A generation ago Americans undertook a revolutionary experiment to redefine marriage. Where historically men and women had sought a loving bond, largely centered on the rearing of children, the new arrangement called for an intimate—and provisional—union of two adults. Now, as Kay Hymowitz argues in Marriage and Caste in America, the results of this experiment separating marriage from childrearing are in, and they turn out to be bad news not only for children but also, in ways little understood, for the country as a whole. The family revolution has played a central role in a growing inequality and high rates of poverty, even during economic good times. The family upheaval has hit African-Americans especially hard, Ms. Hymowitz shows, as Daniel Patrick Moynihan had famously predicted it would. While for decades feminists and academics toyed with the myth of the strong single black mother supported by kinship networks, black men drifted into fatherhood without being husbands, without even becoming part of a family, while black children were left behind. When Americans began their family revolution, they forgot to consider what American marriage was designed to do: it ordered lives by giving the young a meaningful life script. It supported middle-class foresight, planning, and self-sufficiency. And it organized men and women around "The Mission"—nurturing their children's cognitive, emotional, and physical development. More than anything, Ms. Hymowitz writes, it is The Mission that separates middle-class kids—who for all their overscheduling are doing very well indeed—from their less-parented and lower-achieving peers. In fact our great family experiment threatens to turn what the founders imagined as an opportunity-rich republic of equal citizens into a hereditary caste society.