  The enormous financial success of online advertising platforms is partially
due to the precise targeting features they offer. Although researchers and
journalists have found many ways that advertisers can target---or
exclude---particular groups of users seeing their ads, comparatively little
attention has been paid to the implications of the platform's ad delivery
process, comprised of the platform's choices about which users see which ads.
  It has been hypothesized that this process can "skew" ad delivery in ways
that the advertisers do not intend, making some users less likely than others
to see particular ads based on their demographic characteristics. In this
paper, we demonstrate that such skewed delivery occurs on Facebook, due to
market and financial optimization effects as well as the platform's own
predictions about the "relevance" of ads to different groups of users. We find
that both the advertiser's budget and the content of the ad each significantly
contribute to the skew of Facebook's ad delivery. Critically, we observe
significant skew in delivery along gender and racial lines for "real" ads for
employment and housing opportunities despite neutral targeting parameters.
  Our results demonstrate previously unknown mechanisms that can lead to
potentially discriminatory ad delivery, even when advertisers set their
targeting parameters to be highly inclusive. This underscores the need for
policymakers and platforms to carefully consider the role of the ad delivery
optimization run by ad platforms themselves---and not just the targeting
choices of advertisers---in preventing discrimination in digital advertising.
