  In January 2019, YouTube announced it would exclude potentially harmful
content from video recommendations but allow such videos to remain on the
platform. While this step intends to reduce YouTube's role in propagating such
content, continued availability of these videos in other online spaces makes it
unclear whether this compromise actually reduces their spread. To assess this
impact, we apply interrupted time series models to measure whether different
types of YouTube sharing in Twitter and Reddit changed significantly in the
eight months around YouTube's announcement. We evaluate video sharing across
three curated sets of potentially harmful, anti-social content: a set of
conspiracy videos that have been shown to experience reduced recommendations in
YouTube, a larger set of videos posted by conspiracy-oriented channels, and a
set of videos posted by alternative influence network (AIN) channels. As a
control, we also evaluate effects on video sharing in a dataset of videos from
mainstream news channels. Results show conspiracy-labeled and AIN videos that
have evidence of YouTube's de-recommendation experience a significant
decreasing trend in sharing on both Twitter and Reddit. For videos from
conspiracy-oriented channels, however, we see no significant effect in Twitter
but find a significant increase in the level of conspiracy-channel sharing in
Reddit. For mainstream news sharing, we actually see an increase in trend on
both platforms, suggesting YouTube's suppressing particular content types has a
targeted effect. This work finds evidence that reducing exposure to anti-social
videos within YouTube, without deletion, has potential pro-social,
cross-platform effects. At the same time, increases in the level of
conspiracy-channel sharing raise concerns about content producers' responses to
these changes, and platform transparency is needed to evaluate these effects
further.
