If asked to describe the typical American who lives paycheck-to-paycheck, you might conjure up the image of a minimum-wage worker struggling to make rent each month.

But a new study out of the Brookings Institution finds that two-thirds of U.S. households living "hand-to-mouth" are actually middle class, with homes and retirement assets. Only about one-third of those just getting by financially are what most would consider poor, with median incomes of about $20,000.



Overall, about 38 million American families live hand-to-mouth, or about one-third of all U.S. households.



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So why do families with decent incomes and assets have to live "hand-to-mouth," spending all their cash with each paycheck? The reason may be that such middle-class consumers are opting to put money into illiquid investments with better rates of return than a savings account, such as real estate or a retirement portfolio, Greg Kaplan, assistant professor of economics at Princeton University, told CBS MoneyWatch.

"This behavior might look on the face to be kind of crazy, but in some of our other work we show it looks like a very sensible thing to do, given the incentives to invest in retirement accounts and real estate," he said.

Those incentives include tax-deferred contributions to retirement accounts and the home mortgage interest deduction.

Middle-class households living hand-to-mouth tend to be older, at about 40-years-old, than poor paycheck-to-paycheck families and hold an average of about $50,000 in illiquid assets, according to the study, which is from Kaplan, Giovanni Violante of New York University and Justin Weidner of Princeton University.

The findings may alarm financial experts, who typically recommend that households sock away about three to six months of income in a savings account to cover emergencies such as a health problem or the loss of a job. Such crises "are a real risk, but they are fairly uncommon," Kaplan said. Americans may be balancing those unlikely risks with the fact that returns on real estate, stocks and bonds far outpace the almost zero rate of return on a savings account.

America isn't alone: More than one-third of the populations of Canada, the U.K. and Germany also live paycheck to paycheck. The one outlier is Australia, which has almost no poor "hand-to-mouth" households, while about one-fifth of the country is comprised of middle-class paycheck-to-paycheck households. That's due to Australia's requirement that employers contribute to their workers' retirement accounts, the study noted.

The study was designed to consider which households are served by fiscal policies such as government transfers -- tax rebates, for instance -- to stimulate the economy.

"To understand how effective it is, you have to think about whether households are likely to spend that income," Kaplan noted. For fiscal policy to get the "biggest bang for the buck," it's important to figure out which households are actually living hand-to-mouth, which turns out to be a large number of middle-class households.

Of course, that might not come as a surprise to many Americans. For those working for companies with 401(k) programs or pension plans, it may seem painless to put aside money for their golden years. And after taking out taxes, retirement savings and other deductions from the median household income of $53,000, there may be little left for middle-class earners to sock away, even if they wanted to.