Richard Wilkinson and Kate Pickett have made great play of the fact that The Spirit Level is based on what they see as a science: ‘social epidemiology’. Leaving aside the dubious nature of their conclusions, it is worth noting that since the field of epidemiology has expanded, its explanatory powers have shrunk. While the ability to examine the spread of infectious disease has been invaluable, employing epidemiology beyond this has been - for the most part - bad news for our understanding of health. It's even worse as a method of understanding society.
The most famous early example of the application of epidemiological ideas is the ingenious manner in which John Snow, a London physician, managed to identify the source of a cholera outbreak in Broad Street, Soho in 1854. Simply by mapping the cases of cholera, he found they were clustered around one particular well. Whether Snow’s intervention to disable the well by removing the handle of its pump really saved the day is a matter of some dispute, but his method of plotting cases and looking for concrete causes was inspirational.
Fast forward to the 1950s, where a classic study by Austin Bradford Hill and Richard Doll identified for the first time (at least, outside Germany) the link between smoking and lung cancer. Bradford Hill and Doll sent a questionnaire to doctors to measure their smoking habits, then followed up the questionnaire some years later. Those doctors who smoked were much more likely to have developed the disease and no one would seriously disagree now that active smoking is a very significant risk factor for lung cancer.
There is a proviso, however: even with smoking, the most famous of these epidemiological connections, most of those engaged in the risky behaviour - smoking - do not die from lung cancer while there are a small number of non-smokers who die from lung cancer, too. Other explanatory factors, like a genetic predisposition to developing certain cancers, a less-than-ideal diet and sheer luck seem to come together, too.
Despite the search for other similar environmental health risk factors, from passive smoking to bacon butties, epidemiology has proven to be a fairly crude tool. Austin Bradford Hill, despite the success of his groundbreaking study, advised that epidemiological methods should be applied cautiously. He listed a variety of tests that should be applied to any putative connection between a risk factor and a disease, summarised in John Brignell’s book The Epidemiologists (see my review here: http://www.spiked-online.com/index.php/site/article/2446/).
1. Strength: Is the association strong enough that we can rule out other factors?
2. Consistency: Have the results been replicated by different researchers, and under different conditions?
3. Specificity: Is the exposure associated with a very specific disease as opposed to a wide range of diseases?
4. Temporality: Did the exposure precede the disease?
5. Biological gradient: Are increasing exposures associated with increasing risk of disease?
6. Plausibility: Is there a credible scientific mechanism that can explain the association?
7. Coherence: Is the association consistent with the natural history of the disease?
8. Experimental evidence: Does a physical intervention show results consistent with the association?
9. Analogy: Is there a similar result to which we can draw a relationship?
When these tests are applied rigorously, nearly all the postulated links between risk factors and harmful effects fall apart. For example, in the case of passive smoking and lung cancer, the association is weak, it is not consistent, exposure is usually assumed rather than measured, and so on. In other words, an association between passive smoking and lung cancer is either non-existent or too weak to worth worrying about.
Let’s now turn to this new field of ‘social epidemiology’. We can see in The Spirit Level that Wilkinson and Pickett’s claims fall flat on their faces when Bradford Hill’s tests are applied. The associations are not strong, rarely rising much above ‘apparently random’; other explanations are usually both available and more plausible than the idea of inequality leading to status anxiety; other researchers, simply by minor tweaks in the data, have seen the associations disappear; and the wide selection of social problems deemed to arise from inequality is a long way from ‘specific’.
As someone who would broadly see themselves as left-wing, I find there is a more pressing problem with The Spirit Level: that it dumbs down the fight between different groups in society over the carving up of the wealth we produce from a genuine struggle for material betterment to a psychological disorder demanding intervention from on high. Seeing inequality in terms of status envy is thus actually disempowering rather than liberating. But regardless of our attitude to the political question of wealth distribution, The Spirit Level is well worth criticising as bad epidemiology, too.