Elizabeth Landau, a 32-year-old single person, wrote in a Scientific American article that a lot of people her age (Millennials) are what she calls "commitment phobes." Publicly, Ms. Landau supports her friends' marriages, "But, privately," she writes, "I feel left behind in what Vanity Fair described as a 'dating apocalypse.' Of course, plenty of single men and women like me don't seek out one-night stands. But I feel like, in the dating-app era, many aren't keen on investing lots of quality time in any particular match when a better one might be a swipe away." Landau continues: "My outlook may have entered a vicious cycle: It's hard to get excited about meeting someone who won't care about you that much."

She is definitely on to something. The Pew Research Center reports that Millennials are significantly less likely to be married than previous generations in their 20s. A Gallup poll found that the percentage of 18 to 29-year-olds who say they are single and not living with a partner rose from 52 percent in 2004 to 64 percent in 2014. Marriage among 30-somethings also dropped 10 percentage points during that decade, while the percentage living together rose from seven to 13 percent.

But why? More than half of the Millennials surveyed by Pew characterize their own cohort as self-absorbed. "Trying to live with somebody else and putting their needs first is more difficult when you have been raised to put yourself first," says San Diego State University psychologist Jean Twenge.