Brian Wansink of Cornell University in Ithaca, N.Y., says people often underestimate the number of calories in an organic snack and over-reward themselves by eating more.
Wansink and colleagues found people who ate cookies labeled as "organic" believed their snack contained 40 percent fewer calories than the same cookies without the label.
The researchers found those most likely to do this were people who report usually buying organic foods and those who say they usually read nutritional information labels.
"An organic label gives a food a 'health halo,'" study co-author Wansink says in a statement.
He advises those who desire a more accurate calorie count to guess and then double the number.
"You'll end up being more accurate, and you'll probably eat a lot less," he says.
The study findings were presented at the Experimental Biology conference in Anaheim, Calif.