The Model of Hierarchical Complexity (MHC) is a developmental model of behavioral complexity. It organizes behaviors into orders of complexity ranging from 0-16. The lowest order, order 1, corresponds to automatic responses to a single stimulus, such as taxes in bacteria. Each order above this is composed of two or more lower order behaviors organized into a new structure. To learn more about the Model of Hierarchical Complexity, please use the MHC workshop (presented in English and Chinese). There is also a Wikipedia page, and an introductory paper describing an older version of the model.