It all comes down to willpower, right?  Strength of purpose.  Muster the resolve to skip dessert, and you have a shot at losing that spare tire hanging off your belly.  Succumb to your temptations, however, and you are simply being weak.

But is it just weakness that causes us to overeat?

A study in Psychological Science suggests that our inability to resist that mouthwatering looking chocolate cake doesn’t arise simply because our willpower is weak but also because, after exhausting our willpower, the cake looks even more mouthwatering to us than it did before.  Our ability to overcome temptation is reduced at the same time that the power of the temptation increases.

In this study, participants first underwent an exercise meant to exhaust their willpower.  They watched a seven minute documentary on Canadian bighorn mountain sheep.  Believe it or not, that documentary on its own doesn’t delete people’s willpower significantly.  Instead, it was distracting words scrolling across the screen that exhausted people’s willpower.  You see, half the participants were told to watch the documentary and read the words if they wanted to, as they scrolled in front of their field of vision.  No willpower needed there.  If you are curious what the word looks like, you look at it.  If not, you don’t.

But the other half of the participants were told specifically not to read the words — they were told to maintain their focus on the sheep.  Nothing but the sheep.  Seven minutes of ignoring words while watching sheep?  Exhausting just to think about it!

And willpower exhaustion was an important part of the study, because previous research has shown that willpower is depletable.  Exert willpower for seven minutes and you have less willpower to draw upon in the near future.

Which leads us to part two of the study.  The researchers placed these participants in an fMRI machine (a brain imager) and flashed pictures of deliciously unhealthy foods.  They wanted to see which parts of people’s brains lit up in front of these tempting delicacies.  I should tell you that the participants in this study were all trying to lose weight, and had all fasted before the study (which, by the way, probably means their willpower was already beginning to be depleted before they began the research).

Here is what happened.  The fMRI images revealed differences across the two groups of participants in their ability to resist temptation.  They found neurologic evidence of depleted willpower among the people who spent seven minutes not reading those pesky words.  But that is not all that the researchers found.  Those people whose willpower had been relatively depleted also showed increased activity in regions of the brain associated with “Q reactivity” — something to do with the OFC portion of their brains.  (Sorry, neuroscience is above my pay grade.)  Basically, the activity in these brain regions revealed that the food pictures looked tastier to depleted participants than it did to non-depleted ones.

Think of it this way.  You’re on a diet.  You have a tough day at work, and an awful commute back home (where it took all your remaining willpower not to flip off that @$&hole who cut in front of you on the highway) and now you open up your fridge to have a healthy salad.  But you see a tempting container of macaroni and cheese.  Not only are you too exhausted to resist the temptation, but the macaroni and cheese actually strikes you as something that would be so delicious to eat!

It might be time to give your spare tire a name.  Because it isn’t going anywhere anytime soon.

Peter Ubel is a physician and behavioral scientist who blogs at his self-titled site, Peter Ubel and can be reached on Twitter @PeterUbel.  He is the author of Critical Decisions: How You and Your Doctor Can Make the Right Medical Choices Together. This article originally appeared in Forbes.