When a case before a domestic court gives rise to EU law questions, this court may (and sometimes must) ask the European Court of Justice to rule on the correct answer. The number of these preliminary references varies considerably between Member States. We set out to design a structural model that allows us to explain these variations. We base the model on the preliminary reference system which allows us to identify the structural and behavioural factors that form a pre-condition for a preliminary reference. Since observable data defining these factors does not exist, we further transform them into sub-components, thereby enabling us to identify proxies such as the size of the general government expenditure on law courts and the duration of EU membership. We perform statistical analyses of the associations between these proxies and the number of preliminary references. On this basis, we find that structural differences may explain about 85 per cent of the variation in preliminary references between the Member States, whereas at most the remaining15 per cent can be attributed to differences in judges’ behaviour.