Oddly Wired
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behavior

The size of a serving unit changes how much we eat.

A 2006 study showed that people ate significantly more when snacks were presented in larger units rather than smaller, individual ones.

The brain uses the 'unit'—a single piece, a container, or a bag—as a heuristic for a reasonable serving size. We stop eating when the unit is finished, regardless of the actual volume of the food.

Context often dictates consumption more than actual appetite.

Where this comes from

Study
Geier, Wansink, & Rozin, 2006, Psychological Science
What the check found
The 2006 study 'Unit Bias: A Heuristic That Helps Consume Less' by Geier, Wansink, and Rozin confirms that unit size acts as a consumption cue, leading people to consume more when presented with a single larger unit.
Confidence
Well replicated

Related

One of these every day, and every one names the study it came from. When a finding is contested, it says so.