I remember (a good few years ago), reading an article/bizarre rant regarding the ‘myth’ of helmet safety.
The argument centred around the rotational effect on the neck and brain during a crash, created by the increased diameter of the head caused by the helmet.
The writer believed that the only real solution was a helmet designed to simulate the sheering effect of the human scalp sliding and separating from the skull in a glancing-type impact. As such a helmet didn’t exist at that point (mid 2000s), he asserted that one should never wear a helmet under any circumstances, as the naked human head provided superior safety mechanisms!
Frankly I thought he was fairly insane at the time, but fast-forward to today and that helmet design is now available to buy:
MIPS
POC MIPS MTB Helmet