On the east coast, electric cars actually create more pollution than their gasoline-burning counterparts, a new study by the National Bureau of Economic Research finds.

Policymakers have long assumed that cars run on electricity, not gasoline, are greener. But “rather than simply accepting the assertion of environmental benefits from electric vehicle use, we conduct a rigorous comparison of the environmental consequences of gasoline and electric vehicles,” the study’s authors write.

Crunching the hard numbers, the study finds that on average, electrical vehicles actually generate more pollution than gas-fueled cars, though it also acknowledges “considerable variation” over different geographical areas.

It matters a lot, the report finds, how the power used by electric vehicles is generated. While the states out West often use natural gas and other green sources for electricity, the Midwest and Northeast rely heavily on coal, which is significantly dirtier. So in much of the nation, electrical cars actually result in more pollution, not less.

So why does this matter? Well, the federal government currently offers a $7,500 subsidy for each electric vehicle purchased, and many states also chip in. This study suggests the underlying assumption for these subsidies—that electrical cars are inherently more environmentally friendly—is often wrong.

The National Bureau of Economic Research’s new report syncs nicely with an earlier one by the Congressional Budget Office, published in 2012. It concluded that though taxpayers will subsidize electrical cars vehicles to the tune of $7.5 billion by 2019, the impact on gas consumption and emissions is basically negligible.

The scientific evidence continues show that subsidies for electric vehicles aren’t yielding the major environmental gains they’d promised. Smart policymakers would pull the plug.