Dieters may try to estimate a meal’s calorie count. Now a study by Northwestern University’s Alexander Chernev finds that even the order in which food is presented—and whether the food is thought of as a vice or a virtue—affects how many calories we think it has. The work will be published in 2011 in the Journal of Consumer Research.

Study subjects were shown a cheese-steak first, which they guessed had on average 578 calories. Or they saw a virtuous fruit salad first, which they guessed was 311 calories. After which they estimated the same cheese-steak as having 787 calories.

But when first shown the vice of a slice of chocolate cake, which they guessed had 416 calories, subjects estimated that the same cheese-steak wasn’t much worse of a vice, at only 489 calories. So estimates of the cheese-steak calorie content went up when it followed fruit salad, but went down when subjects first considered a slice of cake.

An absurd outcome of this was that subjects estimated a cheesesteak and cake combo as having fewer calories than a fruit salad-cheesesteak one. So remember, when you’re counting calories, you really can’t rely on gut feelings.