International Relations. A self-Study Guide to Theory

cause-and-effect-relation. This account of cause was originally influenced by 
the Humean account of the term (David Hume 1748, An Enquiry Concerning 
Human Understanding). Hume’s idea was based on the experience that par-
ticular objects temporally appear together: when A occurred, so did B. Estab-
lishing a causal relationship in the Humean account of cause means discov-
ering the temporal relationship between observed events.
In positivist scientific inquiry, the cause is the independent variable. The 
effect is the dependent variable. Variables (consisting of specific observa-
tion) stand in interrelation: the value of the dependent variable depends on a 

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change in the value of the independent variable. If this occurs, we speak of 
correlation. Correlation indicates the strength and direction of a linear rela-
tionship between two variables. Certain phenomena in international politics 
provide examples of such a correlation, for example the outbreak of a war or 
a conflict which can be observed to be linked to a particular type of political 
system of the conflicting parties. The theoretical statement about the rela-
tionship usually takes the form of an if-then-hypothesis. When proved “posi-
tive”, an explanation indicates the law-like succession of empirical phenom-
ena. Taking up the example mentioned previously, such a “law” has been 
formulated in the theory of inter-democratic peace. This law is based on the 
hypothesis that democracies do not fight each other. It has been empirically 
proved (here by statistical observation with the Correlates of War project at 
the University of Michigan) and is now usually presented as the empirically 
best-proven theory of IR. If we were to formulate this theory as a causal re-
lation in terms of “if, then”, it would read as follows: if international rela-
tions take place between democracies, then there will be no war between 
them”.
However, establishing a correlation between two variables is not a suffi-
cient condition to establish a causal relation. Causal analysis in science is 
more complex. However, for the purposes of this learning unit, the example 
of correlation should be enough to indicate the nature of the specific type of 
causal relation between independent and dependent variables found at the 
heart of positivism: there is a cause for each phenomenon. The effect of a 
phenomenon temporally follows its cause; ultimately, a single cause emerges 
for the effect observed or the explanation is reducible to a single cause. A 
positivist explanation is thus a statement about the interrelation of an (ob-
servable) cause and an (observable) effect. If the (theoretically) hypothesized 
interrelationship between cause and effect can be scientifically verified, the 
explanation counts as valid knowledge (law-like). 
There are two typical forms of law-like explanations in positivist science: 
the deductive-nomological and the inductive-statistical forms of explanation. 
The deductive-nomological model of scientific explanations developed by 
Carl Gustav Hempel and Paul Oppenheim at the end of the 1940s (Hempel 
and Oppenheim 1948: Studies in the Logic of Explanation) was an important 
stage in the development of positivism. Since the 1950s and 1960s, it has be-
come a standard model of scientific explanation. The deductive-nomological 
model is the formal structure of a scientific explanation which is derived by 
way of deduction: from the abstract (the general law as the explanans) to the