Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

There is also the risk that helmets themselves cause or worsen some injuries by icreasing head weight and size, by strangulation or by transmitting forces to the neck etc.

The key point is that the helmet *increased *the diameter of your head, so it'll more likely to come in impact with the ground, but at the same time the helmet will offer some cushioning from the impact, it's a catch-22 after all.