The social discounting paradigm is a powerful means of quantifying altruism in humans, who are typically willing to forgo some amount of personal earnings in exchange for increased earnings for another person. The amount of money that people are willing to forgo decreases with increasing social distance. In this study, we examined variables related to sex, intolerance of uncertainty, and empathy, all of which are theorized to affect the social discounting rate. Participants (27 men and 28 women) completed measures of intolerance of uncertainty, empathy, and social discounting. We found sex differences in psychological predictors of social discounting: in women, empathy (but not intolerance of uncertainty) predicts the social discounting rate, while in men, social discounting is associated with intolerance of uncertainty (but not empathy). Possible neurobiological, social, and cognitive explanations for this sex difference are discussed. Copyright © 2015 John Wiley & Sons, Ltd.