Psychologists have long observed that people conform to majority opinion, a phenomenon sometimes referred to as the ‘bandwagon effect’. In the political domain people learn about prevailing public opinion via ubiquitous polls, which may produce a bandwagon effect. Newer types of information – published probabilities derived from prediction market contract prices and aggregated polling summaries – may have similar effects. Consequently, polls can become self-fulfilling prophecies whereby majorities, whether in support of candidates or policies, grow in a cascading manner. Despite increased attention to whether the measurement of public opinion can itself affect public opinion, the existing empirical literature is surprisingly limited on the bandwagon effects of polls. To address this gap, we conducted an experiment on a diverse national sample in which we randomly assigned people to receive information about different levels of support for three public policies. We find that public opinion as expressed through polls affects individual-level attitudes, although the size of the effect depends on issue characteristics.