The dappled causal world for psychiatric disorders: implications for psychiatric nosology
On Thursday 16th October, Professor Kenneth Kendler delivered his second (and final) Loebel Lecture, entitled ‘The dappled causal world for psychiatric disorders: implications for psychiatric nosology’. You can view it online here or listen here.
Whilst Kendler’s first lecture – summarised by Roger Crisp here – focused on empirical issues, the second lecture was more philosophical. Kendler’s key question in the second lecture could perhaps be formulated as: Given the complex aetiology of mental disorders, how can we best understand and explain how they arise?
Kendler began by providing some historical context. Using as a case study Robert Burton’s 1621 book, The Anatomy of Melancholy, Kendler observed that before the middle of the 19th century, mental disorders were not thought to have a single, clear aetiology; on the contrary, Burton provides a mind-boggling list of possible causes for melancholy. This changed in the late 19th century, when scientific advances meant that various somatic diseases could be explained in terms of elegant, simple causal mechanisms that were based on bacteriology, Mendelian models, and vitamin deficiencies. This paved the way for a reductionist quest to discover a single causal aetiology for all diseases, including psychiatric disorders. However, with the exception of the discovery that general paresis—which resulted in psychosis and other characteristically psychiatric symptoms—is caused by syphilis, attempts to discover simple aetiologies for psychiatric disorders have been unsuccessful.
What does this lack of success tell us? Kendler concedes that it might just mean that we need to keep looking. However, he thinks it more likely that finding simple aetiologies for psychiatric disorders is an unrealistic goal. In order to attain this goal, it would need to be the case that, for each psychiatric disorder, there is a privileged level of explanation: that is, it would need to be the case that the best causal story for any given psychiatric disorder is one couched in terms of neuropathology, or molecular genetics, or environmental stressors, or some other narrowly-focused set of explanatory factors. However, invariably, we need to consider all these explanatory factors and more in order to explain psychiatric disorders. Kendler demonstrates this by identifying a set of criteria—including generalisability, specificity, and generativity—that we expect good causal explanations to satisfy. In the case of many somatic disorders, such as cystic fibrosis, all of these criteria can be met by considering factors at a single level—at the genetic level in the case of cystic fibrosis. However, for psychiatric disorders, we must draw on explanations at various different levels in order to satisfy the criteria: a phenomenon that Kendler terms ‘dappling’. That there is no privileged explanatory level for psychiatric disorders means that there can be no simple, comprehensive causal stories told about them.
What should be the implications of all this for the way in which psychiatric disorders are conceptualised, diagnosed, and treated? Kendler advocates a technique that he calls epistemic iteration, in which researchers make a series of attempts to describe a given disorder, each of which improves on previous ones. He cautions us that the epistemic iteration route is fraught with peril: we risk—and I’m simplifying here—chasing moving targets, or falling victim to a flawed conceptual model of the disorder we’re trying to understand, in which case nothing short of a ‘reboot’ of our conceptual machinery (i.e. a Kuhnian revolution) will help us progress.
To make progress in this area, Kendler argues that we must (1) adopt a pluralistic, broad-minded, and empirically rigorous approach to discovering the dappled causes of psychiatric disorders; (2) identify causal mechanisms, guidance about which can be found in the philosophy of biology literature; and (3) work on tracing aetiologic pathways back into neuropsychological terms and then ‘translating’ these third-person descriptions into the sort of first-person, mental language that captures the subjective experience of patients.
It was a pleasure to hear Professor Kendler’s lectures. The data on which he draws is itself fascinating, the lectures are accessible to those without a scientific background, and he is a highly engaging speaker. Oxford’s Loebel Lectures could not have got off to a better start.
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