Impact of spam exposure on user engagement

In this paper we quantify the effect of unsolicited emails (spam) on behavior and engagement of email users. Since performing randomized experiments in this setting is rife with practical and moral issues, we seek to determine causal relationships using observational data, something that is difficult in many cases. Using a novel modification of a user matching method combined with a time series regression on matched user pairs, we develop a framework for such causal inference that is particularly suited for the spam exposure use case. Using our matching technique, we objectively quantify the effect that continued exposure to spam has on user engagement in Yahoo! Mail. We find that indeed spam exposure leads to significantly, both statistically and economically, lower user engagement. The impact is non-linear; large changes impact users in a progressively more negative fashion. The impact is the strongest on "voluntary" categories of engagement such as composed emails and lowest on "responsive" engagement metrics. Our estimation technique and results not only quantify the negative impact of abuse, but also allow decision makers to estimate potential engagement gains from proposed investments in abuse mitigation.