Year: 2018
Type: letter
Abstract: We thank Professor Pearl for his response1 to our papers on causal inference in epidemiology2,3 and appreciate his concern that, by failing to keeping up with ‘the dazzling speed with which epidemiology has modernized its tools’, we risk being left behind as epidemiology moves into an exclusively DAG-driven phase.1 To be clear, neither of our papers2,3 dismissed the use of directed acyclic graphs (DAGs) and formal causal representations, and we have used them in our own work.4–6 We recognize that DAGs are a powerful tool that can, for example, make the general structure of certain mechanisms generating biases clear, and thus identify when understandings of these can be rendered transportable across study settings.2,7 However, as we also pointed out,2,3 epidemiology and public health are not well served by allowing the ability to construct DAGs to constrain the content and range of substantive questions asked. We find it striking that Pearl did not address these points in his reply, particularly since much of the exchange following our initial paper focused on this issue. Pearl advances a viewpoint that there is only one approach to causal inference: the one he advocates. To Pearl, infallible science is within reach, because his methods are ‘based on a rigorous logic of causation and, therefore [are] as protected from leading to spurious causal inferences as mathematics itself’.1 He appears to have no time for triangulation, which he deems to be an ‘ostensibly unavailable methodology’,1 despite the examples we provided2,3 and many others in the literature.8,9 Similarly, inference to the best explanation (IBE) is dismissed as ‘a romantic aspiration’.1 Surprisingly, Pearl then appears to reframe his discussion by claiming that both triangulation and IBE are possible—but only through the approved application of methods he has developed.1 Being ‘unavailable’ and ‘romantic’ are, seemingly, damnations reserved for those foolish enough to attempt inference using approaches unlicensed by Pearl. Indeed, with regard to causal questions, Pearl goes so far as to claim that ‘until very recently science gave us no means even to articulate them, let alone answer them’.10 Presumably the answers we have been naïve enough to accept, which came from the pre-Pearlian period (with respect to cholera, smoking, LDL cholesterol, etc.) now need to be revisited. What about all that time you spend as an epidemiologist worrying about control of confounding in aetiological studies? Solved: ‘the task of selecting an appropriate set of covariates to control for confounding has been reduced to a simple “roadblocks” puzzle manageable by a simple algorithm’.10 Pearl considers that epidemiology ‘has been a pioneer in accepting the DAG-counterfactuals symbiosis as a ruling paradigm’.1 We remain unconvinced that this has radically transformed the reliability of causal claims in published epidemiology. We reviewed 200+ articles that had cited the most popular package that assists users in constructing DAGs, DAGitty, and found much to be desired in many of them, which sometimes reached highly implausible causal conclusions while following the principles to the letter.3 Since our review, more than 200 further papers have appeared sing this platform. We cordially invite Professor Pearl to review these more recent papers, and look forward to his conclusions regarding the plausibility of causal inference in, and general quality of, work that is apparently following his lead. Even the instigators of this useful tool have themselves expressed some reservations about the conduct of some studies in our pioneering DAG-adopting discipline.11 Pearl repeatedly affirms the infallibility of the structural framework of DAGs and counterfactuals.1,10 This stance, however, rests on the seemingly straightforward and yet, as we discussed,3 highly problematic assumption that existing subject-matter knowledge is what enables construction of reliable DAGs. The infinite regress quickly beckons, as of course that (causal) subject-matter knowledge must have come from somewhere.3 Further, Pearl ignores the fact that there are important questions that relate to the social production of health and disease, not to mention the social production of science, which cannot be usefully disciplined by a linear series of DAGs, even if such DAGs could be meaningfully constructed.2,3 Consider the case of skin colour, that hoary old allegedly obvious marker of ‘race’, used so prominently in the counterfactual disputes we described over whether ‘race’ can or cannot be treated as a ‘cause’ if it cannot be ‘modified’,2,3—with Pearl among those arguing that causality is possible for ‘race’, because ‘the essential ingredient of causation … is responsiveness, namely, the capacity of some variables to respond to variations in other variables, regardless of how those variations came about’.12 Clearly, more than logic is at issue in these debates, just as more than logic is needed to understand the fallacies, for analysing health inequities, of reducing ‘racism’ to ‘race’ to ‘skin colour’. Also at issue is the impact of racism on the social production of science, its ideas and its substantive findings, with this evidence informing the supposedly reliable ‘existing subject-matter knowledge’ that would be used for DAG construction. Underscoring this point is a recent paper in Cell with the cautionary title: ‘An unexpectedly complex architecture for skin pigmentation in Africans’.13 This study reported that genetic control of skin colour is not just by 15 genes, as previously identified by research overwhelmingly conducted (surprise, surprise) on populations of the Global North, but by at least 50 if not more loci, as revealed by the first-ever study of skin colour among the Southern African KhoeSan populations.13 The authors’ inference? – that ‘skin pigmentation is a far more complex trait than previously discussed, analogous to numerous other complex traits discussed in biomedical literature’.13 Inference to the best explanation, anyone? Seeing selection bias requires more than logic alone. We realise that our desire to retain some form of pluralism within causal inference in epidemiology may render us obsolete. However, our opinion is that epidemiology as a discipline requires familiarity with a wide range of techniques for interrogating the ways through which how the world is organised influences the health of those living on it. Certainly, DAGs and counterfactual approaches are an important element of contemporary training in epidemiology, but this should not be at the expense of recognizing the importance of biological, sociological and historical understanding, and questions should be prioritized by what is important, rather than what can be reduced to any particular form of representation. It would be unfortunate to confine questions only to those which can be reduced to algorithms, as the world of public health is much larger than this. There are many excellent presentations of the use of DAGs which recognize limitations as well as strengths.14 We feel these are more likely to help epidemiologists conduct balanced investigations of causes (at all levels) of disease, than inculcation of the belief that there are algorithms of the truth already out there, just waiting to be implemented on data that just happen to be available.
DOI: 10.1093/ije/dyy071
Authors: Nancy Krieger, George Davey Smith
Language: English
Subfield: Statistics and Probability
Field: Mathematics
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W2800362828