E.D. Hirsch's article in the latest City Journal offers the evidence that vocabulary alone is a great predictor of upward mobility. Why? Because it's an indicator of the child's knowledge-resources.
"Vocabulary doesn’t just help children do well on verbal exams. Studies
have solidly established the correlation between vocabulary and
Implication? An old principle: reading can unlock the world, enabling a child from an impoverished background to travel around the globe, and gain the knowledge to achieve great goals. Abraham Lincoln did it, and Ben Franklin, and today's children have so many more resources at their fingertips.
The statistics Hirsch cites regarding what's happened to vocabulary in the past decades are unfortunately disturbing, but in line with all the rest of the research on educational decline.