Much of the research on birth order has focused on individual differences in personality traits, with relatively few studies focused on aspects of social behavior other than sibling conflict. However, one would predict that the differences in parental investment and niche differentiation that shape personality differences between siblings would also influence other social relationships. In particular, middleborns may be more likely to prioritize non-kin relationships. This study investigated the impact of birth order on a number of measures of prosocial behavior. Results suggest that birth order has a moderate effect on prosociality such that later birth orders exhibit greater prosociality. However, both the linear and quadratic effects were significant and the quadratic was negative indicating that the greatest increase in prosociality is seen between first and secondborns, the rate of change decelerates as birth order and prosociality increase. (C) 2016 Elsevier Ltd. All rights reserved.