Economists have long argued that there is such a thing as a market for spouses. The old theory, associated with University of Chicago Nobel laureate, Gary Becker, is that people marry for the same reason that nations trade with one another: comparative advantage. If men are better at earning money in the labor market and women are better at taking care of home and children, it makes sense for them to combine forces within marriage so that they can then specialize in what they each do best. The problem with Becker’s theory is that with the expansion of labor market opportunities for women, it now needs revision.

Along come June Carbone and Naomi Cahn, who have just written a fascinating book providing us with the needed update. Although they are both lawyers, their story has an economic ring to it. The book, entitled “Marriage Markets,” argues that these markets are more segmented by class than in the past (lawyers now marry other lawyers not their secretaries). One can only understand what has happened to marriage if one combines this reality with another one which is the fact that growing income inequality has produced a shortage of women at the top end of the income distribution (where top earners are still primarily male) and a shortage of men at the bottom end (where less skilled men can no longer find good jobs in manufacturing, for example).

Within these segmented markets, sex ratios matter, they contend. It is a simple matter of supply and demand. Among the bottom one third, there are few “good” men for women to marry so they are choosing to go it alone rather than take responsibility for both earning the bulk of the income and handling most if not all of the household chores while putting up with a lot of other problems such as infidelity, substance abuse, or domestic violence. At the top end, in contrast, men want to marry highly-educated professional women and there aren’t quite enough of those to go around. So these women can afford to be much pickier; they can demand that their husbands help with child care, remain faithful, not drink too much, and in general be “good” husbands.

Women in the middle class are beginning to look more like those at the bottom than those at the top because changes in the economy have impacted men in this middle third almost as much as those at the bottom.

As an economist who has also written a book on these same issues (Generation Unbound, forthcoming in September), I was impressed with the Carbone and Cahn story. Their book is far richer and more nuanced than I can capture in this short summary. Still, I suspect they would agree that a purely economic theory falls short as an explanation of the dramatic transformation of family life in the U.S. in recent decades. Social norms, women’s changing roles, and sexual liberation have to be factored into the equation. Research from the new field of behavioral economics (a special focus of my new book) has taught us that people often do things that are not in their own best interest, much less that of their children. As a result many are drifting into sex and parenthood before they are ready to be parents, much less make a long-term commitment to the other parent.

Half of all babies born to women under the age of 30 are now born outside of marriage. Most of these births were unplanned. Many are cohabiting at the time of the baby’s birth but they typically break up with the child’s father before the child has reached school age and often go on to have children with a new romantic partner. This means that a large proportion of children are growing up without the kind of stable environment that two committed parents provided in the past.

Carbone and Cahn argue that this situation will cast a long shadow on the well-being of the next generation. They call for more supports for young adults, greater investment in children, and better family planning. I generally agree with their proposals, but would put more emphasis on the need for young adults to plan more and drift less. A child’s education begins in the home. No improvement in public policy can compete with what only families can provide. Without a strong start in life, children do not do well in school and typically never catch up to their more advantaged peers. Whether the source of the problem of family breakdown is economic or not, the consequences surely are.