A randomized controlled trial was used to evaluate the effects of a prosocial behavior after-school program called Mate-Tricks for 9- and 10-year-old children and their parents living in an area of significant socioeconomic disadvantage. The children were randomly assigned to an intervention (n = 220) or a control group (n = 198). Children were compared on measures of prosocial behavior, antisocial behavior, and related outcome measures. The trial found adverse effects on four outcomes among the intervention group compared to the control group: antisocial behavior increased on two different measures (d = 0.20) and (d = 0.18), child-reported liberal parenting increased (d = 0.16), and child reported authoritarian parenting also increased (d = 0.20). In addition, parental participation was significantly associated with several program outcomes. It was concluded, that group based after-school behavior programs may have the potential to cause iatrogenic effects and must be designed, piloted, evaluated and implemented with a high degree of care.