"This was no 'equal opportunity' recession or an 'equal opportunity' recovery," said Andrew Sum, director of the Center for Labor Market Studies at Northeastern University. "One part of America is in depression, while another part is in full employment."
The findings follow the government's tepid jobs report this month that showed a steep decline in the share of Americans working or looking for work. On Sunday, President Barack Obama stressed the need to address widening inequality, warning that proposed budget cuts will worsen the gap.
"The folks in the middle and at the bottom haven't seen wage or income growth," Obama said on ABC's "This Week."
While the link between income and joblessness may seem apparent, the data are the first to establish how this factor has contributed to the erosion of the middle class, a traditional strength of the U.S. economy.
Based on employment-to-population ratios, which are seen as a reliable gauge of the labor market, the employment disparity between rich and poor households remains at the highest levels in more than a decade, the period for which comparable data are available.
(Read more: Richest 1% earn biggest share since Roaring '20s)
"It's pretty frustrating," says Annette Guerra, 33, of San Antonio, who has been looking for a full-time job since she finished nursing school more than a year ago. During her search, she found that employers had become increasingly picky about an applicant's qualifications in the tight job market, often turning her away because she lacked previous nursing experience or because she wasn't certified in more areas.
Guerra says she now gets by doing "odds and ends" jobs such as a pastry chef, bringing in $500 to $1,000 a month, but she says daily living can be challenging as she cares for her mother, who has end-stage kidney disease.
"For those trying to get ahead, there should be some help from government or companies to boost the economy and provide people with the necessary job training," says Guerra, who hasn't ruled out returning to college to get a business degree once her financial situation is more stable. "I'm optimistic that things will start to look up, but it's hard."
Last year the average length of unemployment for U.S. workers reached 39.5 weeks, the highest level since World War II. The duration of unemployment has since edged lower to 36.5 weeks based on data from January to July, still relatively high historically.
Economists call this a "bumping down" or "crowding out" in the labor market, a domino effect that pushes out lower-income workers, pushes median income downward and contributes to income inequality. Because many mid-skill jobs are being lost to globalization and automation, recent U.S. growth in low-wage jobs has not come fast enough to absorb displaced workers at the bottom.
Low-wage workers are now older and better educated than ever, with especially large jumps in those with at least some college-level training.
"The people at the bottom are going to be continually squeezed, and I don't see this ending anytime soon," said Harvard economist Richard Freeman. "If the economy were growing enough or unions were stronger, it would be possible for the less educated to do better and for the lower income to improve. But in our current world, where we are still adjusting to globalization, that is not very likely to happen."