in which revolutions entail not only mass mobilization and regime change, but also more or less rapid and fundamental social, economic and/or cultural change, during or soon after the struggle for state power. Jack Goldstone defines a revolution as an effort to transform the political institutions and the justifications for political authority in society, accompanied by formal or informal mass mobilization and non-institutionalized actions that undermine authorities. Political and socioeconomic revolutions have been studied in many social sciences, particularly sociology, political sciences and history. Among the leading scholars in that area have been or are Crane Brinton, Charles Brockett, Farideh Farhi, John Foran, John Mason Hart, Samuel Huntington, Jack Goldstone, Jeff Goodwin, Ted Roberts Gurr, Fred Halliday, Chalmers Johnson, Tim McDaniel, Barrington Moore, Jeffery Paige, Vilfredo Pareto, Terence Ranger, Eugen Rosenstock-Huessy, Theda Skocpol, James Scott, Eric Selbin, Charles Tilly, Ellen Kay Trimberger, Carlos Vistas, John Walton, Timothy Wickham-Crowley, and Eric Wolf. Scholars of revolutions, like Jack Goldstone, differentiate four current 'generations' of scholarly research dealing with revolutions. The scholars of the first generation such as Gustave Le Bon, Charles A. Ellwood, or Pitirim Sorokin, were mainly descriptive in their approach, and their explanations of the phenomena of revolutions was usually related to social psychology, such as Le Bon's crowd psychology theory. Second generation theorists sought to develop detailed theories of why and when revolutions arise, grounded in more complex social behavior theories. They can be divided into three major approaches: psychological, sociological and political. The works of Ted Robert Gurr, Ivo K. Feierbrand, Rosalind L. Feierbrand, James A. Geschwender, David C. Schwartz, and Denton E. Morrison fall into the first category. They followed theories of cognitive psychology and frustration-aggression theory and saw the cause of revolution in the state of mind of the masses, and while they varied in their approach as to what exactly caused the people to revolt (e.g., modernization, recession, or discrimination), they agreed that the primary cause for revolution was the widespread frustration with socio-political situation. The second group, composed of academics such as Chalmers Johnson, Neil Smelser, Bob Jessop, Mark Hart, Edward A. Tiryakian, and Mark Hagopian, followed in the footsteps of Talcott Parsons and the structural-functionalist theory in sociology; they saw society as a system in equilibrium between various resources, demands and subsystems (political, cultural, etc.). As in the psychological school, they differed in their definitions of what causes disequilibrium, but agreed that it