Background: Articles whose authors have supplemented subscription-based access to the publisher's version by selfarchiving their own final draft to make it accessible free for all on the web (``Open Access'', OA) are cited significantly more than articles in the same journal and year that have not been made OA. Some have suggested that this ``OA Advantage'' may not be causal but just a self-selection bias, because authors preferentially make higher-quality articles OA. To test this we compared self-selective self-archiving with mandatory self-archiving for a sample of 27,197 articles published 2002