This article aims to explain why the notion of mania puerperalis, or puerperal insanity, was not used in the Netherlands to exonerate women accused of infanticide, in contrast to other countries. It applies the concept of ‘travelling knowledge’ as an approach to the history of forensic medicine, pointing to the fields of medicine and law as contact zones inviting, promoting or barring the transmission of knowledge. Although the notion of mania puerperalis was known in the Netherlands from 1822 on, psychiatric expertise was not requested in actual court cases of infanticide for several reasons. First, physicians and legal scholars continued to doubt the existence of this form of mania. Moreover, it was not always directly connected to infanticide. Also, the specific formulation of the law strongly determined what medical evidence was needed in court cases. Not only did the Code Pénal generally emphasise material evidence, the laws on infanticide specifically mentioned fear and therefore an additional reference to the mental condition of the accused was not needed. Most importantly, the article argues that the existing vocabulary on emotion, both vernacular and medical, already allowed for an analysis of psychic components.