In the 2000s, Finnish political decision-making has been connected more closely with academic research based on the idea of evidence-based decision-making. However, little attention has been paid to the research disciplines and methodologies that the idea constitutes as relevant for public policy even though the composition of this knowledge base may greatly influence policy. We study the relationship between this knowledge base and policy-making by addressing the changes in the knowledge that has steered Finnish corporate tax policy since the early 1990s. We argue that evidence-based decision-making in the Finnish tax policy has led to an increased emphasis on the cause-effect relations derived from neoclassical economics. In particular, we show how behavioural assumptions drawn from neoclassical economic theory played an important part in the justification of the 2014 corporate tax reform. The background assumptions have hardly surfaced to the public debate, which has made it difficult to assess the validity and possible deficiencies of these assumptions. In conclusion, we call for more attention to the assumptions underlying evidence-based policy. These background assumptions need to be explicated to the general public in order to facilitate critical debate and transparency. In the specific field of corporate tax policy, research ought not to be based only on economic analysis, but also on legal knowledge of the different regulatory environments and in-depth empirical observation of the factors the influence the choices made by firms.