THE PURSUIT OF HAPPINESS:

To the relief of many parents, educators, and clergy members who care about the health and well-being of young people, teens are launching their sex lives later. From 1991 to 2017, the Centers for Disease Control and Prevention's Youth Risk Behavior Survey finds, the percentage of high-school students who'd had intercourse dropped from 54 to 40 percent. In other words, in the space of a generation, sex has gone from something most high-school students have experienced to something most haven't. (And no, they aren't having oral sex instead--that rate hasn't changed much.)

Meanwhile, the U.S. teen pregnancy rate has plummeted to a third of its modern high. When this decline started, in the 1990s, it was widely and rightly embraced. But now some observers are beginning to wonder whether an unambiguously good thing might have roots in less salubrious developments. Signs are gathering that the delay in teen sex may have been the first indication of a broader withdrawal from physical intimacy that extends well into adulthood.

Over the past few years, Jean M. Twenge, a psychology professor at San Diego State University, has published research exploring how and why Americans' sex lives may be ebbing. In a series of journal articles and in her latest book, iGen, she notes that today's young adults are on track to have fewer sex partners than members of the two preceding generations. People now in their early 20s are two and a half times as likely to be abstinent as Gen Xers were at that age; 15 percent report having had no sex since they reached adulthood.

Gen Xers and Baby Boomers may also be having less sex today than previous generations did at the same age. From the late 1990s to 2014, Twenge found, drawing on data from the General Social Survey, the average adult went from having sex 62 times a year to 54 times. A given person might not notice this decrease, but nationally, it adds up to a lot of missing sex. Twenge recently took a look at the latest General Social Survey data, from 2016, and told me that in the two years following her study, sexual frequency fell even further.

Some social scientists take issue with aspects of Twenge's analysis; others say that her data source, although highly regarded, is not ideally suited to sex research. And yet none of the many experts I interviewed for this piece seriously challenged the idea that the average young adult circa 2018 is having less sex than his or her counterparts of decades past. Nor did anyone doubt that this reality is out of step with public perception--most of us still think that other people are having a lot more sex than they actually are.

When I called the anthropologist Helen Fisher, who studies love and sex and co-directs Match.com's annual Singles in America survey of more than 5,000 unpartnered Americans, I could almost feel her nodding over the phone. "The data is that people are having less sex," she said, with a hint of mischief. "I'm a Baby Boomer, and apparently in my day we were having a lot more sex than they are today!" She went on to explain that the survey has been probing the intimate details of people's lives for eight years now. "Every year the whole Match company is rather staggered at how little sex Americans are having--including the Millennials."

New research published Monday by the Institute for Family Studies reveals that married couples who have only had sex with their spouse throughout their lifetime are more likely to report a happier marriage than those who have had multiple partners.

The study, authored by University of Utah professor Nicholas Wolfinger, found that both men and women reported higher levels of satisfaction in their marriage if their only sexual partner was their spouse.

"In sum, the surprisingly large number of Americans reporting one lifetime sex partner have the happiest marriages," Wolfinger writes. "Past one partner, it doesn't make as much of a difference."

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