“The Open Access Availability of Articles from Highly Ranked Religious Studies Journals: A Study of Ten Journals”, Theological Librarianship, April 2018.
A study of ten highly-ranked religious journals in mid 2015, aiming to find 377 paywalled articles in free self-archived and repository form (by 2016). The choice of top-titles means it was slanted toward health/medical (Journal of Health Care Chaplaincy, Journal of Religion and Health, Psychology of Religion and Spirituality, Intl. Journal for the Psychology of Religion, Journal for the Scientific Study of Religion were five of the ten), and so it probably shouldn’t be taken as a measure of trends in the core humanities. I suspect they would have had less success with leading paywalled journals in church history, art history, theology, music, ethnography etc.
Nevertheless, the results are interesting…
“OA versions were found for 132 (35 percent) of” [the 377 articles, and] “results indicate that using both Google and Google Scholar to search for OA religious studies journal scholarship yields better results than only using Google or Google Scholar.”
Of course one can’t guarantee that what’s online a year after publication will still be online a decade later, if self-archiving. It would be interesting to see a long-term study of the “rot rate” or “404 rate” in OA unofficial self-archiving of paywalled articles over the period of a decade. I’d suspect it would be around 40%.