  Mentoring is traditionally viewed as a dyadic, top-down apprenticeship. This
perspective, however, overlooks other forms of informal mentoring taking place
in everyday activities in which developers invest time and effort, but remain
unacknowledged. Here, we investigate the different flavors of mentoring in Open
Source Software (OSS) to define and identify implicit mentoring. We first
define implicit mentoring--situations where contributors guide others through
instructions and suggestions embedded in everyday (OSS) activities--through
formative interviews with OSS contributors, a literature review, and
member-checking. Next, through an empirical investigation of Pull Requests
(PRs) in 37 Apache Projects, we build a classifier to extract implicit
mentoring and characterize it through the dual lenses of experience and gender.
Our analysis of 107,895 PRs shows that implicit mentoring occurs (27.41% of all
PRs include implicit mentoring) and it does not follow the traditional dyadic,
top-down apprenticeship model. When considering the gender of mentor-mentee
pairs, we found pervasive homophily--a preference to mentor those who are of
the same gender--in 93.81% cases. In the cross-gender mentoring instances,
women were more likely to mentor men.
