Objective:The goal was to study the utility of nonverbal facial expressions as a research tool for assessing pain in persons with intellectual disabilities. Biases and stereotypes related to age, gender, physical attractiveness, and intellectual disability that may influence the ability of observers to evaluate pain reactions were also examined.Design:Facial reactions to an intramuscular injection of 40 adults (mean age = 49.6 years) with an intellectual disability were videotaped and objectively examined using the Facial Action Coding System. Self-reported pain ratings were obtained using a Colored Visual Analogue Scale for pain. Pain reactions were also rated by untrained observers.Results:A significant proportion of participants (35%) was unable to provide valid self-report. The intensity of objectively coded facial activity as well as observer-rated pain intensity showed significant increases from baseline to injection segments. Observers' pain ratings were primarily determined by the intensity of facial activity and were not significantly affected by stereotypes based on perceived level of intellectual disability, gender, age, or physical attractiveness.Conclusions:The findings support the validity of both objectively coded and observer-rated facial expressions of pain as research tools in treatment outcome studies involving persons with intellectual disabilities. Self-report has substantial limitations for the assessment of pain in this population.