Hierarchy theory: An overview

In both the natural and the artificial worlds, complex systems are often hierarchically organized. In other words, they tend to be structured in layers or levels. The rates of interaction within components at any hierarchical level are much faster than the rates of interaction among components. Also, higher levels tend to be larger and slower whereas lower levels tend to be smaller and faster. This fundamental property of complex systems is called near-decomposability. Hierarchy theory is a general theory that aims to simplify the description, and thus improve the comprehensibility, of complexity by taking advantage of near-decomposability. In this chapter, I provide an overview of the theory, focusing on its core concepts and tenets. These include the following topics: definitions of hierarchy, hierarchical levels, ordering of hierarchical levels, vertical and horizontal structures, neardecomposability and the empty world hypothesis, the basic triadic structure, hierarchy and scale, the observer’s role. I also discuss some common criticisms on hierarchy theory, and conclude with some comments on the nature and future of the theory.