More than 2.3 million adult children are living with their parents in California, a 63 percent increase since the Great Recession began seven years ago and a phenomenon straining budgets and pushing some families to the brink of poverty, according to a study released Thursday.

Researchers from UCLA and the Oakland-based Insight Center for Community Economic Development say job losses, home foreclosures, and divorces are among the factors driving hundreds of thousands of adults to return to their childhood homes. In many cases, those homes are headed by parents who are approaching or in retirement, and are living on fixed incomes themselves.

"There were 433,000 older adults, age 65 and over, who housed approximately 589,000 of those adult children," the researchers said in the report released by the UCLA Center for Health Policy Research. Those figures were generated from federal census statistics, said the report's lead author, Steven P. Wallace.

Of those adult children living with older parents, researchers found that only about 42 percent were employed, putting a significant strain on the parents to provide not only housing, but also necessities like food, medical care, and transportation for them. To do so for just one adult child, the study found, costs the average couple about 2 1/2 times more per year than they would pay just to take care of themselves.

Exacerbating that problem is the already high cost of living in California, said Mr. Wallace, who is also the UCLA center's associate director.

While qualification for many public assistance programs is based on federal poverty level standards, he said, those standards are calculated on the average cost-of-living for the country as a whole.

Under federal poverty level standards, for example, an older couple renting a home would be expected to cover all expenses on $14,710 a year. But researchers found in 2011, the most recent year for which statistics were available, that just renting a one-bedroom apartment in San Francisco would cost $17,580 a year.

They estimated that an older couple living with one adult child would need about $54,000 a year to live in San Francisco. In Southern California's Orange County, the cost would be about $52,000, and in San Diego County it would be $48,691. In less expensive areas like San Bernardino and Riverside counties, it would still be about $35,000.

Among other things, the study recommends Supplemental Security Income benefits and food assistance programs for older people be increased, more housing for seniors be provided and that low-income adult children be encouraged to apply for health insurance through the state's Medi-Cal program.

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In the meantime, Wallace said California residents shouldn't expect adult children will move out any time soon.

"Right now the economy isn't generating jobs fast enough to soak up a lot of the people who have either lost their jobs or are coming into the job market," he said. "The near future looks like the numbers are likely to remain stable or grow."