The first peer-reviewed study of virginity pledgers (by sociologists Peter Bearman of Columbia and Hannah Brueckner of Yale) found that in the year following their pledge, some virginity pledgers are more likely to delay sex than non-pledgers; when virginity pledgers do have sex, they are less likely to use contraception than non-pledgers. This study found, however, that virginity pledges are only effective in high schools in which about 30% of the students had taken the pledge, meaning that they are not effective as a universal measure. Their analysis was that identity movements work when there is a critical mass of members: too few members, and people don't have each other for social support, and too many members, and people don't feel distinctive for having taken the pledge. This study was criticized for not being able to conclude causality, only correlation, a criticism which applies to all studies of virginity pledges thus far.