In reply to Shani:
Large Cohort-1 of mixed helmet and non-helmet wearing cyclists: gives overall accident rate of X
.... change law requiring helmet wearing
... leads to fall in participation
Smaller Cohort-2 of helmet wearing cyclists: gives overall higher accident rate of Y
Without seeing the data, as the Cohorts have changed you can't simply conclude that wearing a helmet leads to 'more' injuries. The injury rate of a Cohort-3 subset (always wore a helmet anyway) within the earlier larger Cohort-1 is likely to have stayed the same, and was probably at a higher rate as they undertake inherently 'riskier' riding than the 'casual' riders who stopped when helmets became compulsory.
Cycling demographics also have also changed, and more MTB use has increased injury, so it will also depend on the historical dataset used for Cohort-1 that they are comparing with the data for Cohort-2.
https://www.researchgate.net/publication/321016664_Mountain_Biking_Injuries
The amount of new MTB riders who get hurt as they progress quickly is pretty high amongst the people I know who've enter the sport.
Pro/Experienced Level riders also appear to injury lower body more, as they have learnt not to crash on their head and avoid OTBs: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6543986/
Post edited at 12:11