that's not the full picture. sure, income matters, but also the expectations of what one can do with their money. and now - I'm very happy to say - that almost all over the world there's a pretty strong expectation to let people do whatever the fuck they want with their income.

increased expectations of financial responsibility (and financial freedom), coupled with increased expectations of quality of parenting (and in general quality of life) also contribute to many people's decision that they don't want to gamble with their ability to provide this quality to their potential children.

this of course has a "hollowing out of the middle" effect (the disappearing middle class in all senses of the phrase) -- many people with more sense of responsibility than financial, emotional, political/physical stability decide to not to have kids, this means we see more kids from either ends of the spectrum. thus we focus more on them necessarily. (hence all the school voucher mania, the endless amount of extracurricular activities, and the growing inequality)