Full face vs Open face. The pic below is from a study in Germany that shows the areas of impact on a full face helmet. Based on left side and right side totals, you're looking at 35% of impacts in the chin area.
Even with all of the above evidence though, I still go without a helmet most of the time around town. There's no justification for doing it. I just like riding most of the time without one on. For long trips, I take a 3/4 helmet with a built in face shield, but that's more for wind/debris protection and to alleviate fatigue, than it is for protection.
