class inclusion
the concept that a subordinate class (e.g., dogs) must always be smaller than the superordinate class in which it is contained (e.g., animals). Jean Piaget believed that understanding the concept of class inclusion represented an important developmental step. Children progress from classifications based on personal factors, perceptual features, and common function to classifications based on hierarchical relationships; for example, a monkey is a primate, a mammal, and a vertebrate animal. See concrete operation.