The era of systems thinking in political science dates from the mid-1950s. David Easton gave currency to the term political system in 1953, and major statements on how to study such systems appeared a short while thereafter. Input-output analysis, general systems theory, equilibrium models all competed for attention. Even though students of foreign governments and relations among states were especially stimulated to abandon exclusively institutional and historical studies for brave new ways to comprehend political reality, there were signs even before the end of the decade that emergent fields of comparative politics and international politics were to part company in their preferred systemic formulations.

In comparative politics the structural-functional orientation popularized by Gabriel Almond prevailed as the convenient tool for investigating not just governments but anything political. In principle all political systems were subject to comparison because they all, regardless of form, share certain distinctively political functions. In practice this meant comparison of diverse systems nonetheless sharing important features qualifying them to be called states. The result was an ample but bounded universe of items for comparison.

Structural-functionalism inspired what has come to be called the comparative movement, penetrating most areas of political inquiry except, peculiarly enough, international relations. The stated object of comparing systems is theory-building. While failing to create theories of any consequence, the effort was marked by a surge of conceptual clarification, taxonomic ordering and empirical enrichment. Substantial though these gains are over previous scholarship, they succeeded mostly in fuelling rising epistemological expectations that structural-functional comparison could never fulfil.

Opinion leaders in political science turned increasingly to neopositivist epistemology prevailing in philosophy of science, which provided damning critiques of anything functional and legitimated the quest for properly hypothetico-deductive theory. Whether the rise of neopositivism and the quest for formally stated theory as the basis of ‘real’ science are benighted misadventure or passage to enlightenment cannot as yet be told. It can only be said that theory development in any form, or by any name, must be preceded by a period of conceptual and taxonomic growth. This would indeed seem to have been the pattern with all major advances in human understanding.