There’s a long term debate about the effecacy of helmets. this article from 2014 summarizes it pretty well. All the studies, both in favor and against are relatively weak compared to what we might expect, but this is epidemiology, not biology.
The biggest indicator is simply that countries with heavy helmet use have more head injuries per 100,000 miles ridden than those with low helmet use. Even that is a correlation, but causality is unclear.
Surely that’s like what happened in WWI (I think) where they found making soldiers wear helmets created more head injuries than before. They almost stopped using them before realising that less soldiers were returning dead so they just increased medic capacity to handle it.
Wearing a helmet is going to result in a head injury in an accident which would otherwise have caused death.
