<p>Research review articles over the years have established that studies on teaching critical thinking suffer from methodological and theoretical deficiencies, making results unreliable. The methods have been criticized for not handling threats to internal validity (designs without control groups or proper randomization) and the theories have been criticized for conceptual vagueness. To address this, the study reported on here developed an experimentally robust intervention design to test a new theory for teaching critical thinking—the variation theory of learning and teaching. This theory focuses on opening up new aspects of the world for the learner through systematic variation in these aspects. In this study, an aspect of critical thinking was targeted: the aspect of alternating points of view on societal issues. 92 Swedish 9th grade students and 5 social studies teachers from a school in the commuting area of Gothenburg were randomly assigned to treatment, i.e. variation-theory-based teaching, and control group, i.e. “business-as-usual” teaching. The results were measured through essay tasks, in pre- and post-tests, showing significant gains in alternating points of view in favour of the treatment group, both on the issues taught and novel issues, i.e. transfer. The results suggest that, when put to the test in a methodologically robust intervention, variation theory is a powerful theory for teaching critical thinking, and in addition, it offers a way to theoretically explain the teaching and learning taking place. This suggests a need for further exploration of the theory in relation to research on and teaching of critical thinking.</p>