Construct validity of the Motivation Assessment Scale (MAS) (Durand, Crimmins, The Motivation Assessment Scale 1988) was studied using Rasch analysis data from 67 children (246 MASs), with dual diagnosis of autism and intellectual disability or with intellectual disability only. Results failed to support the proposed unidimensional construct or the original 4-factor structure. Some motivators appear to form a unidimensional construct: "to gain attention", "to gain a tangible object", and "to escape". There was evidence that sensory stimulation represents a different construct. Children with intellectual disability were more apt to be motivated by desire to gain a tangible item or attention. Children with the dual diagnoses were more apt to have sensory stimulation or escape from task demand as a motivator for stereotypic and repetitive behavior.