Consumer Reports does excellent research, I recently discovered, not only their own original research on products but they provide the background literature too:
https://www.consumerreports.org/head-injuries/wear-a-helmet-...
The majority of serious injuries from cycling have one thing in common, says Fred Rivara, M.D., M.P.H., a professor of pediatrics at the University of Washington. “Two thirds of hospitalizations and three quarters of deaths from bicycle injuries are due to head injuries,” he says. “The most effective way to prevent that from occurring is to wear a helmet.”
Indeed, extensive research has demonstrated that a helmet is the best way[0] riders can protect themselves against head injuries—especially those that are potentially fatal.[1]
https://www.consumerreports.org/cro/bike-helmets/buying-guid...
Statistics bear it out: According to the Insurance Institute for Highway Safety, in the majority of bicyclist deaths the most serious injuries are to the head, highlighting the importance of wearing a bicycle helmet. Helmet use has been estimated to reduce the odds of head injury by 50 percent and the odds of head, face, or neck injury by 33 percent.
Tangentially, here's how they test bike helmets:
In our tests we strap helmets onto head forms and use an apparatus that drops the helmets at about 14 mph onto a flat anvil to find out how well they withstand impact. An electronic sensor inside the head form monitors the force that would be transmitted to a rider’s skull in an accident.
We also test the strength of helmet chinstraps, attachment points, and buckles. We drop an 8 3⁄4-pound weight 2 feet so that it yanks on the straps to simulate the force that might occur in a crash.
EDIT: Add a paragraph and footnotes
[0] http://www.cochrane.org/CD001855/INJ_wearing-a-helmet-dramat...
[1] https://www.ncbi.nlm.nih.gov/pubmed/27450862