According to the research they are!
But not massively so, only slightly and the very limited research that has been done is not that good. I prefer to shave with Occam's Razor which means that the "helmet saved my life" didn't. In reality if you factor in the TRL research on which accidents might have been survivable with a helmet (i.e. no other fatal injuries) you come down to 2 or 3 helmet saved my life stories a year as the maximum which could be true.