Abstract
Decisions are often governed by rules on adequate social behaviour. Recent research suggests that the right lateral prefrontal cortex (rLPFC) is involved in the implementation of internal fairness rules (norms), by controlling the impulse to act selfishly. A drawback of these studies is that the assumed norms and impulses have to be deduced from behaviour and that norm-following and pro-sociality are indistinguishable. Here, we directly confronted participants with a rule that demanded to make advantageous or disadvantageous monetary allocations for themselves or another person. To disentangle its functional role in rule-following and pro-sociality, we divergently manipulated the rLPFC by applying cathodal or anodal transcranial direct current stimulation (tDCS). Cathodal tDCS increased participants’ rule-following, even of rules that demanded to lose money or hurt another person financially. In contrast, anodal tDCS led participants to specifically violate more often those rules that were at odds with what participants chose freely. Brain stimulation over the rLPFC thus did not simply increase or decrease selfishness. Instead, by disentangling rule-following and pro-sociality, our results point to a broader role of the rLPFC in integrating the costs and benefits of rules in order to align decisions with internal goals, ultimately enabling to flexibly adapt social behaviour.
Highlights
Likewise, in the social domain, norms about fairness, morality, or pro-sociality often demand to restrict selfishness
To analyse whether brain stimulation affected internal goals or intrinsic behaviour, we first looked at the decisions of participants when they were free to choose and not confronted with a conflict between social motives – i.e. the ‘me’ block and the ‘other person’ block under no rule
When an arbitrarily imposed rule coincided with this internal goal, people followed the rule overwhelmingly, irrespective of brain stimulation
Summary
In the social domain, norms about fairness, morality, or pro-sociality often demand to restrict selfishness. In line with this interpretation, the LPFC has been broadly associated with adaptive behaviour that enables humans to flexibly react to external stimuli in order to implement internal goals, rather than just follow fixed stimulus-response patterns or arbitrary rules[12,16,17,18,19,20,21] and integrates thought and action in the pursuit of these goals[18,19,20,21,22,23,24,25] Resonating with this interpretation, while brain stimulation over the right LPFC shifted decisions towards more selfishness or more pro-sociality depending on the stimulation, it did not affect the underlying fairness perception[9,12,13,14]. Difficult moral dilemmas, that require to find a compromise between norms and welfare maximisation, have been associated with greater LPFC activity[28], pointing to a value-based integrative function of the LPFC
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