Since prison culture often emphasizes punishment and retribution for not cooperating with fellow prisoners, the fact that they often cooperate is not surprising. My guess is that if one did this with people who had just been convicted of crimes but had not yet been in prison for a long time, one would see less cooperation.
Article at http://www.businessinsider.com/prisoners-dilemma-in-real-life-2013-7#ixzz2ZxwzT6nj, seems revelant to a lot of the discussion here.
There've been studies about people who consider themselves to be relatively successful are less cooperative than people who consider themselves relatively unsuccessful. The study referenced in that article seems to bear this out.
So if you want the other party to cooperate, should you attempt to give that party the impression it has been relatively unsuccessful, at least if that party is human?