More complex behaviors characterize multiple system models. The four highest stages in the MHC are not represented in Piaget's model. The higher stages of the MHC have extensively influenced the field of positive adult development. Some adults are said to develop alternatives to, and perspectives on, formal operations; they use formal operations within a "higher" system of operations. Some theorists call the more complex orders of cognitive tasks "postformal thought", but other theorists argue that these higher orders cannot exactly be labelled as postformal thought.