This was the most shocking statistic I read this weekend: 58 percent of first births in lower-middle-class households are now to unmarried women. Meanwhile, two in five of all births are to unwed mothers, an all-time high, according to the Centers for Disease Control and Prevention.

Why?

The thesis of this fascinating article in the Wall Street Journal says the real mystery here isn't "Why so many babies?" but rather "Why so few marriages?" -- particularly among less-educated men and women.

This is a complex economic mystery that we've explored often at The Atlantic, but we can take a big bite out of it by focusing on three factors: (1) The changing meaning of marriage in America; (2) declining wages for low-skill men; and (3) the declining costs of being a single person.

A New Union
Marriage has changed. Once upon a time, the typical marriage, as Justin Wolfers has explained, involved special roles for the husband and wife. He would work. She would stay home. It was an efficient arrangement where opposites attracted. Men who wanted to be executives would marry women who wanted to be housewives. And, since almost half of women had no independent earnings 40 years ago, there were a lot of women who just wanted to work at home and raise a family.

Several factors mussed up this traditional union. Today women expect to work much, much more than they used to -- and they do. They make up the majority of new college graduates and their labor participation rate has soared over 60 percent. Since 1950, hours of work by married women have increased by roughly a factor of three, according to the Minneapolis Fed.

Now that women are better educated, with greater control over both their fertility and their earnings, modern marriage has changed from an arrangement where men marry for a housewife to a "hedonic" model where both partners can be the breadwinner. As marriage has shifted from opposites-attract to like-attracts-like, researchers have found that sorting has increased all along the educational scale. College graduates are more likely than ever to marry college graduates, as Charles Murray has written. High school dropouts are more likely to marry high school dropouts.

Think of marriage like any other contract or investment. It's most likely to happen when the gains are big. So we should expect marriages among low-income Americans to decline if women perceive declining gains from hitching themselves to the men around them.

That's precisely what we've seen...

Cheap Wages, Cheap Technologies
Low-skill men have had a rough two generations. The evaporation of manufacturing work has gutted their main source of employment, while globalization has held down their wages. Marriage has declined the most among men whose wages have declined the most. Here's a remarkable graph from the Hamilton Project comparing change in earnings (the RED LINE) and change in likelihood to be married (the BLUE BARS).

In a dating pool where poor women are more likely to be surrounded by men with low and falling fortunes, more women have ditched a union for good economic reasons: It could be a financial drain. In The Truly Disadvantaged,  William Julius Wilson, argued that "high rates of unemployment and incarceration meant that the local dating pool was populated by unmarriageable men--and the result was that women chose to live independently."

It is hardly easy to do anything with earnings near the poverty level. But it is relatively easier to raise a child and keep up a home with modern household innovations. The connection between Lunchables, detergent and marriage rates is not often made. But perhaps it should be. The development of time-saving technologies -- cheap prepared foods, cheap clothes, machines to wash, dry, and vacuum -- has not only encouraged more women to seek work, but also made it relatively easier for single parents to raise a child. Put starkly, technology makes it cheaper and easier than ever to be single. It makes marrying a financially unstable man even more risky.

That women find themselves drifting "unintentionally" into parenthood with men they have no intent of marrying creates another generation of problems. Children raised in two-parent households are more likely to go to college, more likely to be employed, and more likely to earn a high wage. The rise of unwed mothers might be logical for many of these women. But there is too much evidence that it deepens the divide between the haves and have-nots in America.