Z:gnu-www-ja-frank--21819d-In one version of the experime/en

In one version of the experiment (the &ldquo;unlimited&rdquo; version), subjects were told that they could make promises not to defect, but they were also told that the anonymity of their responses would render such promises unenforceable. In two other versions of the experiment (the &ldquo;intermediate&rdquo; and &ldquo;limited&rdquo; versions), subjects were not permitted to make promises about their strategies. The latter two versions differed from one another in terms of the length of pre-game interaction, with up to 30 minutes permitted for the intermediate groups and no more than ten minutes for the limited groups. All groups were given an extensive briefing on the prisoner's dilemma at the start of the experiment and each subject was required to complete a questionnaire at the end to verify that he or she had indeed understood the consequences of different combinations of choices.