Americans across the income spectrum still highly value marriage, sociologists have found. But while it used to be a marker of adulthood, now it is something more wait to do until the other pieces of adulthood are in place — especially financial stability. For people with less education and lower earnings, that might never happen.

College graduates are more likely to plot their lives methodically — vetting people they date until they’re sure they want to move in with them, and using birth control to delay childbirth until their careers are underway.

Less educated people are more likely to move in with boyfriends or girlfriends in a matter of months, and to get pregnant at a younger age and before marriage. This can make financial and family stability harder to achieve later on.

“It starts with moving in together quickly, for economic exigency reasons as opposed to relationship reasons,” Ms. Sassler said. “Then struggling with making ends meet and trying to manage this with a partner just elevates the challenges.”

Evidence shows that the struggles of men without college degrees in recent years have led to a decline in marriage. It has been particularly acute in regions where well-paying jobs in male-dominated fields have disappeared because of automation and trade.

In a working paper published in July, three economists studied how the decline in manufacturing jobs from 1990 to 2014, across industries and regions, “contributed to the rapid, simultaneous decline of traditional household structures.”

Labor market changes made men less marriageable, they concluded. There were fewer available men, because unemployment was associated with a rise in incarceration or mortality from drugs and alcohol. The men who were left were less desirable, because they lacked income and were more likely to drink to excess or use drugs.