After watching Melissa Guth's youngest children for the day, a grandparent drops them off at Guth's home in Monroe, Wis. (M.P. King/AP)

As the number of children living with grandparents has risen in recent decades, the profile of caregiver grandparents has also evolved into a more diverse tapestry, with grandparents filling in the gaps in increasingly nontraditional family structures, according to a report released Tuesday.

In particular, as rates rise for divorce and remarriage, single parenting, and other nontraditional family structures, older Americans have been stepping in to help their offspring with childcare, said the study, which was conducted by US 2010, a research project on changes in American society funded by Brown University and the Russell Sage Foundation, a New York-based social science research center.

More than 40 million Americans, or 13 percent of the U.S. population, are 65 or older, with that figure expected to increase to 20 percent by 2050, and more than 90 percent of them are grandparents. They are more diverse, better educated, expected to live longer and better off financially than previous generations, says the report, “Diversity in Old Age: The Elderly in Changing Economic and Family Contexts,” which uses data from the Census Bureau; the Health and Retirement Study, a University of Michigan longitudinal study funded by the National Institute on Aging; and the University of Michigan’s Survey of Consumers, as well as other sources.

Across the United States, nearly 7.8 million children are living in homes with grandparents pres­ent, 4.9 million live in grand­parent-headed households and 2.6 million live in homes where the grandparents say they are the primary caregivers, said Amy Goyer, AARP’s expert on multigenerational and family issues, adding that 1 million children living with a grandparent do not have either parent in the house at all.

“We saw a big uptick with the recession, because grandparents have always been a safety net,” Goyer said, adding that nearly 20 percent of grandparents with grandchildren in the house are living in poverty.

Grandparents and other older relatives are compared by some social scientists to the National Guard, because they “come in and help when they’re needed,” said Judith Seltzer, director of the California Center for Population Research at UCLA and one of the report’s authors. “They do it because that’s what family members do,” she said.

But who exactly does it, to what degree and under what circumstances vary across race, ethnicity, gender, socioeconomic levels and family structure, the report said.

More than in the past, the grandparent stage is now seen as a distinct life stage, with older adults generally more likely to have finished raising their own offspring by the time they become grandparents. They are also more likely to help their children financially the older they are; younger grandparents tend to help more with child-care, the report said.

High rates of divorce, remarriage, and cohabitation and childbearing outside marriage weaken families bonds, including intergenerational relations , the report said, noting that such circumstances are most prevalent among poorer, less-educated families.

At the same time, remarried parents and divorced fathers are less likely to help their adult children financially; women tend to have closer ties to their grandchildren than men do. Almost one-third of grandmothers who live with their grandchildren are the primary caregivers, the report said.

African American and Hispanic grandmothers are more likely than white grandmothers to live with grandchildren, and African American grandmothers are more likely than Hispanic grandmothers to be the primary caregivers, the report found.

Often, being a primary caregiver for grandchildren is not part of the grandparents’ plan.

Willette Taylor, 61, of the District has been raising her 9-year-old grandson, Nayor, on her own since her 23-year-old daughter was killed when he was an infant; Taylor’s husband died a few months later.

For a while, Taylor’s parents stepped in, but they were too frail to help much. Now unemployed, she is searching for work and living off Social Security benefits from her husband and daughter.

“I miss some things” about a child-free life, Taylor said. But, she said, she is comforted by how much her grandson reminds her of her daughter. “I gotta do what I gotta do,” she said. “I wouldn’t have it no other way.”

Pat Owens, 70, of Thurmont, has been the primary caregiver for her 17-year-old grandson, Michael, since he was 4. When she started, she said, there were few resources for people in her situation; in 2001, she started Grandfamilies of America, an organization that provides resources for caregiver grandparents and other relatives, and she has pushed for legislation to give family members priority when placing children whose parents cannot care for them.

Owens took over care of her grandson when her daughter, who was using drugs and drinking heavily, flitted in and out of her parents’ house and eventually left.

“I really just envisioned you get married, you have a family, everybody’s happy, like all the fairy tales you read,” Owens said.

As families grow smaller and more fractured, the study “raises some important questions as to how the family safety net will hold up, on both sides,” Seltzer said.

Even as older people with fewer children can devote more time to individual grandchildren, she said, grandparents are themselves more likely to be left without a family caregiver as they age, especially among families with high rates of divorce and unmaried cohabitation.

“So this group that has the greatest need for help are going to have the weakest family bonds,” she said.