Here is an article from a doctor who has actually researched this topic and is able to look at the issue from a broader perspective, taking into account the damaging effects on health from discouraging cycling.
“The best evidence is that [a helmet] doesn’t make any difference to serious head injury when riding a bicycle …
initial research used to back the mandatory laws was “deeply flawed and criticised”. Some newer findings, he said, showed that these laws could increase the chance of serious injury. “On a society-wide basis, it seems as though the compulsory wearing of helmets is diminishing the number of people riding bicycles” he said. “The number one health concern is heart attacks and obesity. “Anything that can be done to decrease that would be a good thing.” “
This is a refreshing change from the scaremongering we typically get, that usually results in more people being scared of cycling.