The effect of portion size and unit size on food intake: Unit bias or segmentation effect?

The "unit bias" has been proposed as an explanation for the portion-size effect; people consider a single unit to be an appropriate amount to eat and thus eat more when served a larger unit than when served a smaller unit. We suggest that the unit bias might be better characterized as a "segmentation effect," such that people eat less when a unit of food is separated into smaller subunits, but may eat more than a single unit. Furthermore, we suggest that portion-size effects should be independent of this segmentation effect.

In Study 1, female participants (n = 87) were served either a small or large portion of food that was either presented in the form of a single unit or multiple individually wrapped units. In Study 2, female participants (n = 42) were served a fixed portion of food that was either presented in the form of a single unit or multiple units presented on separate plates.

Across both studies, there was no evidence that participants prefer to eat a single unit. Participants served multiple smaller units did eat less than did participants served a single larger unit, even when the overall portion size was the same, but the amount eaten was consistently more than a single unit. Furthermore, perceived norms of appropriate intake mediated the effect of unit number on food intake.

These findings suggest that a segmentation effect, rather than a unit bias, is driving people's food intake, with implications for designing interventions aimed at reducing excessive food intake.