Physics/Mathematics
Social Sciences
Biological Sciences
The citation impact advantage is found in all fields
 analyzed so far, including articles (self-archived
in any kind of  open-access website or archive) in
social sciences (above  right) biological sciences
(below right) and all fields of Physics (self-archived
in ArXiv, below). Note  that the percentage of
published articles that have been  self-archived
(green bars) varies from about 10-20%from field
to field and that the size of the open-access citation
impact advantage (red bars) varies from about
25% to over 300%, but it is always positive.
http://opcit.eprints.org/oacitation-biblio.html