Science writer Ben Goldacre and statistician David Spiegelhalter say that issues surrounding arguments for and against cycle helmets are so complex that they appear to be in conflict with the British Medical Association’s official policy, “which confidently calls for compulsory helmet legislation.”

The pair joined forces to address what is perhaps the most contentious of cycling topics – a subject they freely admit they “both dread questions about” – and, specifically, the issue of whether studies can conclusively settle the debate either way.

Their main conclusions after outlining some of the problems associated with trying to establish the benefit or otherwise of helmets through scientific means?

“The current uncertainty about any benefit from helmet wearing or promotion is unlikely to be substantially reduced by further research,” and, “we can be certain that helmets will continue to be debated, and at length.”

Goldacre, who besides being the author of Bad Science and Bad Pharma, is Wellcome research fellow in epidemiology at the London School of Hygiene and Tropical Medicine, and Spiegelhalter, Winton Professor of the Public Understanding of Risk at the University of Cambridge, were writing in the British Medical Journal (BMJ).

At the outset, they say: “We have both spent a large part of our working lives discussing statistics and risk with the general public. We both dread questions about bicycle helmets. The arguments are often heated and personal; but they also illustrate some of the most fascinating challenges for epidemiology, risk communication, and evidence based policy.”

They identify two broad areas that science seeks to address when it comes to cycle helmets: “At a societal level, ‘what is the effect of a public health policy that requires or promotes helmets?’ and at an individual level, ’what is the effect of wearing a helmet?’ Both questions are methodologically challenging and contentious,” they add.

Goldacre and Spiegelhalter single out one recent study, led by Jessica Dennis at the University of Toronto, which held that compulsory helmet laws in various Canadian provinces had achieved only a “minimal” effect on hospital admissions for head injuries related to cycling.

The pair acknowledge that other studies have reached different conclusions, but describe the one conducted by Dennis as having “somewhat superior methodology—controlling for background trends and modelling head injuries as a proportion of all cycling injuries.”

By contrast, they say, case-control studies, which often find reduced rates of head injury among cyclists wearing helmets compared to those who do not, “are vulnerable to many methodological shortcomings” – for example, “if the controls are cyclists presenting with other injuries in the emergency department, then analyses are conditional on having an accident and therefore assume that wearing a helmet does not change the overall accident risk.”

Other variables they identify and describe as “generally unmeasured and perhaps even unmeasurable” include the fact that people who choose to wear helmets may be more risk-averse than those who do not, plus whether there is an element of “risk compensation” in play among those forced to wear helmets in places where they are required by law.

They run through some of the issues that opponents of helmet compulsion make, including that making them mandatory negates the positive health benefits, but again outline that the issue is more complicated than it appears on the face of it, citing a study that identified “two broad subpopulations of cyclist,” each of which would react differently to the introduction of compulsory helmet laws.

That study, carried out by the Institute of Transport Economics in the Norwegian capital, Oslo, described the country’s cyclists as comprising “one speed-happy group that cycle fast and have lots of cycle equipment including helmets, and one traditional kind of cyclist without much equipment, cycling slowly.”

The Norwegian study added: “With all the limitations that have to be placed on a cross sectional study such as this, the results indicate that at least part of the reason why helmet laws do not appear to be beneficial is that they disproportionately discourage the safest cyclists.”

The BMJ article says that “statistical models for the overall impact of helmet habits are therefore inevitably complex and based on speculative assumptions,” and that “this complexity seems at odds with the current official BMA policy, which confidently calls for compulsory helmet legislation.”

“Standing over all this methodological complexity is a layer of politics, culture, and psychology,” they say – whether that be anecdotal evidence of acquaintances who avoided injury through wearing a helmet, or “risks and benefits may be exaggerated or discounted depending on the emotional response to the idea of a helmet.”

They also point out that the Netherlands and Denmark, for example, have high rates of cycling but low rates of helmet wearing and cyclist casualties, which they suggest results from deployment of decent infrastructure, legislation aimed at protecting riders, and cycling itself being viewed as “a popular, routine, non-sporty, non-risky behaviour.”

Goldacre and Spiegelhalter do however see something of value in the helmet debate, but it’s not related to the actual wearing or non-wearing of one, or whether they should be made mandatory.

“The enduring popularity of helmets as a proposed major intervention for increased road safety may therefore lie not with their direct benefits – which seem too modest to capture compared with other strategies – but more with the cultural, psychological, and political aspects of popular debate around risk,” they say.