Younger Americans in their late 30s are now the group most likely to doubt they will be financially secure after retirement, a major shift from three years ago when baby boomers nearing retirement age expressed the greatest worry.

The survey findings by the Pew Research Center, released Monday, reflect the impact of a weak economic recovery beginning in 2009 that has shown stock market gains while housing values remain decimated.

As a whole, retirement worries rose across all age groups — roughly 38 percent of U.S. adults say they are "not too" or "not at all" confident that they will have sufficiently sized financial nest eggs, according to the independent research group. That's up from 25 percent in 2009.

But the concerns are increasing the greatest among younger adults approaching middle age, whose equity in their homes represents most of their net worth. About 49 percent of those ages 35-44 said they had little or no confidence that they will have enough money for retirement, more than double the 20 percent share in that age group who said so in 2009.

Baby boomers born between 1946 and 1964 also reported having more retirement anxieties than before, but now to a lesser degree compared to their younger counterparts. About 43 percent of Americans ages 45-54 expressed little or no trust in their retirement security, up from 33 percent in 2009. Among Americans ages 55-64, the share expressing little or no confidence was 39 percent, up from 26 percent.

Broken down by smaller groups, the Pew analysis found that retirement worries peaked among adults in their late 30s; a majority, or 53 percent, of Americans ages 36 to 40 lacked confidence that they will have large enough nest eggs. Just three years ago, it was baby boomers ages 51 to 55 who had the most anxiety over whether their income and assets would be sufficient.

Richard Morin, a senior editor at Pew who co-authored the report, said the shift in attitudes was somewhat surprising.

"I think most people would expect those on the cusp of retirement — ages 55 to 64 — would be the most concerned about financing their retirement, (so) the finding that the peak is now occurring among adults roughly 20 years younger is notable," he said. "Moreover, the wealth data showing those approaching or in early middle age had lost the most in the past decade suggests that their concerns are not misplaced."

Morin said that it is hard to predict whether 30-somethings will continue to express the most retirement worries in the years to come, but said it was a "real possibility" given that housing values aren't expected to fully recover anytime soon.

The latest findings come as the presidential campaigns focus most often on retirement issues such as Social Security and Medicare when appealing to older voters. In recent weeks, President Barack Obama has pounded Republican challenger Mitt Romney and his running mate, Rep. Paul Ryan, saying their plan to replace Medicare with vouchers won't keep up with health care costs. Ryan has sought to reassure seniors by saying that he and Romney won't alter Medicare for those in or near retirement.

An Associated Press-LifeGoesStrong.com poll in late 2011 also found that concerns about retirement were increasing across all age groups, a reflection of the continuing hard economic times.

According to the Pew report, the inflation-adjusted net worth of Americans ages 35 to 44 fell roughly 56 percent from 2001 to 2010, the sharpest decline for any age group and more than double the 22 percent rate of decline for boomers ages 55 to 64. Net worth, also referred to as wealth, is the sum of all assets such as a house, car, stocks and 401(k)s, minus the sum of all debts including mortgage, credit card debt, car and tuition loans.

In dollars, the median wealth of Americans ages 35 to 44 fell by $56,029 to $43,698 over the past decade. In contrast, those ages 45 to 54 and 55 to 64 lost about $50,000. The median wealth of those 65 and older over the past decade increased slightly — the only age group to experience a gain.

The 35 to 44 age group has been hit the hardest in terms of wealth because they were the ones most likely to have purchased a home at bubble prices during the housing boom, only to see values shrivel in the housing bust. This younger to middle-aged group also largely stayed out of the stock market from 2001 to 2010 and as a result missed out on the stock run-up that began in 2009, according to Pew's analysis of Federal Reserve data.

The S&P 500 index peaked above 1,500 in October 2007 but then fell to a closing low of 676.53 in March 2009. It has risen significantly since then, closing above 1,200 in December 2010 and is now back above 1,400.

Broken down by education and income, adults holding a high school diploma or less were less likely to express confidence in their retirement finances than college graduates, 53 percent vs. 71 percent. Those with family incomes of less than $50,000 also were less confident compared to those making $100,000 or more, 51 percent vs. 79 percent.

The Pew study is based on interviews with 2,508 adults by cell phone or landline from July 16 to 26, as well as an analysis of the Survey of Consumer Finances, which is sponsored by the Federal Reserve. The Pew poll has a margin of error of plus or minus 2.8 percentage points, larger for subgroups. The AP-LifeGoesStrong.com poll was conducted Oct. 5-12, 2011, by Knowledge Networks of Palo Alto, Calif.

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AP Deputy Director of Polling Jennifer Agiesta contributed to this report.

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Online:

Link to Pew report: http://tinyurl.com/96295h5

AP-LifeGoesStrong.com poll from 2011: http://tinyurl.com/8v8qrwh