http://www.nytimes.com/2001/07/29/business/a-bicycling-myste...
> The number of head injuries has increased 10 percent since 1991, even as bicycle helmet use has risen sharply, according to figures compiled by the Consumer Product Safety Commission. But given that ridership has declined over the same period, the rate of head injuries per active cyclist has increased 51 percent just as bicycle helmets have become widespread.
...
> Many specialists in risk analysis argue that something else is in play. They believe that the increased use of bike helmets may have had an unintended consequence: riders may feel an inflated sense of security and take more risks.
Cars are more reckless around bicyclists and drive closer to them when they see they are wearing a helmet:
http://www.nytimes.com/2006/12/10/magazine/10bike.html
We also see here that bicycles are the lowest risk of head injury while traveling streets and sidewalks:
http://www.howiechong.com/journal/2014/2/bike-helmets
Turns out you're much more likely to suffer a head injury in a car, or walking than on a bicycle. Should helmets be mandated for driving and walking? Evidence suggests it would make more sense for those activities.
Wearing helmets increases the likelihood of neck injuries: http://www.sciencedirect.com/science/article/pii/S0001457500...
Feds had to stop claiming bike helmets prevent 85% of head injuries (a commonly cited statistic) after their statistical analysis was found to be bogus:
http://greatergreaterwashington.org/post/19036/feds-will-sto...