DOI: 10.5061/dryad.37pvmcvmk
title: Experimental data: Inferring strategies from observations in long iterated prisoner’s dilemma experiments

description: 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.
description: The data were collected for two treatments wherein participants
 played long IPD in two different pairwise configurations, i.e. fixed
 partners (FP) and shuffled partners (SP). The data from these experiments
 were collected in Brussels, Belgium, at the Brussels Experimental
 Economics Laboratory (BEEL), part of the Vrije Universiteit Brussel (VUB).
 Twelve sessions were held, in total 188 participants were recruited. The
 group of participants consisted of 48\% female and 52\% male. The average
 age in all sessions was 22+-8 years old (see the Supplementary Information
 for detailed data about the participants). The experiments were approved
 by the Ethical Commission for Human Sciences at the VUB (ECHW2015_3). The
 participants played the IPD on individual isolated laptops, designated to
 avoid any type of communication or arrangements by the participants, who
 remained anonymous throughout the experiment. At the end of the
 experiments, subjects were asked optional questions about their age,
 gender, and information about the game they just played (see the
 Supplementary Information for the detailed questions and answers). The FP
 treatment consisted of 6 sessions where 92 participants played pairwise
 IPD with the same opponent. The SP treatment consisted of 6 sessions where
 participants played the same IPD as FP but with a different opponent each
 round, hence the "shuffled" name. The IPD parameters are R = 3,
 T = 4, S = 0, P = 1.  At the beginning of the
 experiment, every participant read a detailed instruction document with a
 small test at the end to make sure they understood the game dynamics (see
 Supplementary Information). At the end of the experiment, participants
 were given their gains (mean = 7.85 euros, SD = 1.67 in the SP treatment
 and mean = 10.61, SD = 3.87 in the FP treatment) and a show-up fee of 2.5
 euros.
description: Please refer to the file README.txt for a full detail of the
 columns used in the dataset.