Background: Evidence of treatment foster care (TFC) and group care's (GC) potential to prevent delinquency and crime has been developing. Objectives: We clarified the state of comparative knowledge with a historical overview. Then we explored the hypothesis that smaller, probably better resourced group homes with smaller staff/resident ratios have greater impacts than larger homes with a meta-analytic update. Methods: Research literatures were searched to 2015. Five systematic reviews were selected that included seven independent studies that compared delinquency or crime outcomes among youths ages 10-18. A similar search augmented by author and bibliographic searches identified six additional studies with an updated meta-analysis. Discrete effects were analyzed with sample-weighted preventive fractions (PF) and 95% confidence intervals (CI). Results: Compared with GC, TFC was estimated to prevent nearly half of delinquent or criminal acts over 1-3 years (PF = 0.56, 95% CI 0.50, 0.64). Two pooled study outcomes tentatively suggested that GC in homes with less than ten youths may prevent delinquency and crime better than TFC, p = 0.08. Study designs were non-equivalent or randomized trials that were typically too small to ensure controlled comparisons. Conclusions: These synthetic findings are best thought of as preliminary hypotheses. Confident knowledge will require their testing with large, perhaps multisite, controlled trials. Such a research agenda will undoubtedly be quite expensive, but it holds the promise of knowledge dividends that could prevention much suffering among youths, their families and society.