This is a tricky question to ask correctly. One attempt has already been closed as subjective and argumentative. There is no one correct answer, for sure. However, there are interesting reasons that I believe are worth recording here.
Ideally include supporting documentation/research. I will vote up for quality of answer, not based on whether I agree with the answer.
For: If you fall on top of your head, it will protect it (also side impacts if wearing a full-face helmet while riding downhill mountain bike)
No, this is not an overly simple reply. It's what it's designed for, it's what it does well and it's why you should wear one.
Feel free to edit with documentation about head protection, but I think the number of people on here (myself first) who can say "I fell and my helmet saved my head" are worth any research.
Against compulsory: where it is compulsory it discourages people from cycling -> they drive instead -> more cars+fewer cyclists make it more dangerous for cyclists.
Personally I wear one, but this is the main argument against compulsory helmet.
Against: There has been some research that showed:
So by wearing a helmet you may increase the risk from car drivers not giving you enough space!
For: Often your friends or family will think are you safer when you wear a helmet; wearing a helmet placates your dearest-and-nearest.
Therefore:
It might make sense to wear a helmet if it's not a discussion one cares that passionately about either way, for these reasons.
Against legal requirement: In many countries that have made it a legal requirement to wear a bicycle helmet, the number of people cycling has reduced. Given that the risk of death due to poor health as a result of lack of exercise is a lot greater than the risk of death due to cycling without a helmet; it never makes sense to require people to use a bicycle helmet.
Against: It may not really be necessary. If I'm riding very casually with no vehicles, a fall would be similar in effect to falling while jogging. Both at college where we had roads closed to cars as well as riding around the farm.
Of course, this implies not riding at speeds where a failure (rider or mechanical) would cause a major problem.
Against: helmet hair (messes up your hair)
Depends on hair style, what you use to style your hair, how much you sweat, etc...
For: Visibility. A brightly-colored helmet is more visible than a cyclist's head. (Admittedly, a cycling cap can do this job just as well as a helmet.)
Against while commuting: It is useless.
Studies have observed that in accidents killing cyclists, the injuries were such that a helmet wouldn't have saved them. Anyway the risk of a head injury in a bike accident is about 15%, which is the same in car accident and in walk accident. Who would promote wearing helmets for car drivers or pedestrians? A documented article can be found on the site of the french organization promoting urban cycling. In summary:
Casual bicycling is not especially dangerous.
What little data exists doesn't demonstrate a correlation between increased helmet use and increased cyclist safety.
Mandatory helmet laws decrease bicycling substantially.
Cycling risks are far lower than risks associated with lack of exercise.
In countries where more people ride bicycles, bicycling is safer.
Conclusion: helmet laws and "fear mongering" around bicycling hurt public health
Against: A lot of people get the idea that cycling is very dangerous when they get the impression that you should always wear a helmet. In fact the health benefits from cycling outweigh the risks many times over.
So by wearing a helmet and/or telling other people it is a good idea to wear a helmet you are increasing the number of people that will die (at a early age) from heart attacks etc.
(note: I'm completely ignoring the cost of the helmet itself, as that's a very minor factor compared to less-direct opportunity costs)
The benefit of wearing a helmet is to partially mitigate against the risk of death or traumatic brain injury. The other benefits (visibility, attachment points, placation) are incidental. The costs are primarily messy hair, minor discomfort and inconvenience.
From what I can tell looking at the data, the odds per year of a crash that leads to death or traumatic brain injury that could be prevented by a helmet are roughly one in a million. I'm assuming going less than 15mph (24 km/h) riding safely, etc. (factoring out minors under 18, riding drunk, people going the wrong way with no lights, etc.)
Getting any kind of truly solid numbers is difficult. It's not possible to conduct a true scientific/empirical study, so the science that's out there is either purely based on studies of dummies or can't properly correct for self-selection bias (the strong possibility is that cyclists that ride safely are more likely to be wearing a helmet).
A summary of US DOT statistics put together by the IIHS shows 714 cyclist traffic deaths for 2008. 26% of them were drunk at the time, 36% were after 9pm and before 6am.
And while those numbers only include people that actually ride cycles (the population of cyclists is smaller than the total population), it also includes people that ride the wrong way, weaving across traffic, without stopping at lights/signs, drunk, with no lights, after dark. And it includes people going quite fast (where a helmet's more likely to help in case of a fall). I'm trying to make guesses on the odds for a typical transportation/commuter cyclist who is unlikely to get over 15mph.
So I'm figuring the odds of a commuter cyclist riding in a generally safe manner being killed cycling are actually lower (less likely) than being hit by lightning (1:500,000).
Of the 525,960 average minutes in a year, I spend roughly 1400 making my hair look "good" (a few minutes most mornings washing and styling my hair, plus time for a haircut every 5 or 6 weeks). So apparently I value having my hair not look messy to the tune about about 0.266% of my life. Let's factor in comfort a bit and round that up to 0.3% (one third of 1%, or about 3 thousandths). So the annualized benefit is roughly 1/1,000,000 of my life and the cost is 1/333 of my life, so the cost is about 3,000 times the benefit.
I'm fond of living and consider severe brain damage almost as bad as death, so I'm going to assign a value to my life of $10,000,000 (well above my expected remaining lifetime earnings). The odds seem to be about 1/1,000,000, so the benefit is $10 per year. I spend more than $10/year on my hair (shampoo, hair gel, haircuts, etc), clothes that make me comfortable, etc ($500 seems about right). I also place probably about a $50/month ($600/year) value on comfort. It's harder to place specific monetary values on my time (the primary cost), but again it seems like the cost of wearing a helmet is at least 1,000 times the benefit.
If I'm engaging in a riskier riding activity (going fast, anything "technical", long enough rides that I'll get tired, riding at night, riding in wet conditions, etc), the benefit of wearing a helmet increases and it's likely math like mine works out in favor of a helmet.
If the opportunity cost goes down (bald/shaved head, long ride so I'll be sweaty and need a shower afterwards anyways, rain so I'll need a hood anyways, cold so I need a hat anyways, riding home in the dark so nobody sees my messy hair afterwards), again the math works more in favor of a helmet.
If the risk is increased and the opportunity cost goes down (long sweaty ride, ride home in the dark, rainy ride), the math goes much more strongly in favor of a helmet.
For: A helmet is a great place to mount lights, a mirror, or a rear blinky.
Against: In a crash with a glancing blow, a helmet increases leverage of the impact forces around your neck. A twist-around-the-neck injury is very different from a straight-on-impact injury.
For Scientific research. (I may be able to do an against or at least inconclusive as well)
A review of studies: REVIEWER'S CONCLUSIONS: Helmets reduce bicycle-related head and facial injuries for bicyclists of all ages involved in all types of crashes including those involving motor vehicles. http://www.ncbi.nlm.nih.gov/pubmed/10796827
A case control study: We conclude that bicycle safety helmets are highly effective in preventing head injury. Helmets are particularly important for children, since they suffer the majority of serious head injuries from bicycling accidents. http://www.ncbi.nlm.nih.gov/pubmed/2716781
Another review regarding competitive cycling: The scientific evidence that bicycle helmets protect against head, brain and facial injuries has been well established by 5 well designed case-control studies. http://www.ncbi.nlm.nih.gov/pubmed/9587180
A review regarding bicycle helmet legislation: Head injuries related to bicycle use are common and can be serious. They can be prevented or reduced in severity with helmet use; however, education has resulted in modest helmet use in most developed countries. http://www.ncbi.nlm.nih.gov/pubmed/16595420
These are just a few. There is a complaint present in several studies about the lack of randomized controlled trials. There are also numerous studies regarding the use of helmets by children. An issue that comes up is that it's difficult to determine whether children are using properly fitted/properly worn helmets.
For: it might be legally required.
This depends very heavily on jurisdiction and may depend on age as well. (in my jurisdiction it's required for minors under age 18 but not for adults)
For: Wearing a helmet provides a sliding layer if you crash and slide. Would much rather have my helmet slide against the pavement than my head!
(not my head)
http://www.flickr.com/photos/ian_crowther/2992217413/in/photostream/
FOR helmets AGAINST focusing only on helmets
It isn't so much that there is a negative aspect to WEARING helmets, obviously they can do little to hurt and a lot to help.
The biggest problem is that the overall focus of bike safety instruction and legislation is on helmets.
We seem to think that simply slapping a helmet on ourselves or our children is enough to secure our/their safety. We neglect to teach or focus on much more relevant bike safety issues. Like funding dedicated bike lanes/roads for (sub)urban riding, or better educating drivers about cyclists on the road
Clearly, you would be much better off if you used your head instead of just covering it with styrofoam, but doing both is definitely the preferred choice.
For
Against
Just a note about that last point - I hear that one a lot, and my usual response is that by a huge margin most accidients and collisions that result in injuries sugnificant enough to require medical attention (and therfore be tracked and reported) are not collisions or accidents involving automobiles. The most common is a 'fall' - meaning you hit something in the road or path, or otherwise lost control and fell over all on your own.
For: When swooped by birds (such as magpies) in spring, it is harder for them to take away a bit of flesh.
For: A helmet is unlikely to help you if you hit your head very hard on an object. However it reduces the likelihood of you getting a minor head injury if you fall of your bike and hit your head, or ride into a tree branch at slow speed. Therefore wearing a helmet may save you having to spend a few hours in A&E while they check you out, you may also avoid an overnight hospital stay for observation.
Against: Time to put, take off This depends on your point of view.
Against: Inconvenient to carry when off-bike.
Against: Does nothing to stop brain detaching from skull membrane which is the most important damage to avoid (cuts are mere flesh wounds)
Against: False sense of security/moral superiority If polystrene was that good then they would make lorries and formula one cars out of the stuff. Look at a big lorry coming towards you and now imagine that a cheap bit of Chinese polystyrene is going to give you Superman-esque powers of invincibility.
Against: The Dutch and Danes do not wear them, although they do buy them for children they cycle with. Although road conditions may be different in Copenhagen, they still have fast moving automobiles.
Against: Most helmets do not fit properly I have sold several hundred helmets in my time, including more than a hundred on one day (at a trade show) hence I have experience of getting them to fit, which they can be made to do. However, most helmets that I see worn are not adjusted correctly and with one's little finger you can typically flip the lid back from being on the top of the head. With a bit more force, e.g. from the rider's mass decelerating from twenty miles an hour, the ill-fitting helmet could harm the rider, strangling them. This is particularly the case with children's helmets.
Against: Reassures mother She grew up when the car was king, she continues to trash the planet with her automobile, she has no intention of ever riding a bike again, she completely believes the FUD about helmets and has the occasional road mishap herself - sound familiar? Of course you can do as you are told - by her - after all, mum knows best, doesn't she? She actually has what psychologists call 'projection' and 'denial' on the go. There comes a time when you should tell her to take a hike, or at least a bike.
Against: Anecdotal rubbish by the helmet trusters We have all heard about so-and-so's little Johnny that would have had his head split open had he not been wearing a helmet, haven't we? You cannot argue against people and their anecdotal evidence, even if little Johnny was riding back from the pub pissed on unfamiliar roads having not been much of a cyclist in the first place. If you have not hit so much as a kerb in 20+ years then you can still be deemed irresponsible by the helmet trusters.
Against: Poor quality helmet product. Only the MET helmets have straps that are soft and lie flat on the side of one's face. Giro and Bell helmets have straps that want to dig in at a 90 degree angle, typically coupled with a buckle that can painfully pinch the skin.
For: Keeps head warm in winter a helmet can keep your head out of the cold and rain.
For: Great for off-road cycling off road with any type of bravado is a bit silly without a lid.
For: Professional road racing You need a lid to road race. Unfortunately for Wouter Weylandt in this year's Giro d'Italia his helmet did not prevent his skull cracking after a crash and he is with us no more. However, no lid, no race.
For: The benefits of wearing a helmet are low; however the cost is also low. So why not wear one?
Against: It's much colder in winter to wear a helmet with an under-helmet cap than a warm winter hat. (Of course, it's much more dangerous to bike in winter, so this argument is a bit of a moot point...)
For: When mountainbiking, you can hit and push aside branches
Against: When not cleaned properly, they get a terrible smell.
Against: Ecological impact (Power consumption, CO2, chemicals) of helmet production.
For: Statistics - Bicyclist deaths by helmet use, 1998-2008
"Ninety-one percent of bicyclists killed in 2008 reportedly weren't wearing helmets" http://www.iihs.org/research/fatality_facts_2008/bicycles.html
Note - This statistic was provided to me in a handout at a workplace safety and first aid workshop conducted by the Red Cross. Cycling was included in the safety section because the company has a considerable number of bike commuters. Both the company and the Red Cross promote helmet usage based on this and similar statistics.
Against: Sunburn on your forehead in strange patterns, potentially painful if your helmet sloshes around a bit. (Sunburn on your entire head in lines if you're bald.)
For: Less total sunburn on your head.
For: You will usually cycle along the road, close to the kerb. When you fall off, you are likely to fall into the kerb on your head. It happend to a friend of mine, and his helmet was dented. If he didn't wear helmet, it would have been his head.
Against: You'll live longer whether or not you wear a helmet
(insisting that one needs to wear a helmet every time I step on a bike decreases overall life expectancy since it will result in less riding)
Using myself as an example: Cycling is the bulk of the exercise I get, and I probably have a genetic predisposition to heart problems. The odds of a heart attack (or other life-threatening health problem exacerbated by lack of exercise) if I don't cycle are probably much higher than the odds of getting killed not wearing a helmet while cycling. If I felt I had to wear a helmet absolutely every time I got on a bicycle, I'd cycle less and actually reduce my life expectancy.
From what I've seen of the statistics (may have to dig up the per-passenger-mile stats stuff sometime) of death per mile of cycling are lower than per mile of being in a car or per mile of walking.
In other words: the purpose of a helmet is increasing my likely life span, and requiring or over-emphasizing helmets may work contrary to that purpose.
(Much like another response I made here, if the cycling is particularly risky the odds move in favor of a helmet. This argument only works for safe vehicular cycling below about 15mph)
For Emergency stop where you happen to be on a patch of black ice, you go sideways - ditto a patch of spilled oil on rain-soaked pavement.
Both wipeouts were totally sudden, no chance to land properly. When my shoulder hit pavement, head snapped over, top/side of helmet went "THUNK!".
Just got up & rode away.
Helmets are less ugly than brains spilled onto the street.
For: Wow... Ride your bike down the road at 20 miles an hour, obeying rules of the road. Have a pickup truck roll through a stop sign an pull directly into your right-of-way path. Hit the pickup, go over the hood and land on the pavement. Note the extremely loud sound inside of your skull: the sound of the helmet absorbing the impact and breaking in 7 places just as it was designed to do, spreading the impact accross 7 different break points instead of one concentrated concussion-inducing, possibly brain-damaging location on your skull. Get up and walk away with only a headache in stunned amazement that people don't want to be inconvenienced by wearing a helmet. Ok, you don't need to wear a helmet 99.99% of the time. But when your head hits the pavement, well, it was totally worth the 25 years of inconvenience. Its up to you, gambler. Odds are pretty good in your favor... But I submit: you'd gladly go back and wear your helmet a thousand times if you could undo the accident you were just in...