  Software bots have been facilitating several development activities in Open
Source Software (OSS) projects, including code review. However, these bots may
bring unexpected impacts to group dynamics, as frequently occurs with new
technology adoption. Understanding and anticipating such effects is important
for planning and management. To analyze these effects, we investigate how
several activity indicators change after the adoption of a code review bot. We
employed a regression discontinuity design on 1,194 software projects from
GitHub. We also interviewed 12 practitioners, including open-source maintainers
and contributors. Our results indicate that the adoption of code review bots
increases the number of monthly merged pull requests, decreases monthly
non-merged pull requests, and decreases communication among developers. From
the developers' perspective, these effects are explained by the transparency
and confidence the bot comments introduce, in addition to the changes in the
discussion focused on pull requests. Practitioners and maintainers may leverage
our results to understand, or even predict, bot effects on their projects.
