In a 1986 JASA article, Paul Holland reported that he and Don Rubin had once made up the motto, “NO CAUSATION WITHOUT MANIPULATION.” The idea is that even in an observational study, causal inference cannot proceed unless and until the quantitative analyst identifies an intervention that hypothetically could be implemented (although Professor Holland accepts the idea that the manipulation may be not ever be carried out for physical or ethical reasons). The idea of studying the causal effect of things that we as human beings could never influence is incoherent because such things could never be the subject of a randomized experiment.
My question: do we really adhere to this principle? Take the one causal link established via observational studies that pretty much everyone (even Professor Freedman, see below) agrees on: smoking causes lung cancer. Has anyone ever bothered to imagine what manipulation to make people smoke is contemplated? Aren’t we pretty sure it wouldn’t matter how we intervened, i.e., however it happens that people smoke, those who smoke get lung cancer at a higher rate? (It might matter what they smoke, how much they smoke, perhaps even where and when, but what got them started and what keeps them at it?) If folks agree with me on this, what’s left of Professor Holland’s maxim?
Paul W. Holland, Statistics and Causal Inference, 81 J. Am. Stat. Ass’n 945, 959 (1986)
David Freedman, From Association to Causation: Some Remarks on the History of Statistics, 14 Stat. Sci. 243, 253 (1999)