I should mention that my system can be modified: if I consistently see that (despite punishment, and attempts to equalise) my partner is continuing to choose to default every-so-often, I can back-deduce that he's gaining more from defaulting than I am. If this is true, then I can increase the number of times I require him to go into debt before I'm willing to continue cooperating.
Right now it looks like playing against me: Default + Default (if I'm trusting) adds to 4+1 = 5 points for every two rounds (compared to 3+3=6 for cooperation). However, I might be wrong, and he received 8 for defaulting successfully (so 8+1=9 being superior to 6).
I can tell how much he values defaulting by seeing how many times he's willing to be defaulted against into order to compensate it.