There was a thread some time back about
http://www.ukclimbing.com/forums/t.php?t=467447&v=1#x6479629 with various people pointing out that there are have been a number of cases of people getting their heads bashed while sport climbing (inverting in falls or having things dropping on their heads).
The angle of this thread is different: why do few people wear helmets sport climbing, whether leading or belaying or both? (I usually wear one for belaying and rarely wear one for leading). Is it
- Reasonable calculation of low risk (very low probability of serious injury/death)
- Conformity (anchoring in what the group does to fit in) despite a low but still substantial probability of serious injury/death)
- Personal experience (not having witnessed or not having experienced a sport climbing head injury)
The most important elements in making it more reasonable to not wear a helmet while sport climbing seem to me to be when the route is more overhanging, well bolted, on solid rock and belayed by someone who knows what they are doing. When it gets less steep, more run out, on more friable rock and with a less expert belayer, the helmet makes more sense.
FWIW I usually wear one for sport belaying and rarely wear them for leading unless it's less steep etc.
I think conformity plays a big part in the actual pattern more generally, so there will be a tipping point towards wearing them once it becomes 'normal'. The pressure of conformity means that a great deal of supposed rationality is actually rationalisation (I am not suggesting that I am immune to that).