Introduction to the model of hierarchical complexity.

This special issue presents a collection of papers based on the Model of Hierarchical Complexity, a non-mentalistic model of developmental stages. The model is different from previous proposals about developmental stage (e.g., Inhelder & Piaget, 1958). Instead of explaining behavior change across age as being due to the development of mental structures or schema, this model instead posits that task sequences form hierarchies that become increasingly complex. Because less complex tasks must be completed and practiced before more complex tasks can be acquired, this accounts for the developmental changes seen. Furthermore, previous theories of stage have confounded the stimulus and response in assessing stage by simply scoring responses and ignoring the task or stimulus. The Model of Hierarchical Complexity separates the task or stimulus from the performance. This short introduction to the special issue will describe this model in some detail, as each of the papers to follow will rely on it. Tasks One major basis for this developmental theory is task analysis. The study of ideal tasks, including their instantiation in the real world, has been the basis of the branch of stimulus control called Psychophysics. Tasks are defined as sequences of contingencies, each presenting stimuli and each requiring a behavior or a sequence of behaviors that must occur in some non-arbitrary fashion. In the present use of task analysis, the complexity of behaviors necessary to complete a task can be specified using the complexity definitions described below. One examines behavior with respect to the analytically known complexity of the task.