Sexual offender treatment for reducing recidivism among convicted sex offenders: a systematic review and meta‐analysis
Authors: Schmucker Martin , Lösel Friedrich 
Published: July 2017

Sexual offender treatment programs to reduce reoffending have been implemented in many countries as part of a strategy in managing this offender group. However, there are still controversies regarding their effectiveness.This review integrates findings from six experimental and 21 quasi‐experimental studies that compare groups of treated sexual offenders with equivalent control groups. These studies tested whether treated sexual offenders differed from the control groups in sexual and other reoffending.Included studies compare official recidivism rates of treated sexual offenders with a comparable group of sexual offenders that have not been subjected to the respective treatment. Quasi‐experimental studies were included only if they applied sound matching procedures, where the incidental assignment would not introduce bias, or where they were statistically controlled for potential biases. The treatment had to explicitly aim at reducing recidivism rates.The review summarizes 27 studies containing 29 eligible comparisons of a treated group and a control group, containing data for 4,939 treated and 5,448 untreated sexual offenders. The studies come from seven different countries, but more than half of the studies have been carried out in North America. All eligible comparisons evaluated psychosocial treatment (mainly cognitive behavioral programs). No studies on pharmacological/hormonal treatment were found which meet the inclusion criteria.On average, there is a significant reduction in recidivism rates in the treated groups. The odds to sexually reoffend were 1.41 lower for treated compared to control groups. This equals a sexual recidivism rate of 10.1 percent for treated offenders compared to 13.7 percent without treatment. The mean rates for general recidivism were higher, but showed a similar reduction of roughly a quarter due to treatment.The results from the individual studies were very heterogeneous, that is individual study features had a strong impact on the outcomes. Methodological quality did not significantly influence effect sizes.Cognitive‐behavioral as well as studies with small samples, medium to high risk offenders, more individualized treatment, and good descriptive validity revealed better effects. There was no significant difference between various settings. We found significant effects for treatment in the community and in forensic hospitals, but there is not yet sufficient evidence to draw conclusions regarding the effectiveness of sex offender treatment in prisons.Plain language summaryTreatment of sexual offenders reduces reoffending, but more research needed to identify effective interventionsTreatment can reduce reoffending (recidivism) rates of sexual offenders. But the results of individual studies are too heterogeneous to draw a conclusion on the general effectiveness of sex offender treatment.What is this review about?Sexual offender treatment programs to reduce reoffending have been implemented in many countries as part of a strategy in managing this offender group. However, there are still controversies regarding their effectiveness.This review integrates findings from six experimental and 21 quasi‐experimental studies that compare groups of treated sexual offenders with equivalent control groups. These studies tested whether treated sexual offenders differed from the control groups in sexual and other reoffending.What are the main findings of this review?What studies are included?Included studies compare official recidivism rates of treated sexual offenders with a comparable group of sexual offenders that have not been subjected to the respective treatment. Quasi‐experimental studies were included only if they applied sound matching procedures, where the incidental assignment would not introduce bias, or where they were statistically controlled for potential biases. The treatment had to explicitly aim at reducing recidivism rates.The review summarizes 27 studies containing 29 eligible comparisons of a treated group and a control group, containing data for 4,939 treated and 5,448 untreated sexual offenders. The studies come from seven different countries, but more than half of the studies have been carried out in North America. All eligible comparisons evaluated psychosocial treatment (mainly cognitive behavioral programs). No studies on pharmacological/hormonal treatment were found which meet the inclusion criteria.Does treatment of sexual offenders reduce recidivism?On average, there is a significant reduction in recidivism rates in the treated groups. The odds to sexually reoffend were 1.41 lower for treated compared to control groups. This equals a sexual recidivism rate of 10.1 percent for treated offenders compared to 13.7 percent without treatment. The mean rates for general recidivism were higher, but showed a similar reduction of roughly a quarter due to treatment.The results from the individual studies were very heterogeneous, that is individual study features had a strong impact on the outcomes. Methodological quality did not significantly influence effect sizes.Cognitive‐behavioral as well as studies with small samples, medium to high risk offenders, more individualized treatment, and good descriptive validity revealed better effects. There was no significant difference between various settings. We found significant effects for treatment in the community and in forensic hospitals, but there is not yet sufficient evidence to draw conclusions regarding the effectiveness of sex offender treatment in prisons.What do the findings of this review mean?Overall, the findings are promising, but there is too much heterogeneity between the results of individual studies to draw a generally positive conclusion about the effectiveness of sex offender treatment. Applied cognitive‐behavioral foundation of treatment has relatively good potential, but other features, like the risk of the treated offenders or including individualized treatment, significantly affect treatment success.More well documented randomized trials and high‐quality quasi‐experiments are needed, particularly outside of North America. In addition, there is a clear need of more differentiated process and outcome evaluations.How up‐to‐date is this review?The study pool of the present analysis was based on the broad search of 2,039 documents from a review published in 2005, updated to cover studies issued prior to 2010. More recent studies were evaluated in an appendix and mostly showed similar findings as in our review. This Campbell Systematic Review was published in July 2017.Executive summary/AbstractBackgroundSexual offender treatment programs to reduce reoffending have been implemented in many countries as part of a strategy in managing this offender group. However, there are still controversies regarding their effectiveness.ObjectivesA meta‐analysis of relatively well‐controlled outcome evaluations assessing the effects of treatment for male sexual offenders to reduce recidivism is conducted. The aim is to provide robust estimates of overall and differential treatment effects.Search methodsWe searched a broad range of literature databases, scanned previous reviews and primary studies on the topic, hand‐searched 16 relevant journals, carried out an internet search of pertinent institutions, and personally contacted experts in the field of sex offender treatment. In total, we identified more than 3,000 documents that were scanned for eligibility.Selection criteriaStudies had to address male sexual offenders and contain an outcome evaluation with a treated group (TG) and an equivalent control group (CG). Apart from randomized controlled trials (RCTs), also quasi‐experimental designs were eligible if they applied sound matching procedures, statistically controlled for potential biases or the incidental assignment would not introduce bias. The studies had to evaluate therapeutic measures aiming at reducing recidivism. Both, psychosocial and organic treatment approaches were eligible. Case reports were not eligible and sample size had to be at least n =10. To be eligible, studies had to report official recidivism data as an outcome and provide sufficient information for effect size calculation. There were no restrictions with regard to country of origin or language and both published and unpublished documents were eligible.Data collection and analysisFor each study/comparison we coded general features, characteristics of the sample, treatment variables and methodological features. As most studies reported their results in terms of recidivism rates, we chose the odds ratio (OR) as effect size measure. If results on treatment dropouts were provided, we merged them with the treatment group results (“intent to treat” analysis). All statistical analysis of effect sizes applied a random effects model.Results29 comparisons drawn from 27 studies met our inclusion criteria. This study pool comprised 4,939 treated and 5,448 untreated offenders. A quarter of the studies were retrieved from unpublished sources. Most studies appeared since 2000 and more than half came from North America. The evaluations mostly addressed cognitive‐behavioral sex offender treatment. No study on hormonal treatment met the inclusion criteria. Only about one fifth of the comparisons were RCTs and matching designs were rare as well. The follow‐up periods ranged from 1 to 19.5 years (M = 5.9 years). Most frequently recidivism was defined as a new conviction and with only one exception studies presented data on sexual reoffending.Overall, there was a positive, statistically significant effect of treatment on sexual reoffending (OR = 1.41, 95% CI: 1.11 to 1.78, p < .01). The mean effect equates to 26.3% less recidivism after treatment (sexual recidivism rate of 10.1% in treated sex offenders vs. 13.7 % in the control groups). There was a comparable effect on general recidivism (26.4% less recidivism in treated groups; OR = 1.45, 95% CI: 1.15 to 1.83, p < .01). The overall effects were robust against outliers, but contained much heterogeneity.Cognitive‐behavioral programs showed a significant effect. Two RCTs on Multi‐Systemic Therapy (MST) which also contains many cognitive‐behavioral elements revealed a particularly large effect. Other intervention types showed weaker or no effects. There was a rather clear trend for better treatment effects of more individualized programs. There was no significant difference between various settings. We found significant effects for treatment in the community and in forensic hospitals, but there is not yet sufficient evidence to draw conclusions regarding the effectiveness of sex offender treatment in prisons.The overall methodological quality of the studies was not significantly related to effect size. It should be noted, though, that we could not demonstrate a significant effect on sexual reoffending for the few RCTs in our study pool. Sample size was not linearly related to effect size but small studies with fewer than 50 participants had larger effects. This may suggest publication selection bias. However, studies from unpublished sources did not reveal weaker effects compared to published studies. The strongest methodological moderator was descriptive validity. Most studies lacked a detailed documentation of offender variables so that only few analyses could target this factor. With regard to offender characteristics we found no significant treatment effect for low risk participants. In contrast, medium and higher risk groups benefitted from treatment. Although the treatment of adolescents fared somewhat better than for adults, this difference was not significant. It made no difference whether offenders entered treatment voluntarily or on a mandatory basis.Authors’ conclusionsOverall, the findings are promising, but there is too much heterogeneity between the results of individual studies to draw a generally positive conclusion about the effectiveness of sex offender treatment. However, the results reveal information that is practically relevant: For example, our review confirms that cognitive‐behavioral programs and multi‐systemic approaches are more effective than other types of psychosocial interventions. The findings also suggest various conditions of success such as more individualization instead of fully standardized group programs, an advantage of treatment in the community or therapeutic settings instead of prisons, a focus on medium to high risk offenders, early treatment of young sexual offenders, and measures to ensure quality of implementation.Overall, and particularly with regard to moderators, the research base on sex offender treatment is still not yet satisfactory. To enable more definite answers, more high‐quality research is needed, particularly outside North America. There is a clear need of more differentiated process and sound outcome evaluations on various types of interventions (including pharmacological treatment), specific characteristics of programs, implementation, settings and participants and research methods.