The problems of establishing both quantitative and qualitative deficits in the behavioural profiles of various diagnostic subcategories of developmentally-disabled children are discussed in a developmental context. Examples of clinical experimental studies from the literature concerned with the syndrome of early-onset psychosis are used to illustrate the importance of methodologically controlling for maturational variables. Methods of appropriate matching of control groups are suggested which should facilitate the delineation of specific and general deficits in subgroups of developmentally-delayed children. © 1981, Sage Publications. All rights reserved.