The growing popularity of social bookmaking applications like flickr and del.icio.us present new challenges to system designers because the effects of social psychological factors on users' tag choices have not been examined. The social psychological principle of social proof is particularly applicable to social bookmarking because it predicts that the tags applied by users will be more similar to each other if they are provided with a list of suggested tags. This study examines the effect of social proof on tag selection by comparing the degree of similarity between tags provided by a sample group and a collection of suggested tags provided to the treatment group. The results indicate that social proof can have an effect on users' tag selection. The conclusion briefly examines the beneficial effect of social proof on the quality of social bookmarking applications and other collaborative tagging applications.