In 2014 a neurosurgeon went on record stating that bicycle helmets make no difference to the safety of the cyclist. I've long held this view. Being a motorcyclist, I wear a full-face helmet. Those are solid shells, with shatter-resistant face shields, filled with high density polystyrene, surrounded by a strong loop in the chin bar to help with structural integrity. From direct personal experience I can tell you motorcycle helmets absolutely do save lives. I wouldn't be writing this entry if I'd not been wearing my helmet in the two crashes I've had. But I've long questioned the flimsy little foam things that cyclists wear. The don't cover the sides of the head, they offer no protection to the face and they have no structural integrity (you can snap most of them in two just with your hands - even the expensive ones).
In January this year, another study was completed that largely came to the same conclusion, but discovered in addition that cyclists take more risks when wearing a helmet because they think they're safer (the same is true of car drivers who drive in airbag-laden cars that are soundproofed - they take more risks because they think they're safer).
A study published in The British Medical Journal last year looked at hospitalisations in 11 countries with varying helmet laws, and found that wearing helmets did not lower injury rates.
Considering the various studies on this issue and the real-world data, you have to ask whether it's worth wearing a bicycle helmet at all.
I'll support motorcycle helmet laws to the end of time because data proves time and time again that they are worth it. But when all the science shows that bicycle helmets almost do more harm than good (because of their placebo effect) I'd be quite happy to support a call to make them optional rather than mandatory.