Detecting potential reference list manipulation within a citation network

Although citations are used as a quantifiable, objective metric of academic influence, cases have been documented whereby references were added to a paper solely to inflate the perceived influence of a body of research. This reference list manipulation (RLM) could take place during the peer-review process (e.g., coercive citation from editors or reviewers), or prior to it (e.g., a quid-pro-quo between authors). Surveys have estimated how many people may have been affected by coercive RLM at one time or another, but it is not known how many authors engage in RLM, nor to what degree. Examining a subset of active, highly published authors (n=20,803) in PubMed, we find the frequency of non-self citations (NSC) to one author coming from one paper approximates Zipfs law. We propose the Gini Index as a simple means of quantifying skew in this distribution and test it against a series of "red flag" metrics that are expected to result from RLM attempts. We estimate between 81 (FDR <0.05) and 231 (FDR<0.10) authors are outliers on the curve, suggestive of chronic, repeated RLM. Based upon the distribution, we estimate approximately 3,284 (16%) of all authors may have engaged in RLM to some degree, possibly opportunistically. Finally, we find authors who use 18% or more of their references for self-citation are significantly more likely to have NSC Gini distortions, suggesting their desire to see their work cited carries over into their peer-review activity.