This study investigated the linguistic components of Piaget's class-inclusion task. First, hierarchical classification is examined from both Piagetian and linguistic theory points of view. Then, two general characteristics of child thinking that relate to the different interpretations of the responses to classification questions are discussed: (1) the tendency to overgeneralize rules, and (2) the lack of analytic aptitude. The considerations of these characteristics lead to the conclusion that children "understand wrongly" in the class-inclusion tasks because of grammatical constraints. In the third section, five research experiments designed to evaluate the grammatical constraint hypothesis are presented: (1) a methodological study to establish two slightly different forms of the class-inclusion question; (2) a study of grammatical aids to correct comparison; (3) a direct test of the child's use of grammatical clues; (4) a study to determine the independence of class-inclusion performance and the ability to compare part and whole in other ways; and (5) a study to examine performance for two types of numerical comparisons. The results of the experiments and some possible conclusions are also discussed. (Author/SDH)