Twenty-four Princeton University undergraduates viewed color photographs of different social groups, as well as images of objects such as the space shuttle, a sports car, a cemetery, and an overflowing toilet.

The pictures, say psychology researchers Lasana Harris and Susan Fiske, were designed to elicit emotions of pride, envy, pity, or disgust, as derived from the Stereotype Content Model, which predicts differentiated prejudices.

Medial prefrontal cortex, or MPFC, brain imaging determined if the students chose the correct emotion illustrated by the picture.

The MPFC is only activated when people think about themselves or another human. When viewing a picture representing disgust, however, researchers say no significant MPFC brain activity was recorded, providing evidence that while individuals may consciously see members of social out-groups as people, the brain processes social out-groups as something less than human.

The scientists say they believe brain imaging provides a more accurate depiction of such prejudice than verbal reporting usually used in research studies.

The findings will appear in a forthcoming issue of Psychological Science.