The emphasis on continuity has directed RM theorists to explore similarities between SMs and formal organizations. The foundations for this line of research were provided by Zald and Ash (1966), McCarthy and Zald (1973, 1977a, 1977b) and Zald and McCarthy (1987). By applying theories of formal organizations to the study of SMs they have exposed the wide variety of organizational arrangements that modern SMs have developed. They suggest that the development of democratic-participatory organizational forms, which is almost taken for granted by NSM theory, is only one possible outcome among many. The main variables that affect the organizational structures of SMs are the following: the nature of the movement and of its goals (expressive/instrumental, single/multiple issue), the type of recruitment process (individual or bloc), the role of leaders in the formative stages, and the influence of third parties.