We can’t stop eating.

If you think you can order a gallon-size popcorn and stop eating when you’re full, think again: Visual cues, like portion sizes, have a huge effect on how much food we eat. 

In this classic study, groups of four university students ate bowls of soup for lunch. For two people in each group, the bowls of soup refilled automatically (and imperceptibly slowly) through tubes hidden underneath the table. For the other two in each group, a waiter came by if their portions ran low.

The students with the auto-refill bowls ate 73 percent more soup than the others, but didn’t think they had—and didn’t feel more full. All of the students also underestimated how many calories they’d consumed, and those with the refilling bowls were much further off the mark.

Thankfully, it’s unlikely that your local diner has auto-refill dinnerware, though family-style food spreads, passed hors d'oeuvres or (sneaky) free refills could snare you in a similar trap.  

We judge our fullness by visual cues—like whether our plates are empty—more readily than by how we feel. Outsmart yourself: Order the small popcorn or a half portion at a restaurant, or divvy up big bags of snacks into little plastic baggies. Or pile your plate with beauty-boosting fruits and vegetables to trick yourself into eating extra. 

Read More by Brian Wansink:
Meet the Mind: Brian Wansink
Low-Fat Labels Invite Overeating
Economy-Size is a Diet Buster
If We Can Reach It, We’ll Eat It