Previous systematic reviews have focused on youth gang membership and interventions (Hodgkinson et al. 2009;Klein and Maxson 2006). The Campbell Collaboration, for example, has published two systematic reviews on the involvement of young people in gangs (Fisher et al. 2008a, b), and more recently one on predictors of youth gang membership in low-and middleincome countries (Higginson et al. 2014). These reviews did not consider the factors relating to membership in other types of groups involved in crime, namely OCGs.'Organized criminal' group shall mean a structured group of three or more persons, existing for a period of time and acting in concert with the aim of committing one or more serious crimes or offences established in accordance with this Convention, in order to obtain, directly or indirectly, a financial or other material benefit.