Hierarchical models in cognition: Do they have psychological reality?

Many theoretical models of cognition are based on the assumption that cognitive processes and representations are hierarchically structured. The issue of whether these hierarchies are mere constructs or conceptual tools, or whether they have psychological and neurological reality, is addressed. Theories across a range of cognitive domains are reviewed and four kinds of evidence are considered: behavioural, neuropsychological, ontogenetic, and logical. Logical constraints are identified in relation to operating principles of control, access, economy and analogy. The general characteristics of hierarchical structures and their advantages and disadvantages are discussed.