Mobile madness

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: more likely than average to be regular users, and therefore were not representative of the population at large.

Moreover, the definition of ‘regular mobile-phone use’ was itself questionable. Anyone who had used a phone just once a week for at least six months qualified. That is a pretty low rate of usage. If phones really do cause cancer, but only at high exposure, employing such a generous definition of regular use means that the effect might be diluted into undetectability.

Another potentially serious flaw is that participants asked in 2001-02 about their mobile use a decade earlier will have been using analogue, not digital, handsets. That would lead to a different pattern of exposure and therefore of potential risk.

How the shift from analogue to digital phones might have changed things is anybody’s guess. But both the selection bias and regular-phone-use definition might be expected to mask any adverse effect of phones. The most serious flaw in the study, however, is generally reckoned to be recall bias, which would tend to work in the opposite direction, by suggesting problems with phones that do not, in fact, exist.

Recall bias happened because the study was retrospective rather than prospective. In other words it looked at what people had done in the past rather than following their behaviour into the future. In practice, that meant asking them about past behaviour, and relying on the accuracy of their memories.

Even a healthy person would probably have difficulty recalling exactly how often he used his mobile phone a decade ago, and which ear he routinely held it to. Someone subsequently diagnosed with a brain tumour might easily be biased, consciously or unconsciously, to exaggerate the former and misstate the latter. And that would be enough to account for the fact that several of the single-country studies found that 10 years’ mobile use was associated with an increased risk of brain tumours on the side of the head that people said was exposed to their handsets. It would also—more significantly—account for the studies’ weird suggestion that the same amount of use stops tumours developing on the other side of the head.