Oh yes, thanks @
davefb for reminding us that this thread was originally about skiing vs cycling!
I wonder if you clicked on srw's original link and read the NYT article? (I have to declare here that I have never been skiing, so all I can do is refer to that article and others about Schumacher's accident, as well as drawing on what I know about traumatic brain injuries through my work.) The article is pretty definite about the fact that skiing helmets offer little or no protection against concussion, closed head injuries, traumatic brain injuries, or rotational neck/brain injuries (although the last one seems more speculative than the others). They offer some protection against scalp lacerations and skull fractures, but these are not brain injuries and are less likely to be serious or life-threatening. The 'perception of risk' factor seems to be crucial in that, among a certain population who are already risk-seekers, wearing a helmet might be leading them to be more reckless with regard to their own safety. That applies to other realms of behaviour and other alleged safety equipment as well of course.
The article's summing-up is interesting:
"Seventy percent of snow-sports fatalities involve men in their late teens to late 30s, according to the ski area association. That is the same population that most often engages in high-risk behaviors like driving fast. Head injuries remain the leading cause of deaths in skiing and snowboarding, Shealy said, with about 30 in the United States each year.
“The helmet does a very good job at protecting against skull lacerations and skull fractures, but it doesn’t seem to have much effect on concussions or T.B.I.’s,” Shealy said, referring to traumatic brain injuries. “Our guess is that this is due to the fact that those injuries are occurring at such a high magnitude of energy that they overwhelm what a helmet can do for you.”