The notion that malign invisible forces may cause disease exerts a powerful grip on the popular imagination in modern society. Such beliefs have recently been manifested in our practice area in the court case involving charges of abuse of a child accused of witchcraft and in a campaign against the siting of a mobile phone mast near a local school. Although fears of the adverse effects of electromagnetic fields are currently in the ascendant, I still see one representative of an older tradition: a patient who tapes little magnets at various sites around her body in the belief that this relieves her rheumatic symptoms. However, the old-fashioned faith in the healing power of magnetic and electrical forces has given way to a distinctively modern preoccupation that these forces, now appropriated by science and technology, and manipulated by sinister capitalist corporations, are damaging to health, particularly to the health of children.
Given the prevailing cultural climate, the recent British Medical Journal headline, ‘Power lines may be linked to childhood leukaemia’ guaranteed that the accompanying research findings would have a major public impact.1 In fact, as an editorial commentary made clear, there was little evidence that the increased risk of living closer to power lines was due to magnetic fields.2 Furthermore, the increased risk may reflect some other factor that varies geographically (such as the familiar fact that homes tend to be close to power lines in relatively deprived and highly industrial areas). Even if the effect were causal, it could only account for ‘a tiny proportion’ of cases of leukaemia (estimated as 5 out of 400–420 a year). But, like their colleagues in the wider media, the BMJ subeditors don't like to let the facts spoil a good headline.
The latest (of many) studies attempting to link electromagnetic fields from different sources (power cables, microwave ovens and mobile phones) to a range of human diseases is a striking illustration of one of the great fallacies of contemporary epidemiology – the ‘fallacy of van Helmont's tree’, as described by the late Irish psychiatrist Maurice O'Connor Drury.3 The Belgian scientist Jean Baptiste van Helmont (1577–1644), a pioneer of quantitative methods in the natural sciences, conducted a famous experiment with his newly developed and highly accurate weighing scales. After weighing precisely a quantity of earth, an earthenware pot and an ash sapling, he planted the tree and carefully covered the soil. Every day he watered the plant, but allowed nothing else to enter the pot. Some months later, when the plant had grown 100–fold, he carefully removed the plant from the pot and separated it from the soil. On weighing again, he found that, while the weight of the pot and the soil remained the same, that of the plant had increased dramatically. His conclusion, which he considered irrefutable on the basis of his figures (although he agreed that it seemed paradoxical), was that all parts of the plant — the leaves, the branches and the stem, — were composed of nothing but water.
With the retrospective wisdom conferred by subsequent scientific discoveries — of photosynthesis and nitrogen fixation — the source of van Helmont's error is clear, and so also is the moral of the story. When little is understood about a biological system, precise methods of studying it may yield misleading results. Although O'Connor Drury was writing specifically on the limitations of randomised controlled trials in psychiatry, his comments also apply to other ‘imperfectly understood’ conditions, such as cancer.
Given the failure of numerous studies of ‘extremely low frequency magnetic fields’ to demonstrate a causal link with malignant disease — and in the absence of any plausible biological mechanism through which they might cause such disease — further research into this area would be a scandalous waste of resources.4 It seems no more likely to yield useful results than the quest for evil spirits among the children of newly arrived immigrants from Africa.
- © British Journal of General Practice, 2005.
