Background: Unwarranted exclusion of people with uncertain or impaired decision-making capacity from participation in research violates principles of justice and fairness and adversely impacts the health and welfare of these populations. Methods: We conducted a cross-sectional study of institutional review board (IRB) policies for investigators and IRB members at 94 top-funded U.S. research institutions to better understand the guidance they provide to investigators who work with populations that have a wide range in decisional capacity. We collected data from publicly available websites and used deductive and inductive methods to develop our coding framework. Results: We found that 41.5% of institutions had policies that require exclusion of people with uncertain or impaired decision-making capacity unless inclusion is scientifically justified. Only 5.3% had policies that require inclusion of these populations unless exclusion is scientifically justified. Eligibility criteria depended upon the risks of research in 54.3% of policies. Guidance on obtaining consent or assent was provided in 77.7% of policies and 44.7% provided guidance on assessing decision-making capacity. 30.9% of policies required that the IRB include a member who is knowledgeable of the needs and concerns of people with uncertain or impaired decision-making capacity when it reviews research pertaining to that population. Conclusion: Some IRB policies at U.S. research institutions may be unfairly excluding people with uncertain or impaired decision-making from research participation. Institutions should review their IRB policies to ensure that these policies protect adults with uncertain or impaired decision-making capacity from harm but also do not exclude them from research unfairly.