Sometime in the third millennium BC, if not before, some entrepreneurial warrior donned a helmet to protect his brain from blows to the head. He may have been mocked as a coward, but soon enough copper and bronze helmets became a common sight on ancient battlefields. As Homer reminds us in the Iliad, these protective contraptions were not always effective:

The first to hurl, Great Ajax hit the ridge of the helmet's horsehair crest — the bronze point stuck in Acamas' forehead pounding through the skull and the dark came swirling down to shroud his eyes.

Nonetheless, this innovation in protective armour proved to have real staying power. Knights, for example, became very attached to their iron helmets in medieval times – even if they lost some of their manly imagery by wearing padded versions – and war helmets remained popular well into the early modern era.

Admittedly, helmets did drop out of fashion between the 17th and 19th centuries, but steel helmets were reintroduced during the Great War to protect soldiers from shrapnel and synthetic helmets are now widely used by modern day troops.

Warriors may have worn the first – and, if the Iliad is anything to go by, the most elaborate – helmets, but they were certainly not alone in their fondness for skull protection.

By the beginning of the 20th century a number of helmets had been devised to protect motorbike riders from injury and it is now possible to purchase protective head gear for a wide array of different occupational and recreational activities such as skateboarding, snowboarding, and skydiving.

It is somewhat difficult to calculate what the "helmet industry" is worth, but if we bear in mind that 1.2 million ski helmets were sold in 2007-2008 it is reasonable to conclude that total annual revenues may be counted in the billions of dollars.

Why does any of this matter from the ethicists' perspective? Frankly, if the evidence demonstrated that all of these different types of helmets were highly effective and if no one was forced to wear them there would be little to argue about. But this is not the case and so the ethicists' interest is piqued.

Currently, the most ferocious debate about the effectiveness of helmets – and the legitimacy of forcing competent adults to wear them – centres on cycling. It is not entirely clear why, but it may have something to do with the growing popularity of cycling as a sport combined with the visibility and ubiquity of "commuter cyclists" in our everyday lives. Whatever the reason, the debate about the relative effectiveness of cycle helmets is fierce and the debate about their mandatory use is even more so.

No one denies cycle helmets can protect cyclists from skull and brain injuries in some accidents. Instead, the debate focuses on how effective helmets are. Some researchers suggest that helmets reduce the risks of head and brain injury by as much as 63-88%. Others are less optimistic, claiming that the real figures are closer to 58-60%. Curnow has even argued that there is insufficient evidence to conclude that helmets provide any significant protection against serious injury to the brain.

In summary, the majority of researchers think that bike helmets provide some protection, but there is little consensus as to how effective they are.

This really matters because governments are increasingly showing a penchant for creating legislation that would force adults to wear helmets on pain of legal penalty. Australia took the initiative in the early 1990s when cycle helmets became compulsory in every state. Similar legislation now exists in a number of provinces in Canada and in a number of states in the United States, while Slovenia, Sweden, and South Korea have enacted laws requiring children to wear protective headgear. The Northern Ireland assembly also approved a cycle helmet bill back in January 2011 though it seems to have lost some legislative steam since then.

The ethical problems associated with legislation prohibiting adults from cycling without helmets are relatively obvious. First, John Stuart Mill's "Harm Principle" suggests that we should not interfere with competent adults who wish take risks with their own health. Second, even if we do not always agree with the letter of Mill's "law" we still have sound liberal reasons to avoid paternalism unless the risks we wish to prohibit are significant and unless there is a highly effective way of reducing them with little infringement of liberty.

Of course, some will argue that cycle helmet legislation conforms to these latter requirements. However, it is not clear that helmets provide sufficient protection to warrant the claim that they are highly effective and, as a keen cyclist, I would argue that the right to cycle bare-headed is by no means trivial.

I concede that cycling "sans helmet" will lead to higher costs to society in some situations. This is because a number of non-helmeted cyclists will require medical treatment following cycle accidents which they would not need if they always donned protective helmets. However, the total costs involved here are dwarfed by the costs generated by those who smoke, drink excessive amounts of alcohol, eat unhealthily and fail to exercise regularly.

As such, it seems mighty odd to create legislation prohibiting people who are engaged in a healthy activity from taking a relatively small risk of creating a relatively small cost while allowing other people to engage in highly risky activities that will generate enormous social costs. Indeed, the whole thing smacks of discrimination against the cycling minority.

So where does all this leave us? Some will read the above and continue to advocate mandatory cycle helmet legislation. It is hard to know what to say to such people other than to ask them whether they would also agree to defend a compulsory pedestrian helmet law. This might seem like nothing more than a bad April Fools' joke. However, thousands of pedestrians are injured and killed each year and many of those who suffer the worst injuries do so because of head trauma. As such, pedestrian helmets could make a real difference to people's health and significantly reduce healthcare costs to boot.

Perhaps a very committed helmetologist will claim that a pedestrian helmet law is justifiable. Indeed, the logic of the helmetology argument seems to commit advocates of mandatory cycle helmet legislation to exactly this conclusion. But I imagine that most readers would join me in resisting those who would want to give us all a bad hair day, every day.



• Carwyn Hooper is a lecturer in medical ethics and law at St George's, University of London. This article was first published on The Conversation website