Articles whose authors make them Open Access (OA) by self-archiving them
online are cited significantly more than articles accessible only to
subscribers. 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 2002-2006 in
1,984 journals. The OA Advantage proved just as high for both.