Natural experiments: should we define them broadly or narrowly?

The recently updated UK Medical Research Council's guidance on the use of natural experiments to evaluate population health interventions, published in 2025, adopted a broad definition of natural experiments as any event or process that divides a population into groups that differ in their exposure to the intervention of interest and that is not under the control of a researcher. Many authors adopt narrower definitions that only include cases where exposure is determined in an 'as-if random' or 'plausibly exogenous' way. I explain why we used a broad definition in our guidance, why narrower definitions are hard to apply consistently and what might be gained or lost by adopting a broad or a narrow one.