Comparative government or comparative politics is a method in political science for obtaining evidence of causal effects by comparing the varying forms of government in the world, and the states they govern, although governments across different periods of history may also be the units of comparison. There are several methods at work in comparative government (method of difference as opposed to method of agreement and variable as opposed to case study approaches) but all have in common the explanation of differential changes in dependent variable by the presence of different independent variables in the systems under comparison. The nature of dependent (what is to be explained) and independent variables (what explains the pattern of the dependent variable) in the method is almost unlimited, from government form to electoral system to economic or cultural factors
It has areas of concentration that include topics such as democratization, state-society relations, identity and ethnic politics, social movements, institutional analysis, and political economy. Methodologies used in comparative politics include rational choice theory; and political cultural, political economy, and institutional approaches. Aristotle (with his comparative study of constitutions in Greek states), Jean-Jacques Rousseau, John Locke, Baron de Montesquieu, and Thomas Hobbes are some of the key early thinkers in this subdiscipline.
Another method of comparison looks at the inputs and outputs of the political system. Inputs include socialization, recruitment, interest articulation, interest aggregation, political parties and methods of communicating policy. Outputs are generically rule making, application and adjudication.