Much behavior development is cumulative and hierarchical in that subsequent learning is dependent on prior learning. The behavior or behavioral changes that produce subsequent important behavioral changes are referred to as basic behavioral repertoires or behavioral cusps. This progression of learning is called "cumulative-hierarchical learning," and it may be an important concept for understanding much complex human behavior. Despite its potential importance, there has been little systematic study of the concept within behavior analysis or psychology in general, which limits our understanding of complex human behavior. One reason for the lack of research may be the difficulty in studying cumulative-hierarchical learning and identifying behavioral cusps. Methods to study cumulative-hierarchical learning are described.Complex forms are often built by a much simpler (often a very simple) system of generating factors. Parts are connected in intricate ways through growth, and alteration of one may resound through the entire organism and change it in a variety of unsuspected ways. (Gould, 1980, p. 42)