We all do it and according to Brian Wansink, a food psychology professor at Cornell University, we eat with our eyes and not with our stomach. In his book Mindless Eating, he explains that this unconscious eating even of only 100-200 calories a day, a handful of chips here or an extra serving there, causes us to gain pound after pound, year after year. Instead of heeding our internal cues we unconsciously overeat because of external cues that trigger our appetite. Wansink and his graduate students experimented with this idea by giving movie theater patrons either a medium or large bucket of very stale popcorn. Even though some of them had just had lunch, people who were given the big buckets ate an average of 53 % more than those given the medium buckets.
He writes, …the cues around us -- like the size of a popcorn bucket -- can provide subtle but powerful suggestions about how much one should eat. These cues can short-circuit a person's hunger and taste signals, leading them to eat even if they're not hungry and even if the food doesn't taste very good.”
This can work to our advantage, however, when we are trying to eat more vegetables for instance. Pile them on your plate and guess what, you will eat them.