  While many theoretical studies have revealed the strategies that could lead
to and maintain cooperation in the Iterated Prisoner's Dilemma, less is known
about what human participants actually do in this game and how strategies
change when being confronted with anonymous partners in each round. Previous
attempts used short experiments, made different assumptions of possible
strategies, and led to very different conclusions. We present here two long
treatments that differ in the partner matching strategy used, i.e. fixed or
shuffled partners. Here we use unsupervised methods to cluster the players
based on their actions and then Hidden Markov Model to infer what are those
strategies in each cluster. Analysis of the inferred strategies reveals that
fixed partner interaction leads to a behavioral self-organization. Shuffled
partners generate subgroups of strategies that remain entangled, apparently
blocking the self-selection process that leads to fully cooperating
participants in the fixed partner treatment. Analyzing the latter in more
detail shows that AllC, AllD, TFT- and WSLS-like behavior can be observed. This
study also reveals that long treatments are needed as experiments less than 25
rounds capture mostly the learning phase participants go through in these kinds
of experiments.
