A and B may not be sufficient reasons either. Here is a counter example from my home state: http://en.wikipedia.org/wiki/Kerala_model#Human_Development_... Economically Kerala is a third world economy, but in Human Development Index and birth rates it is more closer to developed economies. The birth rate is at replacement level and still falling even though most people do not have sufficient pensions, most parents are not professionals and unemployment is endemic. So you will have to look beyond GDP and consumerism for falling birth rates.
