A number of readers have commented on the motorcycle report.
First of all, it is indeed clear that the number of motorcycle riders has increased. However, that is irrelevant for examining the relationship between head injuries and helmets. The report examines the incidence RATE and the percentage of accidents involving head injuries - both of these metrics control for increased ridership. Granted it is true that large sample size increases do yield a higher probability in achieving statistical signficance between two variables, I don't think that the sample size has increased that large here and I think the logical correlation between helmet use and head injuries rules out an odd statistical artifact as a possible explanation for the clear increase in head injuries assciated with motorcycle accidents.
I don't really think these "self interest" arguments are very appropriate - but even if that's the angle that can be used to convince someone that motorcylclists should be required to wear helmets - then just look at the associated medical costs. The amount of insurance required for a non-helmet rider is much lower then the average procedure cost involving a severe head injury. Thus, that creates an economic burden for the underinsured that you may pay for.