I think there is a simple explanation for this phenomenon: when you have to work two jobs trying to make ends meet, or work a very tiring job for little money, the odds are that you will not have the time or energy to read to your children. And lots of research indicates that being read to as a child is a crucial determining factor in educational progress and success later in life. If you are wealthy you may work long hours to accomplish that, but you will pay someone else to read to your children. This is, I am sure, not the whole explanation, but it certainly is part of it.
Having said that, apparently we need poor, ugly people to serve as good spouses, and just make up the contingent of nice, unselfish people in society. At least, that’s the conclusion that two articles in Time draw (HT John Gardner).