The observation that typical users of augmentative and alternative communication (AAC) systems vary widely in their characteristics raises issues about the utility of a nomothetic approach for investigating and evaluating important variables, as well as about the value of studying children without disabilities to contribute to the understanding of AAC systems. To provide an initial basis for examining the fruitfulness of the nomothetic approach, the effects of 2 fundamental variables, number of display levels (single vs. dual) and vocabulary abstractness (concrete vs. abstract words), on vocabulary acquisition were examined for children without disabilities and for speech impaired children with cerebral palsy (CP). Children demonstrated the same pattern of acquisition, regardless of disability status. Both groups of children made more errors on the dual-level display than on the single-level display and made more abstract errors than concrete errors. Importantly, the performance of individuals consistently conformed to group performance. These findings suggest that a nomothetic research approach that includes results of children without disabilities can usefully illuminate consequences of important variables in AAC systems. Clinical implications based on these findings were also discussed.