A common inference in behavioral science is that people’s motivation to reach a politically congenial conclusion causally affects their reasoning—known as politically motivated reasoning. Often these inferences are made on the basis of data from randomized experiments that use one of two paradigmatic designs: Outcome Switching, in which identical methods are described as reaching politically congenial versus uncongenial conclusions; or Party Cues, in which identical information is described as being endorsed by politically congenial versus uncongenial sources. Here we argue that these designs often undermine causal inferences of politically motivated reasoning because treatment assignment violates the excludability assumption. Specifically, assignment to treatment alters variables alongside political motivation that affect reasoning outcomes, rendering the designs confounded. We conclude that distinguishing politically motivated reasoning from these confounds is important both for scientific understanding and for developing effective interventions; and we highlight those designs better placed to causally identify politically motivated reasoning.