Abstract Scholars have long investigated connections between types of knowledge use and types of policy subsystem. Yet, most of them focus on the learning function of expert information. The legitimizing function of knowledge—when expertise serves as a substitute for decision (Boswell in J Eur Public Policy 15(4):471–488, 2008)—has attracted less attention. An empirically validated explanation of this function is still missing. This article tests existing hypotheses regarding which features of the subsystem are conducive to the legitimizing function. The demonstration rests upon a case study: France’s Ministry of Agriculture’s commissioning of INRA to carry out a systematic literature review on pain in farm animals. Two types of factors are involved in the legitimizing function of knowledge: environmental mechanisms (an adversarial policy subsystem, concentration of policy authority) and relational mechanisms (coalitions displaying epistemic uncertainty and exerting pressures on the source of policy authority, a policy broker mitigating the conflict between the two coalitions).