This paper analyzes whether advertisers would be better off using data that would
enable them to target users more accurately if the only way they could use this
data is by sharing the data with all advertisers. I present a wide range of
theoretical results that illustrate general circumstances under which an advertiser
can be assured that sharing data will make the advertiser better off. I then
empirically analyze how sharing data would affect Google’s top 1000 advertisers on
mobile apps, and find that over 98% of these advertisers would always be better off
sharing data, even if the data can also help their competitors bid more accurately.