[hugh3]

Parental income and child intelligence are correlated, but the direct causal relationship is (I suspect) extremely weak except at the low-end extreme. The real relationship between these two variables is dominated by parental intelligence: smarter parents have smarter children (with a very strong correlation), and smarter parents earn more money (with a reasonably strong correlation).

Once the parents are picked, earning a little more money won't do much to make the children smarter. If anything it might make them dumber, since those extra hours spent at work are much better spent with your children.