"Pluralistic ignorance, a concept first coined by Floyd Allport (1924, 1933), exists when, within a group of individuals, each person believes his or her private attitudes, beliefs, or judgments are discrepant from the norm displayed by the public behavior of others. Therefore, each group member, wishing to be seen as a desirable member of the group, publicly conforms to the norm, each believing he or she is the only one in the group experiencing conflict between his or her private attitude and his or her public behavior."
Unsurprisingly, the researchers concluded that pluralistic ignorance played a role in encouraging the hookup culture: people tended to think that others were more comfortable with sex than they actually were, and would conform their behavior to that expectation.
As the article notes, pluralistic ignorance has been linked to college-aged alcohol consumption in a similar way. And many administrations have responded accordingly; at Princeton, for instance, we're required to take an online course called AlcholEdu which repeatedly reminds us that not everybody is drinking.
I wonder, though, why colleges don't take a similar approach to hooking up? It's clearly not particularly healthy for the undergrads involved. (Don't believe me? Check out Unprotected.) And the data certainly indicates a discrepancy between actual and perceived student behavior: in 2000, Princetonians estimated that 32.5% of their classmates had had fewer than 2 sexual partners within the past year, whereas in reality 79.4% of students had had fewer than 2 sexual partners in the past year.