The vigor of costly punishments: Kinematic insights into socio-economic decision-making

Movement vigor changes with reward magnitude: movements exhibit shorter reaction time and increased velocity when directed toward more rewarding options. But what if the reward is turned down? In interactive economic games, people often choose to punish unfair proposers, even when the punishment is personally costly. How vigorously do they move in punishing unfair offers? Is vigor proportional to the incurred self-cost? Or does it rather reflect the cost inflicted on the unfair other? Here, we combined experimental economics and kinematics to address these questions. In study 1, we used motion capture to track participants’ hand kinematics while they responded to fair and unfair offers in a motor version of the Ultimatum Game. Kinematic analysis showed that vigor (defined as the negative reaction time and peak velocity) increased with reward magnitude for accepted offers and decreased for rejected offers. Study 2 aimed to disentangle whether the decrease in vigor for rejected offers reflected increased self-cost, decreased other-cost, or decreased effectiveness of punishments (defined as the ratio between other-cost and self-cost). We analyzed the vigor of responses in a punishment task where participants could choose to pay money to financially punish a player who had abused their trust. Results revealed that the effectiveness of punishment was the crucial factor modulating the vigor. Higher effectiveness of punishment yielded faster reaction times and higher peak velocity. These findings imply that, although costly sanctions might be fueled by negative emotions, participants accurately compute the cost of such sanctions.