(The Institute for Research & Evaluation)  This research review found that in applying meaningful standards of effectiveness—criteria that have scientific validity and practical utility for policymakers and parents—to sex education outcomes produces a very different pattern of evidence for school-based CSE than what is typically reported by other research reviews that employ lax definitions of effectiveness. Using this more-credible approach, the claims that school-based CSE has been proven effective and AE is ineffective are not supported by 120 of the strongest and most recent outcome studies of sex education worldwide, the same studies that have been relied upon by the U.S. government and UNESCO in their extensive reviews of CSE research. Three decades of research indicate that comprehensive sex education has not been an effective public health strategy in schools around the world, has shown far more evidence of failure than success, and has produced a concerning number of harmful impacts. The evidence for abstinence education effectiveness in the U.S., though limited, appears more promising—enough to justify additional research.