Response to anticipated reward in the nucleus accumbens predicts behavior in an independent test of honesty

Abstract

This study examines the cognitive and neural determinants of honesty and dishonesty. Human subjects undergoing fMRI completed a monetary incentive delay task eliciting responses to anticipated reward in the nucleus accumbens. Subjects next performed an incentivized prediction task, giving them real and repeated opportunities for dishonest gain. Subjects attempted to predict the outcomes of random computerized coin-flips and were financially rewarded for accuracy. In some trials, subjects were rewarded based on self-reported accuracy, allowing them to gain money dishonestly by lying. Dishonest behavior was indexed by improbably high levels of self-reported accuracy. Nucleus accumbens response in the first task, involving only honest rewards, accounted for ∼25% of the variance in dishonest behavior in the prediction task. Individuals showing relatively strong nucleus accumbens responses to anticipated reward also exhibited increased dorsolateral prefrontal activity (bilateral) in response to opportunities for dishonest gain. These results address two hypotheses concerning (dis)honesty. According to the "Will" hypothesis, honesty results from the active deployment of self-control. According to the "Grace" hypothesis, honesty flows more automatically. The present results suggest a reconciliation between these two hypotheses while explaining (dis)honesty in terms of more basic neural mechanisms: relatively weak responses to anticipated rewards make people morally "Graceful," but individuals who respond more strongly may resist temptation by force of Will.

Keywords: dishonesty; fMRI; moral; morality; nucleus accumbens; reward.

Figure 1.

A, B, Task sequence of MID task (A) and coin-flip task (B). In the MID task (A), the subject observes the trial's monetary value, followed by a variable-duration fixation cross. After the fixation cross, a target square is briefly presented. The subject presses a button while the square is on the screen to get a financial reward or to avoid a financial loss. A feedback message with current and cumulative winnings/losses is presented. This is followed by a fixation interval. In the coin-flip task (B), the subject observes the trial's monetary value and privately predicts the outcome of the upcoming coin-flip. The subject records this prediction by pressing one of two buttons (No-Opportunity condition) or presses one of these buttons randomly (Opportunity condition). The subject then observes the outcome of the coin-flip. The subject then indicates whether the prediction was accurate and observes the amount of money won/lost based on the recorded prediction (No-Opportunity) or the self-reported accuracy (Opportunity). This is followed by a fixation interval.

Figure 2.

Response to anticipated reward in the nucleus accumbens predicts the frequency of dishonest behavior in an independent task (n = 28). The x-axis shows for each subject the mean difference in the nucleus accumbens' BOLD response to reward versus neutral trials during the MID task. The y-axis shows each subject's self-reported % Wins in the Opportunity condition of the coin-flip task, an index of dishonesty. Coloration shows anatomically defined ROIs superimposed on a standard brain. NAcc, Nucleus accumbens.

Figure 3.

Response to anticipated reward in the nucleus accumbens predicts DLPFC activity when refraining from gaining money dishonestly (n = 26). Bilateral DLPFC regions exhibited positive correlations (p < 0.001, uncorrected) between mean response to anticipated reward in the nucleus accumbens (averaged across right and left regions) during the MID task and the difference in mean signal change for chosen (Opportunity) Loss trials versus forced (No-Opportunity) Loss trials. Graphs show time courses of mean decision-related percentage change in BOLD signal during the coin-flip task for the honest, ambiguous, and dishonest groups. Signal time courses are displayed across 6 time-bins of 2.5 s each.