  Personal connections between creators and evaluators of scientific works are
ubiquitous, and the possibility of bias ever-present. Although connections have
been shown to bias prospective judgments of (uncertain) future performance, it
is unknown whether such biases occur in the much more concrete task of
assessing the scientific validity of already completed work, and if so, why.
This study presents evidence that personal connections between authors and
reviewers of neuroscience manuscripts are associated with biased judgments and
explores the mechanisms driving the effect. Using reviews from 7,981
neuroscience manuscripts submitted to the journal PLOS ONE, which instructs
reviewers to evaluate manuscripts only on scientific validity, we find that
reviewers favored authors close in the co-authorship network by ~0.11 points on
a 1.0 - 4.0 scale for each step of proximity. PLOS ONE's validity-focused
review and the substantial amount of favoritism shown by distant vs. very
distant reviewers, both of whom should have little to gain from nepotism, point
to the central role of substantive disagreements between scientists in
different "schools of thought." The results suggest that removing bias from
peer review cannot be accomplished simply by recusing the closely-connected
reviewers, and highlight the value of recruiting reviewers embedded in diverse
professional networks.
