Do open access articles have greater citation impact?

The last few years have seen the emergence of several open access options in scholarly communication which can broadly be grouped into two areas referred to as ‘gold’ and ‘green’ open access (OA). In this article we review the literature examining the relationship between OA status and citation counts of scholarly articles. Early studies showed a correlation between the free online availability or OA status of articles and higher citation counts, and implied causality without due consideration of potential confounding factors. More recent investigations have dissected the nature of the relationship between article OA status and citations. Three non-exclusive postulates have been proposed to account for the observed citation differences between OA and non-OA articles: an open access postulate, a selection bias postulate, and an early view postulate. The most rigorous study to date (in condensed matter physics) showed that, after controlling for the early view postulate, the remaining difference in citation counts between OA and non-OA articles is explained by the selection bias postulate. No evidence was found to support the OA postulate per se; i.e. article OA status alone has little or no effect on citations. Further studies using a similarly rigorous approach are required to determine the generality of this finding.