To characterize comprehension of conditional social-legal rules and broader social reasoning in adults with and without traumatic brain injury (TBI). Participants were 20 adults with moderate-to-severe TBI (11 women) and 21 adults without TBI (13 women), ages 24 to 64 years. Participants completed the Wason task, a test of logical reasoning of conditional rules comprising precautionary rules, social-exchange rules, and legal social-exchange rules. Dependent variables were response accuracy and response time. Across rule categories, TBI group participants were significantly less accurate, F(1, 39) = 9.03, p < .01, semipartial R2 = 0.18, and slower, F(1, 39) = 7.32, p < .01, semipartial R2 = 0.16, than comparison peers. Rule category had no effect on accuracy, but for both groups legal social-exchange rules were associated with longer response times, F(1, 78) = 9.82, p < .01, semipartial R2 = 0.11. Processing speed test scores correlated with accuracy, F(1, 37) = 4.62, p < .05, semipartial R2 = 0.11, and response times, F(1, 78) = 4.45, p < .05, semipartial R2 = 0.14, in both groups. Adults with TBI underperformed their uninjured peers in both accuracy and speed of comprehending on precautionary rules, social-exchange rules, and legal social-exchange rules. These differences were attributable in part to differences in processing speed both within and between groups. Results highlight the potential costs of TBI-related cognitive problems in social-legal contexts, suggest the need to better accommodate individuals with TBI who are already involved in legal systems, and identify cognitive mechanisms for future study of social-legal rule comprehension in TBI and other populations with cognitive impairments. (PsycINFO Database Record (c) 2019 APA, all rights reserved).