“The quality of mercy is not strain’d, It droppeth as the gentle rain from heaven”. This quotation from William Shakespeare’s The Merchant of Venice was still used in the 1950s as the motto of the Boston Juvenile Court. It tended to replace the traditional repressive function of the Law by an ideological function expressed by Love. The ideology of compassion and child’s interest, modernized and more or less dechristianized, ensured the smooth running of juvenile courts; it was combined with a certain dose of violence varying from one country to another. While comparative history has been supplemented for several years by studies on Europe and Canada, almost nothing exists on the relationship between France and the United States. Thanks to newly discovered court records, it is possible to address the history of this novel form of justice, which was defined as “neohumanist” by the French juvenile judge Jean Chazal in 1952, and which achieved worldwide success in the twentieth century. Given the magnitude of the task, the focus will be on the comparison of two dynamic and pioneering juvenile courts, Boston and Paris, during the 1940s and 1950s. The article will successively analyze legal frameworks, populations targeted and judges’ decisions, indicative of new ways of thinking and of controlling.