Utilizing multiple methods, confidence was increased that committing a robbery in a group had significant effects on the process of decisionmaking by real robbers who committed real crimes. Persuasion and group obligation was shown to contribute to the decision to offend. The study also found that two variables that might lead to confidence in criminal situations: planning and sense of control were greater in groups than among lone offenders. The analysis suggests that interaction with others contributed to crime groups that were highly persuasive, and maintained a perception of control, more so than solo actors. Robbery is frequently committed in groups, yet previous literature gives only passing attention to the role of co-offending. The effects of co-offenders on criminal decisions and the form crime takes when committed by groups were largely unknown. This study examined the extent to which decisionmaking and action were modified depending on whether or not offenders who committed robbery worked alone or with co-offenders. Tables, notes, and references