This is taken from a comment on BikeRadar, posted by an employee of the CTC: http://www.bikeradar.com/blogs/article/naughty-duffy-eh-20525
“* The evidence from places where helmet use has been increased significantly, notably through helmet laws (e.g. in Australia, New Zealand, parts of the USA and Canada etc) is that cycle use has declined drastically, and that safety for the remaining cyclists has not detectably improved, in some cases it appears to have got worse.
* There is very little evidence about the reason(s) for this apparently counter-intuitive lack of benefits from helmet-wearing, however there are plenty of possible explanations. For one thing, helmets offer at best only very limited protection, they are (and can only be) designed for minor knocks and falls, not impacts with moving traffic. And then there are a whole host of possible reasons why the wearing of helmets may make cyclists more likely to hit their heads in the first place, potentially negating or outweighing whatever (at best limited) benefits a helmet might provide in the event of such an impact.
* For instance, it is known that some people, including young children as well as teenagers, “risk-compensate” when using helmets, i.e. act less cautiously. Drivers may also risk-compensate – one small-scale study has found that they leave less space when overtaking a cyclist with a helmet than one without. By effectively increasing the size of the head, a helmet may also turn what would otherwise have been mere glancing blows or even complete “near misses” into very serious neck injuries or “rotational force” injuries of the kind most likely to result in brain damage. Or, by reducing the numbers of cyclists, pressure to wear helmets may also be counter-productive by reducing the “safety in numbers” for those cyclists who remain. There are other possible factors but these are the main ones.
I explained all this very patiently to him. I also sent him links to CTC’s main helmet page (http://www.ctc.org.uk/DesktopDefault.aspx?TabID=4688 and http://www.ctc.org.uk/DesktopDefault.aspx?TabID=4641) and to the Bicycle Helmet Research Foundation (www.cyclehelmets.org).”