In November 2011, a senior American pediatrician suggested that there was enough evidence to warrant restricting acetaminophen (paracetamol) use among children at risk of asthma, despite inadequate evidence for a causal inference. His argument was based on an ethical principle. However neither his argument nor the evidence he surveys are sufficient to warrant the recommendation, which therefore has the status, not of a sensible precaution, but a stab in the dark. I have written to the editors of Pediatrics to explain why – the link is here:
The theoretical point underlying this is one under-emphasized in both philosophical and epidemiological thinking, namely, that causal inference is something rather different from making a prediction based on the causal knowledge so obtained. The temptation to suppose that we have even a hunch what we happen when we restrict acetaminophen use on the basis that we have a hunch that it causes asthma is fallacious. It all depends on what consequences the non-use of acetaminophen has, and that in turn depends on the form that non-use takes. The point is familiar from philosophical studies of counterfactuals, but those studies arguably either do not offer much of practical use for epidemiology or else have not received an epidemiological audience. (I favour the former option, although I realise many philosophers will disagree.)
The result is a common fallacy of reasoning which we might call The Causal Fallacy: epidemiologists, policy makers, and probably the public assume that because we have causal knowledge, we have knowledge of what will happen when we manipulate those causes. In practice we do not. (This under-appreciated point has been emphasized by Sander Greenland among epidemiologists and Nancy Cartwright among philosophers, and as I see it tells heavily against the programme of manipulationist or interventionist theories of causation.) Establishing whether an exposure such as acetaminophen is a cause of an outcome such as asthma is not sufficient to predict the outcome of a given recommendation on the use of acetaminophen, for the simple reason that more than one such policy is possible, and each may in principle have a different outcome.