This may be a factor, but ....
We tax productive people up to their eyeballs to subsidize a lavish, by any historical standard, lifestyle for the incompetent. Fertility rates fall in developed countries for a number of reasons, but punitive taxation is one, and it especially targets the middle and upper classes. Immigration is a second factor, for reasons that Jason Richwine got fired for noticing, and being rash enougth to commit to print.