Purpose: To promote gains in cognitive complexity (measured by integrative complexity, IC) associated with recognition of validity in others' viewpoints/values, supporting peaceful conflict reduction. Design: Assessment of effectiveness of "Living Well with Difference" (LWWD) course designed to promote critical thinking about contested social issues. LWWD was delivered to 199 secondary school students in England, comprising eight hours of sessions in curriculum time. Findings: Results using Paragraph Completion Tests showed that IC increased in the intervention condition in comparison to the smaller control condition IC gains. Resilience scores did not show significant gain, although it correlated moderately with IC gain. Research Limitations: Unequal numbers of control--intervention groups, non-random school sample, and a moderate amount of missing data are potential limitations. Practical Implications: The discussion explores the possible contribution that LWWD can make to citizenship education and 'deep' critical thinking that engages with emotions and values, complementing prevention curricula in the light of EU recommendations.