Following the US example, European scholarship has seen more and more interdisciplinary or multidisciplinary academic work being carried out over the last few decades, not only in criminal law but also relating to private law and civil procedure. In such studies ‘extralegal’ knowledge from, for example, psychology, sociology and economics, is combined with existing legal insights and transformed into ‘novel’ legal knowledge. This has often led to new thoughts on how to organize our legal landscape and to new public policy issues and solutions. An intriguing question underlying these studies is whether it is in fact possible – and if so, how, why and when – to leap from such ‘extralegal’ insights to normative legal conclusions. How and when can any researcher step over from, for example, empirical psychological facts to legal normative value judgments (as one is required to do from a legal end, for instance as a judge ruling on a case)? What, if anything, allows anyone to do so? What are the conditions under which it would be safe to say that one could cross over from one side to the other? By reviewing the existing methodological literature on this topic and by linking up with ideas about the (analogous) use of comparative law materials, this paper – methodological in nature – tries to come up with a workable ‘method’ for crossing the border between social science disciplines and the law. As it turns out, a due process approach is the best available option. This approach asks of judges, practitioners and scholars to become familiar with the methodology of the social sciences. That hurdle might be overcome by using court-appointed experts to evaluate the usefulness of the extralegal materials. The judge would thus resort to an expert to advise him on how to be a decent gatekeeper when it comes to the possible use of insights from social sciences.