So what does it mean to be "middle class?" There is no precise answer to that question. We can talk about the median annual income for a family of four, which varies state by state - from about $50,000 in Arkansas to more than $90,000 in Connecticut, according to the U.S. census. But for many Americans, being "middle class" refers to a set of cultural values and aspirations. It's not so much about how much they make, as it is about how they grew up, and how they feel connected to American society - and to the American dream. Consider the case of Kathleen Murray, 51 years old, who grew up in Boston in a middle class family.

"My Dad worked for the same company for 40 years," she says, with a trace of wistfulness.

These days, lots of people who describe themselves as middle class say it used to be different. They tell you of a middle class world of a generation or two ago that was more secure, more financially stable. Murray recalls how her father, who maintained the lines for the phone company, raised three kids on just one salary.

Documentary Link New PEW research on the middle-class in America

"We never wanted for everything," she says. "We had good clothes, good books. We all had good college education, and we lived a very comfortable life." And Murray says these days her parents are enjoying a comfortable retirement. Her father gets a pension - "a good pension" - and his health care is still paid for by his former employer.

"And they were very pleased," says Murray, "to see their children step up to a more upper middle class world."

Murray, who is single, was doing pretty well working in financial services in Boston until she was laid off earlier this year. She's still looking for work, but she's more fortunate than many. She was making more than 120-thousand dollars a year, received a good severance package, can still afford her health insurance, and has quite a bit of money saved for retirement. Yet, she's worried.

"I'm in my middle 50s," she says. "And I watch my retirement savings shrink considerably from what it was at the front part of the year."

Like millions of middle class Americans who don't have a pension, Murray has watched her 401-K plan lose over 30 percent of its value in the past few months.

"It's gone," she says. "And I happened to meet my financial planner last week, and I jokingly said, I'm working until 70. He said, "you're working for sure beyond 60 - that's for sure."

Kathleen Murray's concerns about her financial security and the future are shared by an increasing number of Americans these days, including Sidney Fuller-Jones and her husband, Amos Jones, of Dorchester, Massachusetts. Sidney and Amos have twins, Jamari and Nia, who are six years old. We met the family at one of Jamari's practices on a cool autumn evening in Dorchester, where Sidney and Amos beamed with pride at the sight of their young son dressed in his black helmet, shoulder pads, black shirt and white padded pants. "These little boys, they have heart," Sidney declared. "And it just warms my heart!"

But beyond the obvious pride for her son, Fuller Jones is worried about her difficult financial circumstances.

"It's been tough," she says.

Not long ago, Sidney Fuller Jones and her husband, Amos, had a toehold on the middle class ladder. Both of them are college educated. Both of them worked for the state Department of Public Health. Together, they earned about 80-thousand dollars a year - a little below the median income for a family of 4 in Massachusetts. But when Amos got sick with heart disease and kidney failure, it fell to Sidney to keep the family going.

"There was a point when our phone didn't stop ringing," Fuller Jones says. "And it was literally bill collectors because we were behind. It's been a struggle."

Last winter the family had to turn to ABCD, a Boston-based anti poverty agency, just so they could fill their oil tank to heat their home.

The Jones family: (seated l-r) Nia, Amos, Sindey and Jamari.

"If not for that help," says Jones, "we would have been literally out in the cold."

Her financial struggles have made her think about how life has changed since her childhood in the 1960s, when she was raised by her mother, who was a school teacher, and her father, who worked for the IRS. The family considered itself middle class, and although she and her siblings grew up in a modest home ("there were no mansions", she says), they had everything they needed.

"I don't remember being without anything," she says. "And I don't remember hearing my parents talking and worrying about bills."

Fuller-Jones says a generation later things are much more difficult for working families like hers.

"It's the economy. It's grueling," she says.

Indeed, while her salary has remained level for the past several years, prices for food, health care, and until recently, for oil, have climbed steadily. And of course, the loss of her husband's salary has been the most difficult blow of all.

"So we're facing difficult, different times," she says.

Kathleen Murray and the Fuller-Jones family offer two snap-shots of middle class life - from upper middle class, to a family struggling to hold on to the bottom rung of the middle class. Despite differing circumstances, they share a growing feeling of financial insecurity, and a sense of wistfulness about what it used to mean to be middle class. Their concerns about their financial security are shared by an increasing number of middle class families across the nation, according to a series of surveys and polls.

Jared Bernstein, a senior economist at the Economic Policy Institute in Washington, D.C., says a majority of middle income Americans now believe that their kids are not going to fare as well as they did economically.

"And that really strikes at the heart of middle class American values," Bernstein says. "It's the idea that if you work hard, play by the rules, your children will have opportunities to surpass you. That basic value is under siege right now."

And according to Bernstein, there are hard economic data that tell a big part of this story. Bernstein says that middle-income families are actually losing ground economically -- or at least standing still - while prices for the basics continue to climb.

"This is the first time this has happened in the history of the research -- going back to the mid 1900's," Bernstein says.

Family incomes are no longer keeping pace with the demands of the over-all economy, according to Peter Temin, an economist at MIT, who co-authored a recent paper examining post-war income trends. Temin says despite a growing economy over most of the past 30 years, wages for most middle income Americans haven't kept up with inflation and the cost of living.

"It feels like a paradox," he says. Temin says, even though the government reports that incomes have grown since 1980, most people feel as though they have fallen behind. That's because real incomes - what they can actually buy with their earnings - have remained essentially flat.

"That's getting to be about 30 years in which people have not seen an increase," Temin says. "That's a long time. And now with prices rising, it looks like incomes might actually be falling."

At the same time, incomes for the richest Americans have continued to climb. In fact, government data show that today the richest one percent of Americans hold about 22 percent of all income. Jared Bernstein says there's only one year in the history of all the data going back to 1913 that the top one percent held a larger share, and that year was 1928.

"So since the height of the roaring 20s we just haven't seen such an imbalance in the distribution of income," Bernstein says.