James Heckman has done considerable research on what works in early childhood education and its importance for reducing income inequality. Paul Tough uses Heckman to explain how children learn. In his recent book How Children Succeed, he notes
The Perry Preschool Project, in other words, worked entirely differently than everyone had believed. The goodhearted educators who set it up in the sixties thought that they were creating a program to raise the intelligence of low-income children; they, like everyone else, believed that was the way to help poor kids get ahead in America. Surprise number one was that they created a program that didn’t do much in the long term for IQ but did improve behavior and social skills. Surprise number two was that it helped anyway - for the kids in Ypsilanti, those skills and the underlying traits they reflected turned out to be very valuable indeed.
In other words, cognitive ability or aptitude is only part of the reason incomes diverge in the United States. Other factors, all social and much of them attributed to stress experienced during childhood, have lasting effects not only on income, but on marriage, family and other social outcomes as well.
Ira Glass from This American Life does great job of presenting the arguments here. It's well worth the listen.