Much media coverage has focused on some millennials' "failure to launch," the term for when young adults live at home and get a great deal of support from their parents. Human Development and Family Sciences Professor Karen Fingerman wondered about another phenomenon—let's call it failure to latch. She wanted to know: What about the young adults who don't have parental support?

"One in five young adults doesn't have a father in their lives. Six percent don't have a mother, and 2 percent have neither," Fingerman said. "That's a lot of young people."

In a study with colleagues from the University of South Carolina and the University of Michigan, Fingerman looked at the prevalence of parentless young adults in the United States. Examining data on more than 5,000 individuals between the ages of 25 and 32, the team was able to learn why parents weren't present in the lives of so many young people.

The most common reason a young person would not have a mother or father was if one died. However, with fathers, nearly as many young people as had lost fathers had never had a father figure to begin with. Having an incarcerated parent or parents who divorced early in their child's life were other risk factors for not having a relationship as an adult.

Researchers found that not having a parent could predict other things, too. The more educated a young person's parents were, the more likely the young adults were to have a relationship. Women were more likely to have at least one parental relationship compared to men.

The takeaway from the study, Fingerman said: "It's a simple message, but an important one. A huge group of young people don't have parents, but parents are an important source of affection and support for young adults. Those who lack parental relationships are a potentially vulnerable group, and we need to understand what that means for these early years in adulthood."