2.3 Social experiments 

The third (figure 2, C) and fourth experiments (figure 2, D) presented two actors engaged in a social game in which they could either cooperate or defect. Participants were instructed to infer the nature of the second player’s social intention (i.e. cooperative or defective intention), given the action performed by the first player in the previous round (i.e. cooperative or defective action). In both these experiments, the bias was assigned according to the way the second player responded to the strategy adopted by the opponent in the previous round. We chose to bias participants toward a ‘tit-for-tat’ (TFT) mode of reciprocity, rather than toward a particular type of social – defective or cooperative – intention (see Chambon et al., xxxx, for details); meaning that the probability that the second player cooperates if the first player had previously cooperated, or defects if the first player had previously defected, was increased at the expense of the alternative strategies (e.g. always defects, always cooperates). Finally, as in the two previous non-social experiments, both motor intentions (single cooperative or defective act) and superordinate intentions (sequences of motor acts leading to the construction of a cooperative or defective configuration) were considered.