The following is a quote from the Open Access brochure from Create Change
Steve Lawrence, a scientist at NEC Research Institute, analyzed nearly 120,000 computer science articles cited in a standard disciplinary bibliography. When he looked at articles with successively higher levels of impact or citations, he found successively higher percentages of open-access articles, and vice versa. He found the strength of this correlation steadily increased over a decade.
source: Steve Lawrence, “Online or Invisible?” Nature, Vol. 411,No. 6837, p. 521, 2001.