Recent studies suggest that the reported effect sizes of prevention and intervention trials in criminology are considerably larger when program developers are involved in a study than when trials are conducted by independent researchers. This paper examines the possibility that these differences may be due to systematic bias related to conflict of interest. A review of the evidence shows that the possibility of a substantial problem cannot be currently rejected. Based on a theoretical model about how conflict of interest may influence research findings, the paper proposes several strategies to examine empirically the extent of systematic bias related to conflict of interest. It also suggests that, in addition to improved standards for conducting and publishing future experimental studies, more research is needed on the extent of systematic bias in the existing body of literature.