“Women don’t want to take a risk on somebody who’s not going to be able to provide anything,” said Sharon Sassler, a sociologist at Cornell who published “Cohabitation Nation: Gender, Class, and the Remaking of Relationships” with Amanda Jayne Miller last month.

As marriage has declined, though, childbearing has not, which means that more children are living in families without two parents and the resources they bring.

“The sharpest distinction in American family life is between people with a bachelor’s or not,” said Andrew Cherlin, a sociologist at Johns Hopkins and author of “Labor’s Love Lost: The Rise and Fall of the Working-Class Family in America.”