(Bloomberg)--Age discrimination is pervasive in the U.S.,despite laws that prohibit it.

And the older you are, the more discriminationyou face, according to the authors of a National Bureau of EconomicResearch study out Monday.

Older women have it particularly tough.

Baby boomers started turning 65 in 2011, and each day about10,000 more hit that age that has historically been associated withretirement.

These days, though, many are opting to search for jobs tobolster their incomes, which can be a long slog.

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One reason is something that is not often discussed because it'sillegal: discrimination.

Three economists—David Neumark and Ian Burn of the University ofCalifornia at Irvine and Patrick Button of TulaneUniversity—designed a field experiment to try to document howwidespread discrimination is, particularly among workers nearing retirement age.

Using more than 40,000 job applications, they responded to jobads with similar fictional resumes for workers purported to be ages29-31, 49-51 and 64-66.

The study looked at a dozen cities, including New York, Chicago,Los Angeles and Boston.

Whether the jobs were for administrative, sales, securitypositions or janitors, the rates of callbacks—either by telephoneor by email—were much higher for the younger workers than the olderones.

Discrimination, rather than lack of skills, may help to explainwhy older workers have longer periods of unemployment duration.

Long periods of unemployment—six months or longer—have been oneof the lasting problems in the wake of the 2007-2009 recession, thebiggest downturn since the 1930s.

What's more, the bias worsens when gender is considered.

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"We find robust evidence of age discrimination in hiring againstolder women," the economists wrote, citing data on callbacks forsales jobs. "There is evidence of stronger age discrimination forwomen than for men in sales."

The latest study is reminiscent of a 2002 study by University ofChicago economist Marianne Bertrand and Harvard economics professorSendhil Mullainathan.

They found hiring managers were much more likely to call backapplicants with white-sounding names, such as Greg and Emily,rather than black-sounding names, such as Lakisha and Jamal.

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Their findings were “disturbing” and hard to dispute becausethey submitted almost 5,000 applications over a year, Alan Krueger,former chairman of President Barack Obama’s Council of EconomicAdvisers, wrote in a New York Times commentary.

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