First, wearing a helmet is going to alter the kinetics of your head during the accident, possibly for the worse.
Second, wearing a helmet can alter the behavior of people around you in ways that impact your safety. For example, car drivers may drive less carefully around a helmeted rider.
Third, wearing a helmet can alter the behavior of the wearer.
Now, does that mean that helmets don't improve safety? I would guess they probably do. But let's make the argument based on sound reasoning and data, not insults and "common sense".
You had been downvoted at the time of my reply. I don't know why - you make some good points.
> But let's make the argument based on sound reasoning and data, not insults and "common sense".
I strongly agree. When someone talks about homeopathy it's easy to dismiss it as nonsense, and we only need a few studies to tell us it's nonsense. But when someone makes a calm, sensible claim with a reasonable method of action ("helmets protect the brain") it's more important to get good quality science to investigate these claims, to eliminate our biases and to eliminate confounding factors.
Imagine a crash where someone's head scrapes along the ground. Now give that person a helmet. With the wrong materials the helmet will "snag" the ground. This could mean that the head sticks while the body keeps going, risking severe spinal injury.
While the science around driver perception of helmet-wearing cyclists is weak it's a reasonable statement - "drivers give less room to people wearing helmets because they assume a better rider". That could be testable in driving simulators, and that would give use good quality data.
It's nice to hear anecdote. "I wear a helmet because ..." or "I don't wear a helmet because ..." are good things to hear. It gives context to the data and the science. But these are not reasons for everyone to follow the advice.
It's fun to watch a crowd of nominally intelligent people so thoroughly mix up arguments with the conclusions.
If you make a bad argument in favor of wearing helmets, then it's a bad argument regardless of whether helmets are effective. But the moment you call it out, you get nothing but people talking about how irresponsible you are for recommending people not wear helmets.
The exact same thing is going on over in the Arafat discussion. People make crazy arguments about how Arafat (or other people) was killed and then, when called out on it, act like it's the conclusion being attacked, not the reasoning.
First, wearing a helmet is going to alter the kinetics of your head during the accident, possibly for the worse.
Second, wearing a helmet can alter the behavior of people around you in ways that impact your safety. For example, car drivers may drive less carefully around a helmeted rider.
Third, wearing a helmet can alter the behavior of the wearer.
Now, does that mean that helmets don't improve safety? I would guess they probably do. But let's make the argument based on sound reasoning and data, not insults and "common sense".