Generation Unbound: Drifting into Sex and Parenthood without Marriage. By Isabel Sawhill. Brookings Institution Press; 209 pages; $32. Buy from Amazon.com

LAST week the Pew Research Centre, a think-tank, predicted that one in four young Americans will never marry. Among the advantages of this are that you “will get to have sex with a different attractive person every night for the rest of your life” and can live “unfettered by [an] oppressive institution that represents undying love,” suggests the Onion, a spoof newspaper. Isabel Sawhill takes a more serious view. A former budget aide for Bill Clinton who now works at the Brookings Institution in Washington, DC, she has been pondering the state of the family for decades. “Generation Unbound” is clear, concise and admirably fair-minded.

It describes the vast changes that have occurred since the sexual revolution of the 1960s. No-fault divorce has been good for many adults, she writes. Because they can walk away, unhappy partners have more power to demand change. Spousal suicide, domestic violence and the murder of husbands by wives have all fallen as a result. But “children have rights, too,” and the instability of a growing number of American families harms them in all sorts of ways.

At the top of the social scale, a more egalitarian version of traditional marriage is still going strong. Nearly 90% of university-educated mothers get married before having their first child—typically to an equally brainy, high-earning man. Such unions are durable and provide an excellent launchpad for children, who are showered with love and stimulation and grow up to do well in school and the workplace.

Among non-graduate mothers, however, 58% of first births are out of wedlock. And although half of unmarried mothers are still living with the father when the baby is born (and another third are romantically involved with him but living apart), only a third of these couples are still intact by the time the child turns five. By contrast, four-fifths of married couples are still together at this point.

The further down the social scale you go, the more fissile American families are. Children grow up in a “family-go-round” of absent fathers, new stepfathers, new half-siblings and chronic uncertainty. Only a third of single mothers receive any child support from the biological father. Ms Sawhill describes one dad who admits that he has never paid a penny in official child support but insists he is a good provider because he occasionally buys his offspring a pair of trainers.

Scholars on the left tend to blame poverty for family breakdown. Ms Sawhill finds this too glib. Families were much poorer in the 1950s, but they stuck together. Unskilled men’s wages have fallen in recent decades, but not by much. This “cannot explain more than a small fraction” of the change at the bottom. On the contrary, the nuclear family is an effective way out of poverty. A child born in the bottom fifth has an 83% chance of escaping it if his parents remain married, compared with 50% if they were never wed.

Much of the problem is cultural, argues Ms Sawhill. The “feminist revolution seems to have bypassed low-income men.” Male university graduates largely treat their wives as equals; less educated men often don’t. They cannot accept not being master of the household, even if they earn less than their female partner. Increasingly, they opt out of parental responsibilities almost entirely.

Charles Murray, a conservative social scientist, thinks the answer is for educated families to preach the family values they practise. Ms Sawhill doubts that would work. Instead, she suggests several small practical policy ideas, of which the most striking is also the simplest: she urges the wider use of intrauterine devices (IUDs).

Roughly 60% of births to young single women in America are unplanned. Pills and condoms are easy to get hold of, but many couples fail to use them consistently. In one survey 44% of young women agreed that “it doesn’t matter whether you use birth control or not; when it is your time to get pregnant it will happen.” IUDs are 40 times as effective as the pill because they last for years, so you don’t have to remember to take them each morning. Yet only 5% of American women use them, compared with 40% of Chinese women.