In studies of war it is important to observe that the processes leading to so frequent an event as conflict are not necessarily those that lead to so infrequent an event as war. Also, many models fail to recognize that a phenomenon irregularly distributed in time and space, such as war, cannot be explained on the basis of relatively invariant phenomena. Much research on periodicity in the occurrence of war has yielded little result, suggesting that the direction should now be to focus on such variables as diffusion and contagion. Structural variables, such as bipolarity, show contradictory results with some clear inter-century differences. Bipolarity, some results suggest, might have different effects on different social entities. A considerable number of studies analysing dyadic variables show a clear connection between equal capabilities among contending nations and escalation of conflict into war. Finally, research into national attributes often points to strength and geographical location as important variables. In general, the article concludes, there is room for modest optimism, as research into the question of war is no longer moving in non-cumulative circles. Systematic research is producing results and there is even a discernible tendency of convergence, in spite of a great diversity in theoretical orientations.In the early 1930s, when Quincy Wright in America and Lewis Richardson in England began their respective investigations into the causes of war, they were not only unaware of one another's work; they were, of necessity, equally unaware of the radical change that their studies would produce in the field of war and peace research. With perhaps the exception of Jean de Blochwhose Future of War (1899) sought all too successfully to predict what warfare would look like on the basis of a systematic examination of previous warsand Pitirim Sorokin (1937)-whose focus was on the relationship between long cycles in cultural patterns and fluctuations in war and revolution over several thousand years -Wright (1942) and Richardson (1941 and 1960a, b) mark the first traceable efforts to bring scientific method to bear on international conflict. While physical phenomena had been studied in an essentially scientific fashion for several centuries, and biological phenomena for nearly a century, social phenomena had remained largely the domain of theological speculation, moral imperative, and conventional folklore. But even as economics and sociology began to emerge as systematic sciences, followed in due course by the study of national political systems, international politics remained one of the most backward of disciplines. This state of affairs was, and often still is, explained in terms of the intractability of the material; not only does much of the behavior occur in secret, but the material, structural, and cultural conditions associated with the international system are allegedly too spread out in time and in space to permit direct observation. But that explanation is incomplete. Political elites, it can be as...