Not everybody gets to do tightly controlled experiments in the lab. It’s still science, though (provided you do it right). Can you apply it to the study of history? Jared Diamond and James A. Robinson think you can.
In Snow’s case the difficulty was not that manipulative controlled experiments were impossible or illegal, but that they were immoral. In other sciences, manipulative controlled experiments are impossible, so, for example, astronomy, evolutionary biology, epidemiology and historical geology all use natural experiments. If you are studying planets, volcanoes or glaciers, you cannot manipulate them. The same goes for dinosaurs or other things that existed or happened in the past, so manipulative experiments are ruled out in such historical sciences as palaeontology, too. In social sciences such as economics, political science and sociology, manipulative, controlled experiments are ruled out on all three grounds – they are either impossible, or they are immoral or illegal. Investigators have no choice but to test hypotheses using naturally occurring experiment-like variations.
Yet there is one field where natural experiments could be used but seldom are: most historians resist them. That is odd because, after all, many of the sciences that use natural experiments are deeply historical.