“Think before you act” is sage advice for dieters, especially when considering grabbing that holiday cookie, right? A new analysis of 50 studies finds that thinking before you eat can actually undermine your dieting goals. When we think, we often simply come up with reasons why we deserve that extra piece of pumpkin pie.

Jessie De Witt Huberts of Utrecht University says that we are expert rationalizers when it comes to finding a reason to eat more. “People seem to be very creative in coming up with such reasons,” he says. “They can justify having the cake on account that it has been a hard day, or that they will exercise tomorrow, that it is a special occasion, or that it is impolite to refuse. This is when justification processes become a slippery slope – as the reasons are often applied ad hoc, they no longer form strict rules that regulate when you stick to your diet and when you can cut yourself some slack.”

A growing body of research has found that such justification can even come from performing well on a task or from doing good for others. In several studies, people who received positive feedback on a task were more likely to choose an unhealthy versus a healthy option. “What was particularly interesting is how easily convinced participants were by the justifications,” De Witt Huberts says. “Sometimes they merely had to express the intention to help someone, think about doing something altruistic, rather than actually doing it, to justify subsequent hedonic consumption.”

Becoming aware of the justifications you use goes a long way toward helping around the holidays. It’s fine, she says, to indulge on Thanksgiving or other holidays if we consistently stick to our goals, but those special occasions must truly be exceptions.

For more, see “‘Because I Am Worth It’ A Theoretical Framework and Empirical Review of a Justification-Based Account of Self-Regulation Failure,” Jessie C. De Witt Huberts, Catharine Evers, Denise T. D. De Ridder, Personality and Social Psychology Review, published online November 8, 2013 – forthcoming in print February 2014.