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.