In the United States, long considered the land of opportunity, children born into different types of families begin life with very unequal prospects. A growing group of children is being raised in families in which a poorly educated mother begins childbearing at an early age, often outside marriage, and ends up dependent on public welfare. Another group is raised by parents who delay childbearing until they are well-educated, married, and have stable jobs; these children go on to lead more advantageous lives. While virtually everyone talks about the importance of investing in the next...