  Celebrity endorsements are often sought to influence public opinion. We ask
whether celebrity endorsement per se has an effect beyond the fact that their
statements are seen by many, and whether on net their statements actually lead
people to change their beliefs. To do so, we conducted a nationwide Twitter
experiment in Indonesia with 46 high-profile celebrities and organizations,
with a total of 7.8 million followers, who agreed to let us randomly tweet or
retweet content promoting immunization from their accounts. Our design exploits
the structure of what information is passed on along a retweet chain on Twitter
to parse reach versus endorsement effects. Endorsements matter: tweets that
users can identify as being originated by a celebrity are far more likely to be
liked or retweeted by users than similar tweets seen by the same users but
without the celebrities' imprimatur. By contrast, explicitly citing sources in
the tweets actually reduces diffusion. By randomizing which celebrities tweeted
when, we find suggestive evidence that overall exposure to the campaign may
influence beliefs about vaccination and knowledge of immunization-seeking
behavior by one's network. Taken together, the findings suggest an important
role for celebrity endorsement.
