Livre d'Or

The evidence is that the risk of head injury is increased by helmet-wearing because the risk of *collision* is increased by helmet-wearing, and even if the helmet does protect your head (not actually proven in all circumstances, cycle helmets are surprisingly fragile) it does nothing at all to protect the rest of you.

The causation here is that both the cyclist and the drivers around the cyclist are likely to take more care to avoid risky behaviours (like cycling in the dooring zone, or tailgating) if the cyclist appears/feels more vulnerable.