Unilateral hand contractions produce motivational biases in social economic decision making.

OBJECTIVE Unilateral hand contractions have been shown to induce relative activation of the contralateral hemisphere, which is in turn associated with distinct motivational states. Specifically, right hand contraction increases relative left activation and promotes an approach state, and left hand contractions promote relative right activation and withdrawal states. Using the same hand clenching technique, the present study extends this research to examine the incidental role of motivational tendency on interactive economic decision making. METHOD A total of 75 right-handed participants were randomly assigned to 1 of 3 conditions, including withdrawal/left-hand contractions, approach/right-hand contractions, and control/no contraction. Participants completed 2 well-known economic tasks, namely the Ultimatum Game (UG), Dictator Game (DG). RESULTS In the UG, we found that relative to individuals in the withdrawal condition, those in the approach (right-hand contraction) condition made higher monetary offers to human partners who could either accept or reject these offers. Moreover, those in the approach condition rejected significantly more unfair offers from human partners. CONCLUSIONS This study provides the first evidence that hemispheric activation, using unilateral muscle contractions, may play a causal role in biasing social economic decision making. Overall, there results suggest that greater relative left frontal activation promotes reward-maximizing strategies, consistent with an approach motivation, and relative right frontal activation may decrease such strategic tendencies.