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."