We explored functional variables for aggressive behavior in 87 individuals with moderate to borderline intellectual disability who lived in a residential facility. For this purpose we used the Questions About Behavioral Function scale (QABF; Matson, J., & Vollmer, T. (1995). Questions About Behavioral Function (QABF). Baton Rouge, LA: Scientific Publications). Results show that in most clients subscales describing social function (i.e., Attention, Escape/Avoidance, Tangible) had significantly higher mean scores than subscales describing non-social function (i.e., Self-stimulation, Physical discomfort). Except for gender, there were no significant associations between mean subscale scores and client variables, such as psychiatric disorder, age, level of intellectual disability. Female clients had higher mean scores on subscales of Attention, Self-stimulation, and Physical discomfort than male clients. Results of our study suggest that in most cases, aggressive behavior is positively/negatively reinforced by social events. Implications for functional assessment and function-based treatment of aggressive behavior in these clients are discussed.