There’s been exhaustive, and even obsessive, analysis of how the lousy economy has pushed young adults to move back home with their parents, contributed to the multiple-generations-under-one-roof trend, and affected the economy and maybe even modern-day culture.

Less explored is what this “doubling-up” phenomenon means for families with young children. Some research shows that such families heavily rely on moving in with relatives (or non-relatives) to make ends meet in times good and bad. But how many of them do that? How much money do they save, if any? And are the savings meaningful?

It turns out that doubling-up really is a valuable safety net for families with kids, according to a study recently published in the journal Demography. The study tracked 3,000 children born between 1998 and 2000 in 20 large U.S. cities. It found that nearly half of those kids – 49 percent to be exact -- have lived in doubled-up households at least once in their first nine years of life. Doing so saved their families more than $4,000 on average each year, a substantial sum given that the mothers who were interviewed for the study earned $15,000 on average. (Keep in mind this only reflects what the moms earned when their kids were young. As children grow older, a mother’s salary typically increases.)

The mothers followed in the study were single, married or living with a person with whom they were romantically involved when their kids were born. They were interviewed at that time and then again when the children turned one, three, five and nine. The moms were most likely to double-up when the children were young, and less likely to do so as they grew older. By far, single mothers doubled-up the most often in the early years (64 percent).

Source: The Prevalence and Economic Value of Doubling Up, study published in Demography journal

Other studies have attempted to capture the doubled-up population in a single point in time, and concluded that roughly 20 percent live this way. But tracking families for many years showed a very different result. “I would never have expected that half would double-up during a nine-year period,” said Natasha Pilkauskas, a co-author of the study and a professor at Columbia University. “That’s surprisingly high.”

Did the financial crisis heighten the need or desire to double up? Possibly. The rates may have been higher during the recession, Pilkauskas said. But the study does not address that issue. The main takeaway, she said, is that families rely on doubling up in general.

The authors expected that most of those families, because they were young, would move into other people’s homes. Surprisingly, the opposite was true. About 56 percent of mothers said they were bringing people into their own homes. The percentage was even higher – 76 percent – among married mothers. The vast majority of the moms interviewed were renters. Only 16 percent were owners.

When it came to determining the economic value of doubling-up, the study examined only those people who were living in someone else’s home, even if they weren’t paying rent. The authors calculated the difference between what the families should have paid and what they actually paid.

While the savings totaled roughly $4,000 a year on average, married moms reaped more ($5,300), followed by cohabiting mothers ($4,450) and then single mothers ($3,500.) The first two groups probably had higher rents because they had more adults already in the home and therefore needed larger spaces.

The authors – including Irwin Garfinkel of Columbia University and Sarah S. McLanahan of Princeton University – did not reach any conclusions about whether housing policies should encourage or discourage doubling-up. (In the past, some have suggested that those who double-up should count as homeless so that they can qualify for certain homelessness prevention and assistance funds.) This study only says that policymakers need to understand how people really live and what they pay before making big decisions.

The reality these days is that many families and individuals are struggling to pay for housing, whether they own or rent. As a rule of thumb, housing costs should not eat up more than 30 percent of a household’s income. Yet more than a third of U.S. households paid more than that in 2012, according to a report released this year by Harvard’s Joint Center for Housing Studies. That’s an increase of more than 9 million people compared to a decade earlier. About 5.8 million of them put more than half of their income toward housing.