In the update of the UK Medical Research Council’s (MRC) guidance on the use of natural experiments to evaluate population health interventions, published in 2025 (and now under the auspices of the National Institute of Health and Care Research (NIHR) as well as the MRC),1 we kept the broad definition of natural experiments that was used in the first edition.2 Natural experiments are widely used across many disciplines (and also viewed with suspicion by many practitioners of those disciplines),3 and many definitions are available. For evaluation purposes, we defined a natural experiment as an event or process that divides a population into groups that differ in their exposure to the intervention of interest, which is not under the control of a researcher. The second part of the definition distinguishes natural experimental studies that exploit such variation in exposure from trials and other forms of…