This study investigated three issues regarding process interventions in group decision making: (1) [...]how process interventions affect group decision making processes and outcomes; (2) how persistent these effects are, and (3) how the timing (earlier vs. later) and type (directive vs. participative) of process intervention moderate these effects. The key finding was that process interventions had immediate effects on critical group processes, but did not significantly improve decision quality during the task in which the intervention was received. Surprisingly, post-intervention processes continued to improve and resulted in improved decisions in a subsequent task. This "lagged" effect was mediated by the degree to which groups aggregated unique information and avoided advocating for individual members' preferences. Hypotheses regarding the effects of the timing and type of intervention were not supported. However, directive interventions were more effective than participative interventions at reducing the amount group members advocated for their own preferences. Implications for improving group decision making and group process research are discussed.