Although informed consent is critical for all research, there is increased ethical responsibility as individuals with intellectual or developmental disabilities (IDD) become the focus of more clinical trials. This study examined decisional capacity for informed consent to clinical trials in individuals with fragile X syndrome (FXS). Participants were 152 adolescents and adults (80 males, 72 females) with FXS who completed a measure of decisional capacity and a comprehensive battery of neurocognitive and psychiatric measures. Females outperformed males on all aspects of decisional capacity. The ability to understand aspects of the clinical trial had the strongest association with the ability to appreciate and reason about the decision. Scaffolding improved understanding, suggesting researchers can take steps to improve decisional capacity and the informed consent process.