An oldie but a goodie.

"An examination is made of a meta-analysis by Attewell, Glase and McFadden which concludes that bicycle helmets prevent serious
injury, to the brain in particular, and that there is mounting scientific evidence of this. The Australian Transport Safety Bureau (ATSB)
initiated and directed the meta-analysis of 16 observational studies dated 1987–1998. This examination concentrates on injury to the brain
and shows that the meta-analysis and its included studies take no account of scientific knowledge of its mechanisms. Consequently, the
choice of studies for the meta-analysis and the collection, treatment and interpretation of their data lack the guidance needed to distinguish
injuries caused through fracture of the skull and by angular acceleration. It is shown that the design of helmets reflects a discredited theory
of brain injury. The conclusions are that the meta-analysis does not provide scientific evidence that such helmets reduce serious injury to
the brain, and the Australian policy of compulsory wearing lacks a basis of verified efficacy against brain injury.

Source:

The efficacy of bicycle helmets against brain injury
WJ Curnow

The efficacy of bicycle helmets against brain injury