Data here: http://tar.weatherson.org/2013/07/11/how-often-do-journals-cite-other-journals/

I find this information interesting and extremely odd. My initial reaction was that the data reflected what I had seen with JP (constant citing of JP articles, much less citing of other journals), but that isn't the case. In fact, according to the data, only 74 out of 489 articles in the Journal of Philosophy cite other articles from that journal. Frankly, I can't believe that number. It's too low.

Additionally, I find his choice of Journals interesting: were these 18 the most cited journals in Mind, JP, and PR? If so, that is disturbing. But I would imagine that more generalist Journals (Southern Journal of Philosophy, Philosophy and Phenomenological Research, European Journal of Philosophy, etc) would get more play than the specialist journals at the end.