Personal and social rewards differently shape the neural correlates of deceptive decision making

Previous research suggests that deciding whether to lie to another individual often involves a moral dilemma between the temptation to deceive to get a financial (personal) reward and the desire to follow social norms of moral conduct (social reward). A failure to achieve the social reward of honesty is very often associated with a subsequent sense of guilt. Consistent with this, guilt aversion has been considered one of the strongest motivator for honesty. On the other hand, sensitivity to temptation (caused by primary reward) is considered a promotor of dishonest behavior. Here, we aimed at testing the effect of personal reward (temptation) and social reward (guilt aversion) on the neural correlates to lie. We employed the Temptation to Lie Card Game (TDCG) where participants could choose whether to lie (egoistically/altruistically) or not to another player. We actively manipulated personal reward by modulating the economic reward of each trial and the social reward by modulating the level of guilt associated to each decision (by employing low versus high social ranking opponent players). Our findings show that guilt and reward act as opposing forces on the decision to deceive others: sense of guilt decreases the proneness to make egoistic lies and increases the proneness to make altruistic lies. On the other hand, reward does the opposite, pushing participants toward dishonest behaviour. Tellingly, personality traits related to moral cognition significantly modulate deception. Moreover, we found reduced lateralized readiness potential (LRP) amplitude and reduced Mu-suppression (as measured by time-frequency analysis) for egoistic lies in the high vs. low guilt condition. Surprisingly, LRP reduction was found when participants were lying in the high vs. low reward condition suggesting that even if participants were more tempted to lie in this condition (as showed by the behavioural results), the motor preparation of this response was cortically inhibited. Our findings confirm the importance of personal and social rewards in shaping interpersonal deception and provide critical evidence for a role of cortical motor preparation in implementing moral/immoral actions