The Efficacy of the Growing Pro-Social (GPS) Program in Male Prison Inmates: A Randomized Controlled Trial

This was a randomized controlled trial with blind assessments, carried out in nine prisons in three city areas in mainland Portugal (Lisbon, Oporto and Coimbra) and in the Madeira Island.

After the study was approved by the Head of the General Directorate of Reintegration and Prison Services of the Portuguese Ministry of Justice, a list of potential participants who did not meet the exclusion criteria was made available to the research team by psychologists from the justice system. Next, a large sample of participants was randomly selected using a random number table by a research assistant who was blind to any personal information about each inmate. Then, a first meeting between the research team and the randomized inmates occurred, in which researchers invited inmates to participate voluntarily. In this meeting, researchers explained the goals of the study and presented a brief overview of the intervention program. It was also explained to inmates that their participation in the study would not impact their sentencing in any way.

Participants who agreed to participate, gave written informed consent, completed the baseline assessment, and were randomly assigned to treatment conditions (treatment and control groups) using a random number table by a research assistant who was blind to any information about each participant. Afterwards, the research team informed the psychologists in each prison of the result of the randomization so that GPS could be initiated. Participants in the control group were informed that they would be offered the GPS treatment after the study's completion (including the follow-up period).

Besides baseline assessment, participants completed the mid-assessment (after the 20th session of the program), post-treatment assessment (at the end of GPS) and follow-up assessment (12 months after GPS completion). Staff who conducted randomization did not serve as therapists or accessors, and accessors were blind to condition assignment. Respondent-specific codes were used to link the data from one time-point to the next one.

GPS's facilitators were chosen among the psychologists who already had training and experience in delivering the program with inmates (who were not selected for this study). In order to assure program integrity and consistency, facilitators received regular supervision by the research team (including the program's main author) during the time GPS was run in prisons. Moreover, the program's structured and manualized design ensures integrity, at least partially.