You comment and m0nty's about Specialized brand helmets meeting a stricter standard prompted me to do a little helmet research. I was a bit skeptical of the slim Bern helmets. I think simple physics demands that to stop a moving head on impact slowly enough to avoid concussion, one must use a (dorky?) a "spaceship on your head" helmet. There's no getting around that to go from velocity v to 0 without accelerating more than some threshold time requires a certain amount of space.

Now, I don't know if the thresholds chosen by ANSI or Snell or whomever will make a real difference in a crash, but Consumer Reports rated Bern helmets poorly in "impact absorption".

http://bikeportland.org/2012/05/31/nutcase-bern-helmets-rece...