Lots of people wonder whether helmets work to save lives, whether they are better for kids than adults, whether they all work the same, whether they are worth the cost to the purchaser or to the society... I'm not going to even try to answer those questions myself but I'll show you where to look for them.
First of all, we ourselves strongly prefer to wear helmets whenever we ride, but other people feel that cycling is safe enough as it is, like walking, and don't see the need for one. That's fine, too, especially for adults who are making the decision about their own risks and benefits. Nobody is wrong, in our view, though many people seem to demonize those who disagree with them. One of our favorite studies showed that like many cyclists, even neurosurgeons in Germany are divided. Link is here.
Really, the best time to wear helmets might be during a discussion with other bicyclists about helmets.
Although cyclists are very divided on this issue, it is only rarely that you see the actual research on it. So here it is! This is a sampling of some data we've found and some ways to get related sources. The links go to the full article if possible, or to an abstract otherwise. Often you can link to the full article from the PubMed medical database abstracts, from the top right side of the screen. Or go to www.pubmed.com and type in a query like bicycle helmet injury risk for a very complete list. PubMed covers nearly every article worldwide that's important to the medical and medical science research community. Other sites on ncbi.nlm.nih.gov cover more basic science and probably don't have much on bikes. Google is less useful but try it if you want.
To summarize the issues (paraphrasing):
Helmet supporters argue that:
helmets reduce the severity of injuries and save lives of people who would have died of their injuries
This reduction in risk is usually cited as between 65-90 % and is repeated in many studies
kids in particular can benefit from this
helmets are effective for preventing injury, no matter whether it's a crash with a car or with a tree or another rider
Helmet law supporters argue that:
Because they are so convincingly protective, helmets should be mandatory for every rider, or at least for children.
Helmet detractors argue that:
Riding a bike isn't that dangerous compared to other things and you don't need a helmet, based on your low risk of injury
Neither do kids need a helmet, for the same reasons
Helmets decrease bike ridership and make biking seem more dangerous than it is, and when there aren't as many riders on the road each one is less safe
People with a helmet take risks they wouldn't try if they were bareheaded
Few riders in Europe use them and they have low risk of injury
There are a lot of other more important factors than just helmets that affect bike safety
Helmet law detractors say:
The number of bike riders goes down when a helmet law is introduced
This cuts down on the general health of a population and/or brings costs
There is no compelling reason for the State to mandate helmet use and limit individual decision making
The Research (some of it):
Newest available US fatality statistics
Insurance Institute for Highway Safety - Highway Loss Data Institute
Summaries of multiple studies from a helmet supporter:
Harborview Injury Prevention and Research Center
The seemingly most cited article in a medical journal (found 85-88% risk reduction):
A Case-Control Study of the Effectiveness of Bicycle Safety Helmets
Cochrane intervention review (same authors) (2/3 risk reduction):
Systematic reviews of bicycle helmet research
I found articles quickly from Finland, Sweden, the Netherlands, Germany, Britain and Australia, some below. Eastern European and Asian studies of bike injuries are also generally on PubMed, with at least an abstract in English.
Cyclist Injuries Treated in Emergency Department (ED): Consequences and Costs in South-eastern Finland in an Area of 100 000 Inhabitants.
Clinical factors associated with intracranial complications after pediatric traumatic head injury: an observational study of children submitted to a neurosurgical referral unit.
A decrease in both mild and severe bicycle-related head injuries in helmet wearing ages--trend analyses in Sweden.
Comments on Depreitere et al. Lateral head impacts and protection of the temporal area by bicycle safety helmets. J. Trauma. 2007;62:1440.
The current injury situation of bicyclists--a medical and technical crash analysis.
[Bicycle accidents in children in The Netherlands in 1990/1991; time for bicycle helmets]
[Bicycle accidents. A retrospective study 1986-1991]
Bicycle helmet legislation: can we reach a consensus?
The efficacy of bicycle helmets against brain injury.
(search for this author for more articles)
Three lessons for a better cycling future
Arguments against helmet legislation are flawed
Again, this is just a small sample of sources we've been reading lately. Link to PubMed and have a look for yourself! Maybe after looking at the statistics and research you'll begin to see the rationale of the other side's opinions, or at least have more ammunition for your next squabble...