Mark J. McCabe and Christopher M. Snyder have self-archived "Did Online Access to Journals Change the Economics Literature?" in SSRN.
Here's an excerpt:
Does online access boost citations? The answer has implications for issues ranging from the value of a citation to the sustainability of open-access journals. Using panel data on citations to economics and business journals, we show that the enormous effects found in previous studies were an artifact of their failure to control for article quality, disappearing once we add fixed effects as controls. The absence of an aggregate effect masks heterogeneity across platforms: JSTOR boosts citations around 10%; ScienceDirect has no effect. We examine other sources of heterogeneity including whether JSTOR benefits "long-tail" or "superstar" articles more.