Richard Ned Lebow (this volume chapter 4) has recently invoked what might be called a streetcar interpretation of systemic war and change.1
According to him, all our structural theories in world politics both overdetermine and underdetermine the explanation of the most important events – such as World War I, World War II, or the end of the Cold War. Not only do structural theories tend to ﬁxate on one cause or stream of causation, they are inherently incomplete because the inﬂuence of structural causes cannot be known without also identifying the necessary role of catalysts. As long as we ignore the precipitants that actually encourage actors to act, we cannot make accurate generalizations about the relationships between more remote causation and the outcomes that we are trying to explain. Nor can we test the accuracy of such generalizations without accompanying data on the presence or absence of catalysts. In the absence of an appropriate catalyst (or a “streetcar” that failed to arrive), wars might never have happened. Concrete information on their presence (“streetcars” that did arrive) might alter our understanding of the explanatory signiﬁcance of other variables. But since catalysts and contingencies are so diﬃcult to handle theoretically and empirically, perhaps we should be extremely cautious in attempting to test ostensibly nonlinear processes with existing data sets.