  The lack of an engaging pedagogy and the highly competitive atmosphere in
introductory science courses tend to discourage students from pursuing science,
technology, engineering, and mathematics (STEM) majors. Once in a STEM field,
academic and social integration has been long thought to be important for
students' persistence. Yet, it is rarely investigated. In particular, the
relative impact of in-class and out-of-class interactions remains an open
issue. Here, we demonstrate that, surprisingly, for students whose grades fall
in the "middle of the pack," the out-of-class network is the most significant
predictor of persistence. To do so, we use logistic regression combined with
Akaike's information criterion to assess in- and out-of-class networks, grades,
and other factors. For students with grades at the very top (and bottom), final
grade, unsurprisingly, is the best predictor of persistence---these students
are likely already committed (or simply restricted from continuing) so they
persist (or drop out). For intermediate grades, though, only out-of-class
closeness---a measure of one's immersion in the network---helps predict
persistence. This does not negate the need for in-class ties. However, it
suggests that, in this cohort, only students that get past the convenient
in-class interactions and start forming strong bonds outside of class are or
become committed to their studies. Since many students are lost through
attrition, our results suggest practical routes for increasing students'
persistence in STEM majors.

Physics Education