Open Access. The accepted version of this work has been reproduced here according to the publisher's polices.
NOTICE: this is the author’s version of a work that was accepted for publication in Journal of Affective Disorders. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Journal of Affective Disorders, 123(1-3): 317-320, 2010. http://dx.doi.org/10.1016/j.jad.2009.09.015 (Please note: access via this link may only be available with a subscription)