Behind almost all of the disturbing issues raised by Mitt Romney's jaw-dropping tax returns stands one largely unchallenged conservative article of faith. Much lower tax rates for capital gains than income earned through labor, conservatives claim, spur investment, catalyze economic growth and fuel job creation. But if that Republican theology isn't true, then the United States has for decades done nothing more than deliver a massive windfall to the wealthiest Americans needing it least. Unfortunately, that's precisely what the data show. As it turns out, lower capital gains taxes increase income inequality - and not investment - in America.

As Paul Krugman recounted two weeks ago, the historically low capital gains rate enjoyed by Mitt Romney hasn't always been 15 percent. In the not-too-distant past, it reached 39.9 percent and with the Reagan tax reform of 1986 was briefly the same as the top tax rate on income. But successive presidents of both parties lowered the capital gains rate on investment income because they believed, the Washington Post explained, "it spurs more investment in the U.S. economy, benefiting all Americans."

But as Jared Bernstein demonstrated with the chart above, there's no evidence to support that claim. Bernstein found that the business cycle, not acts of Congress, drive investment in the U.S.

Hard to see anything in the picture supporting the view that either the level or changes in cap gains taxes play a determinant role in investment decisions.

Remember, the ostensible reason for the favoritism in tax treatment here is to incentivize more investment and faster productivity growth. But that's not in the data and the reason it's not in the data is because investors aren't nearly as elastic to cap gains rates as their lobbyists say they are (more precisely, they'll carefully time their realizations to maximize their gains around rate changes, but that's not real economic activity-that's tax planning).

Reviewing other analyses, Brad Plummer of the Washington Post concurred with that assessment that low capital gains taxes don't necessarily jump-start investment in the economy:

The top tax rate on investment income has bounced up and down over the past 80 years—from as high as 39.9 percent in 1977 to just 15 percent today—yet investment just appears to grow with the cycle, seemingly unaffected...

Meanwhile, Troy Kravitz and Len Burman of the Urban Institute have shown that, over the past 50 years, there's no correlation between the top capital gains tax rate and U.S. economic growth—even if you allow for a lag of up to five years.

Billionaire Warren Buffett, the inspiration for the "Buffett Rule" advocated by President Obama and his Democratic allies, couldn't agree more. As he told The New York Times last year:

"I have worked with investors for 60 years and I have yet to see anyone -- not even when capital gains rates were 39.9 percent in 1976-77—shy away from a sensible investment because of the tax rate on the potential gain. People invest to make money, and potential taxes have never scared them off."

But if lower capital gains tax rates have had little impact on investment, they have had an outsized impact on income inequality in the United States.

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