...To say that most western riders don't fall head first is something I could not agree with. Even if it were true, falling backward presents the opportunity to roll back and whack your head, in a whiplash type movement, that could be very hard on a human skull.
It probably is true. Dressage riders and western riders tend to hurt their lower back in falls, while jumpers (and anyone riding English with a forward seat) tend to hurt their noggins.
"With regard to admission to spinal units for horse riding accidents, there are far more lumbar and thoracic injuries than cervical in contrast to all other sporting injuries (Table 12) which are almost entirely cervical injuries, indicating that there are different mechanisms involved.17 In all other sporting injuries where the head leads it is almost inevitable that the cervical spine, which is more vulnerable, will be fractured rather than the lumbar or thoracic spine. The only rugby injury in which the thoracic spine was involved was when a drunken rugby player fell downstairs after a game. This would be in keeping with the speculation that in horse riding accidents there are two methods of riding: either jockey style (cross country position) with the head forward, where the rider would be more likely to sustain a cervical injury accompanied inevitably by a head injury, and classical style where the head is held high and the rider would be likely to fall on to the buttocks.8 Spinal injuries resulting from horse riding accidents
It is certainly true that head and neck injuries are strongly correlated with jumping, not dressage.
As an example, one of my two falls was without a helmet. Mia had bolted, I got her stopped, tried to get off of her and she bolted in the dismount. Since my left foot initially moved forward with her - until it came out - I landed on my back, and my hip still hurts 2.5 years later. My body probably instinctively curled to protect my head. The ballcap I was wearing had dust on it, but my head felt fine.
Of course, that is NOT an argument against wearing helmets, which is why I don't like anecdotal evidence. Think how stupid I would sound if I wrote, "Sure was glad I had a ballcap on...saved my life!"
But when another study of eventing shows 88% of head injuries occurred in the jumping phase, 1% in dressage and 11% while the rider was doing ground ops, it seems pretty likely that there is a difference in how a person tumbles from a horse, depending on if he is leaning forward or is settled in the saddle.
A western rider can still split his skull, which is why I usually wear a helmet, and why I wouldn't consider riding on pavement or rocky trails without one.
But when you look at the numbers, I think you can understand why many dressage or western riders are underwhelmed with the need for a helmet, and why a jumper would be insane to ride without one...well, at least, now that helmets are widely available...
I want to add that saddle selection is also a form of risk acceptance/reduction. A stock saddle (Australian or western) gives you a lot more help in staying on when things turn ugly, yet we are mercifully spared from well-meaning people trying to ban English riding, or forbidding kids from doing so.