Full context:   In the early 1970s, Hill and Grunner reported that more than 100 theories of group development existed. Since then, other theories have emerged as well as attempts at contrasting and synthesizing them. As a result, a number of typologies of group change theories have been proposed. A typology advanced by George Smith (2001) based on the work of Mennecke and his colleagues (1992) classifies theories based on whether they perceive change to occur in a linear fashion, through cycles of activities, or through processes that combine both paths of change, or which are completely non-phasic. Other typologies are based on whether the primary forces promoting change and stability in a group are internal or external to the group. A third framework advanced by Andrew Van de Ven and Marshall Scott Poole (1995), differentiates theories based on four distinct "motors" for generating change. According to this framework, the following four types of group development models exist: Some theories allow for combinations and interactions among these four "motors". For example, Poole (see below) found in his empirical research that seemingly complex patterns of behavior in group decision-making result from the interplay of life-cycle and teleological motors.