Java Man Espressopithecus wrote:
> Don't count on it.
>
> You are apparently assuming that the underlying aggregate risk of head
> injuries to cyclists was the same before and after MHLs. Yet I
> understand that the studies also showed that cycling declined
> significantly after MHLs. Are you theorizing that this decline was
> uniform across all risk groups?
Which is why one also corrects for the rate of other injuries, ie
non-head. It is the proportion of HI to other injuries which is
interestign because it doesn't change. If anything we can conclude that
those that most need to wear helmets already are and those who give up
cycling have a very low rate of HI. Loss of these cyclists from the
pool is directly counterbalanced by the extra protection given to those
who take up helmet wearing.
Hmm.. Need a bunch of things to match exactly, every time, in every
jurisdiction.
The null hypothesis is that helmets do nothing useful, so any changes
in population between the wearers and non-wearers do not really affect
the HI rate.
Is there a causal mechanis for this? Yes.
Biomechanical studies by King et al. show that while a helmet is good
at reducing linear accelleration within it's design range, it has
minimal effect on roatational accelleration of the brain.
A much lower degree of rotational accelleration is required to to cause
a similar amount of brain injury (from cadaver studies on human and
other primates dating back to the middle of the last century)
So we have a mechanical cause (or lack of one) that proceeds as
follows.
1. Brain injury is predominantly caused by rotational accelleration of
the brain, resulting in shearing of the connections between brain
cells.
2. Helmets are ineffective as an intervention for this measure.
3. Population studies where helmet wearing rapidly increases show no
change in head injury rates.
It is consistent and it is causal. Just what you asked for.
And it is repeatable. It has been seen everywhere there has been a
sudden step change in helmet wearing (ie a short term longitudinal
study).
And being a causal hypothesis that is supported by multiple
observations, it is predictable that the only effect of an MHL will be
a decrease in cycling, and no change in overall HI rates.
...d