despite our own distaste for countless lives living at near-subsistence rather than our own extreme per-capita wealth (see the Repugnant Conclusion), those many lives will be happy ones (even amidst disaster).
So. Are the inhabitants of Kiryas Joel unhappy?