A recurrent theme in helmet promotion is that "all the evidence says that helmets reduce head injuries"
This is wrong on two levels
First and foremost the research does not say that helmets prevent injury, it says that those cyclists who choose to wear helmets were, in the small groups studied, less likely to suffer head injury - in fact less liekly to suffer injury of any kind, or to have damage to thier bikes in crashes. This does not scale to the population level. In fact, the countries where head injury rates are lowest are the ones with the lowest helmet wearing rates.
Pro-helmet evidence somes from small-scale case-control studies. These all have a problem called confounding - to put it simply, they can't easily distinguish between the chooser and the choice. If the most cautious cyclists are the ones most likely to wear helmets, which seems reasonable, is this likely to affect the results? The evidence which supports helmet use is the same kind of evidence which showed that combined hormone replacement therapy reduces risk of coronary heart disease - except that it doesn't. Subsequent clinical trials found that it actually caused a small increase in risk, but the kinds of women who asked for HRT were more likely to be health conscious on other ways. For such an obvious finding this caused a quite disproportionate amount of consternation in the epidemiological community!
More importantly, none of the other types of evidence support the pro-helmet results, either in magnitude or in sign.
The existence of evidence which conflicts with the pro-helmet line
comes as a surprise to many. The fact that the pro-helmet
evidence is of a type which is notoriously weak is rarely mentioned by
helmet proponents, and the existenc eof conflicting evidence is rarely,
if ever, mentioned at all.
Most of the conflicting evidence comes from whole-population
studies. For example, America's Consumer Products Safety
Commission produced some figures which showed that cyclist head injury
rates have increased as helmet use has risen. The same
happened in Australia after a helmet law was passed. The largest
ever study, by Rodgers of CPSC, which surveyed eight million cyclist
injuries, found that there was a small but not significant increase
in risk of injury and a small and significant increase in risk
of fatality for helmeted riders.
The most widely-studied population may be that of New Zealand, where
a mandatory all-ages helmet law was passed in 1994. This resulted
in a doubling of helmet use to over 95% in a single year, and no
measurable change in cyclist head innjury rates. There was a
numerical drop in head injuries, and an equivalent numerical drop in
non-head injuries, both of whihc are consistent with the large
concurrent observed drop in cycling, but the proporiton of cyclists'
injuries which are head injuries trended no better than for pedestrians.