According to the Bicycle Helmet Research Foundation, “This claim originates from a single source – Thompson, Rivara and Thompson, 1989 – and has never even been approached by real-world evidence anywhere. The research on which the prediction was made has been widely criticised for fundamental methodological errors. The researchers themselves revised their prediction downwards to 69% for head injuries in 1996 (Thompson, Rivara and Thompson, 1996), but this too remains greatly in excess of real-world experience. In places where helmet use has become significant, there has been no detectable reduction in head injuries relative to cycle use.” http://cyclehelmets.org/1027.html

There is lots of information at this website, and links to lots more. Their bottom line:

If wearing a helmet is the difference between you having the confidence to cycle (or to cycle more) or not, you should wear one! The health benefits of cycling outweigh greatly any negative consequences of helmet use.

On the other hand, if wearing a helmet makes it likely that you will cycle less, then the balance of advantage is cycling without a helmet.

If helmet wearing is unlikely to affect the amount you cycle, you may like to consider the following. Interpretation of the data can be controversial, but examination of the wider evidence from places where helmet use has become significant suggests that the following are reasonable conclusions:

If worn correctly, a cycle helmet may afford some protection against minor, largely superficial, injuries to the head.
A helmet is unlikely to offer protection against more serious or life-threatening injuries.
You are more likely to hit your head in a crash if you wear a helmet.
There is evidence that helmeted riders may crash more often (Erke and Elvik, 2007 has found a 14% increase in risk in Australia and New Zealand). This could be particularly the case if wearing a helmet makes you feel better protected.
A helmet may increase the very small risk of the most serious brain injuries that lead to death and chronic intellectual disability.
The likelihood of serious head injury when cycling is extremely small, and hugely outweighed by the health benefits of cycling.

In all cases you should regard learning to cycle skilfully as your most effective defence against injury of any kind.

Note: Reference to choice in wearing a helmet clearly does not apply where helmet use is mandatory.

I love the idea of folks cycling in street clothes although I’ve never quite figured it out myself. I tried it years ago in Boston and the pantyhose always ran and I got catcalls if I tried to wear a skirt, even a skort. I’m just surprised that helmets haven’t caught on in Europe as one more marker of bike power. They are so light-weight and cheap now. Unless your hair is wet when you put them on, they no longer give you helmet hair. The problem must be that they are not electronic. Maybe if iTunes were piped in somehow (in a non-distracting manner, of course, like the car radio). We don’t want them to get smartphone features since cyclists texting would not improve safety. (Did see that by the way the other day: a cyclist with click-in pedals, at the light, balancing on the curb with no feet on the ground and texting with no hands on the handlebars! I guess the back pockets of cycling tops are no longer just for snacks and drink, but also for smartphones)