CNN host Anderson Cooper on Monday became the latest journalist to question so-called "studies" cited by Republican presidential nominee Mitt Romney and his running mate, Rep. Paul Ryan, to prove that their proposal could cut taxes without raising the deficit.

In his "Keeping Them Honest" segment, Cooper observed that Romney had pointed to "six studies" to defend the tax plan, "but neither he nor his running mate Paul Ryan have ever specified which tax deductions they'll cap, which loopholes they'll close, or frankly give out many details at all."

"Despite that handicap, a bipartisan panel of three authors for the Tax Policy Center examined the plan and concluded that there's really no way of making the numbers work, that is unless the middle class pays more. Thousands of dollars more per family, according to the authors," the CNN host added.

But recently Romney told NBC's David Gregory that the Tax Policy Center was biased and that he had "five different economic studies, including one oat Harvard, Princeton and AEI and a couple at the Wall Street Journal" to back his proposal. Romney later increased that total to six studies while facing President Barack Obama at the first presidential debate.

During last week's vice presidential debate, Ryan also said that "six studies have guaranteed, six studies have verified that this math adds up."

"The suggestion is that these are full-blown academic studies," Cooper noted on Monday. "Actually, three are blog posts, one is a Wall Street Journal op-ed. In the Wall Street Journal piece, Martin Feldstein, who's also a campaign advisor, makes the math work but only by using a different definition of middle class than Mr. Romney uses in his own plan."

"In another study cited by Mr. Romney, Princeton economist Harvey Rosen assumes the tax cuts would generate enough economic growth to offset the cost but for many, that is -- that's a rather large assumption. One that's also by the way questioned by many conservative economists as well."

Cooper concluded: "Bottom line, though, that word assume. Every one of these authors in each of these studies or so-called studies is making assumptions. As some may be solid assumptions, others dubious, but they're all just assumptions because neither Mitt Romney nor Paul Ryan nor any of their surrogates have yet come forward with specifics."

Watch this video from CNN's Anderson Cooper 360, broadcast Oct. 15, 2012.