Do you have a link for that study?
At this point, just me. I looked for it, and it probably is one of his books. Harry Dillingham might well be dead, but his books were early sixties, and I attended his sociology of religion class in roughly 1977.
So, I am your link. I heard him say it. And that was his life’s work.
I went back to see what I could find something easily online, and there are two promising books that might have the info in them. And, his being a prof at a major city university, he would also have lived under publish or perish, so I expect it’s in some article, too.