  Context: Contemporary software development organizations lack diversity and
the ratios of women in Free and open-source software (FOSS) communities are
even lower than the industry average. Although the results of recent studies
hint the existence of biases against women, it is unclear to what extent such
biases influence the outcomes of various software development tasks.
  Aim: We aim to identify whether the outcomes of or participation in code
reviews (or pull requests) are influenced by the gender of a developer..
  Approach: With this goal, this study includes a total 1010 FOSS projects. We
developed six regression models for each of the 14 dataset (i.e., 10 Gerrit
based and four Github) to identify if code acceptance, review intervals, and
code review participation differ based on the gender and gender neutral profile
of a developer.
  Key findings: Our results find significant gender biases during code
acceptance among 13 out of the 14 datasets, with seven seven favoring men and
the remaining six favoring women. We also found significant differences between
men and women in terms of code review intervals, with women encountering longer
delays in three cases and the opposite in seven. Our results indicate reviewer
selection as one of the most gender biased aspects among most of the projects,
with women having significantly lower code review participation among 11 out of
the 14 cases. Since most of the review assignments are based on invitations,
this result suggests possible affinity biases among the developers.
  Conclusion: Though gender bias exists among many projects, direction and
amplitude of bias varies based on project size, community and culture. Similar
bias mitigation strategies may not work across all communities, as
characteristics of biases and their underlying causes differ.
