The reason is straightforward: Policy is produced by human actors who are not merely driven by natural impulses or by the compulsion of external factors. In policy research, however, the questions that political scientists are expected to answer are typically backward looking, starting from an explanandum or a dependent variable at the other end of the hypothetical chain of causation. At one end of the social science spectrum are mainstream economics and those variants of political science and sociology that have become committed to the rational-actor approach. Political scientists may, and often do, opt for historically descriptive studies of individual cases that are capable of representing the full complexity of the researcher's understanding of a particular situation. The closest equivalent to experimental designs in empirical research is comparative studies using carefully matched cases selected according to the logic of the "most-similar systems" or the "most-different systems" design.