There was a more recent paper on MMO or F2P design that included photos of rats in skinner boxes in its discussion of designing for player engagement, but for the life of me I can’t find it.
Liz Ryerson on content creators using fringe games as a content mill
There is supposed to be a thread where @Campster, @melessthanthree, @chrstphfr, and @QuestForTori discuss sales impact of the various Errant Signal essays on their games, but I have not been able to find it.