Facial expressions are an important aspect of social interaction, conveying not only information regarding emotional states, but also regarding intentions, personality, and complex social characteristics. The present research investigates how a smiling, compared to a nonsmiling, expression impacts decision making and underlying cognitive and emotional processes in economic bargaining. Our results using the ultimatum game show that facial expressions have an impact on decision making as well as the feedback-related negativity following the offer. Furthermore, a moderating effect of sex on decision making was observed, with differential effects of facial expressions from male compared to female proposers. It is concluded that predictions of bargaining behavior must account for aspects of social interactions as well as sex effects to obtain more precise estimates of behavior.