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

Tester's point makes sense. Helmets spread the impact force and adsorb energy. Spreading the force helps avoid cracking your skull open, but doesn't help with concussion,. Energy adsorption can protect against both concussion and fracture. Polystyrene adsorbs energy as it crushes and crumbles. You can increase the energy it adsorbs by making it thicker and stronger. But if you make it stronger, lesser impacts won't generate enough force to crush it and all the energy will be transmitted directly to the skull, potentially still causing nasty concussions.

As we're playing the anecdotes game: a couple of friends have had crashes that led to worrying concussion (noticeable effect on thinking that took weeks to months to fade). Both were wearing helmets, and i got a look at one of them. The plastic outer skin was slightly scuffed and cracked, but the expanded polystyrene wasn't crushed - it hadn't softened his concussion.

I ride more and further than them, don't wear a helmet, have had a few crashes over the years, but have yet to injure my head.

Don't quote me. That purpose of that post was to abuse Tester and advocate the JPS Lotus colours. Both of which are of more importance than anything else in this thread thus far.