Aggressive behaviors in those with intellectual disability (ID) and autism (ASD) have been linked to a variety of factors including ID level, age, sex, psychiatric disorders, and medical conditions but these factors have not been studied, in large samples, in terms of how they affect the stimuli that trigger aggression. In this survey of 2243 adults, four triggers of aggression associated with frustration, discomfort, change in the physical/social environment, and defensive reactions were analyzed for their relation to ID level, ASD, age, sex, number of psychiatric diagnoses, sleeping problems, seizures, visual impairment, ear infections and gastrointestinal problems. All four triggers were associated with increasing number of psychiatric disorders, with frustration, discomfort, and change intolerance commonly linked to sleeping problems and ASD. Implications for assessment and intervention are discussed.