The assessment of maintaining variables with the goal of developing prosocial intervention plans has become a driving force in the intellectual disabilities (ID) literature. One particularly crucial variable is whether the behavior is low or high rate and whether the challenging behavior is best characterized by self-injury or aggression. Because low rate behaviors present such a challenge to experimental functional analysis, checklist data in the form of the Questions About Behavior Function (QABF) may therefore be particularly useful. The reliability, frequency, and related characteristics of aggression and self-injury of 95 adults with ID were studied. High rate behaviors were reliable, but reliability of QABF functions varied with respect to the challenging behavior assessed. Individual items had higher interrater reliability for aggression compared to self-injury. (Contains 5 tables.)