The Effect of Use and Access on Citations
  It has been shown (S. Lawrence, 2001, Nature, 411, 521) that journal articles which have been posted without charge on the internet are more heavily cited than those which have not been. Using data from the NASA Astrophysics Data System (ads.harvard.edu) and from the ArXiv e-print archive at Cornell University (arXiv.org) we examine the causes of this effect. 
Michael J. Kurtz, Guenther Eichhorn, Alberto Accomazzi, Carolyn Grant,   Markus Demleitner, Edwin Henneken, Stephen S. Murray