Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

Personally, I would expect it to be confirmed that populations of voluntary helmet-wearers would also be at greater risk than compelled populations, as I don't believe that the increase in head injuries in compelled populations is only due to decreasing levels of cycling, but rather than hypothesising it'll be more interesting to see what that research actually says, when it's done. :)

http://www.cyclehelmets.org/1261.html

You would be right.
0.5% of Dutch riders wear helmets, but those helmet wearers account for 13% of hospitalisations.