Discrimination through optimization: How Facebook's ad delivery can lead to skewed outcomes.

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 platformu0027s ad delivery process, comprised of the platformu0027s choices about who should see an ad. It has been hypothesized that this process can 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 platformu0027s own predictions about the relevance of ads to different groups of users. We find that both the advertiseru0027s budget and the content of the ad each significantly contribute to the skew of Facebooku0027s 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.