3.3. Non-social vs. social intentions

Social experiments were characterized by participants’ response over-relying on prior expectations as revealed by responses massively shifting towards preferred intentions (i.e. ‘tit-for-tat” intention (TFT): cooperation if previous cooperation, defection if previous defection) whatever the amount of visual information available from the action scene. This increased reliance on prior knowledge cannot be accounted for by differences in terms of complexity between non-social and social experiments, namely a greater memory load due to the requirement to track two successive intentions – the first and the second player’s ones. Indeed, participants performed equally well, in terms of correct responses and reaction times, in both the social and non-social experiments. Additionally, the effect of facilitation associated with TFT strategy in the motor social experiment cannot be explained by a visual priming effect of the first player’s action on the second player’s one, that could have occurred when the latter performed the same action as the former. Indeed, TFT strategy was also found to be favoured in the superordinate social experiment; yet in this study TFT strategy did not necessarily imply that the action of the first player should be reproduced by the second player.

The dependence of the participants on their prior knowledge appears to reflect some expectations driven by the social context of the task. It is well-known that even basic movements, like the relative movements of geometrical figures, automatically induce participants to perceive the figures as socially interacting (Heider & Simmel, 1944; Castelli, Happé, Frith & Frith, 2000; Scholl & Tremoulet, 2000, for a review), together with eliciting strong expectations about the intentional causes of their movements (striking, kissing, escaping, etc.). Situations identified as involving social interactions are generally prone to trigger specific expectations concerning the way agents are likely to behave in such situations (Frith & Frith, 2006). These expectations may be derived from knowledge of diverse nature, such as that provided by group stereotypes (Fiske, Cuddy, Glick, & Xu, 2002), social-specific naive theories (Ybarra, 2002)or agent’s reputation acquired from experience of reciprocal social interactions (Singer et al., 2006). In the present experiment, increased dependence on prior knowledge for inferring social intentions, regardless of their scope, seems precisely to fall within expectations of this kind. Indeed, during the whole task, increasing the frequency of the second player adopting a TFT strategy amounted to progressively assigning a specific reputation to that player. A bias in the response towards “tit-for-tat” mode of reciprocity reveals that participants did integrate this reputational knowledge and made their response accordingly.

The pervasive effect of these specific expectations is also well illustrated in the baseline condition by the early preference of participants for the TFT reciprocation. Even before being biased in this direction, participants tended to infer more rapidly that the second player was more inclined to mirror the first player’s strategy. This early preference was probabilistically reinforced in the bias session and, as a consequence, exerted a greater influence on the participants’ performance since it persisted even when the reliability of the visual information was high. Indeed, while in the motor non social experiment the very same motor act presented alone was inferred from a much lower amount of visual information, in the social experiment a bias response towards the preferred social intention was still observed for a higher amount of visual information. This shows that the influence of these expectations in the bias session was such that the participants had difficulties in disengaging from their a priori expectations, resulting in predicting a play congruent with prior expectations but counterfactual to perceptual evidence. Similarly, these difficulties could account for the cost in performance associated with the selection of intentions that did not meet these expectations. In the bias session, participants were indeed significantly less accurate and were slower to select a non-preferred intention (i.e. always defect, always cooperate) when this selection required concomitantly inhibiting the competing tit-for-tat intention.