Most supporters of the death penalty agree with one of the major arguments against it, according to a new survey by the Pew Research Center.

The survey of 1,500 US adults, conducted in March, found that nearly two-thirds of Americans who favor the death penalty agree there's a risk it will kill innocent people — one of the major reasons some Americans oppose capital punishment.

Nearly half of death penalty supporters also said it doesn't deter crime. But nearly six in 10 said minorities aren't more likely to be sentenced to death, when in reality they are executed at higher rates.

So why do Americans support the death penalty? Previous Gallup surveys have given a consistent justification: vengeance. In 2014, 35 percent of those in favor of the death penalty for convicted murderers said a variation of "it fits the crime," "eye for an eye," or "they took a life," while 14 percent said "they deserve it." Another 14 percent said the death penalty saves money — even though lengthy court appeals make it more expensive than life in prison. Just 6 percent said it deters crime.

The Pew survey also found that support for the death penalty is decreasing — although the issue, like many others, is getting much more polarized by political party affiliation, with Democrats growing more opposed as Republicans remain in strong support. There was a 37-point gap in support for the death penalty between Republicans and Democrats in 2015, compared to a 16-point gap in 1996.

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Correction: The key in the chart was originally incorrect.