It was already established in TE common sense that skorts and feminine-looking jerseys did provide a bit more space from motor vehicles passing by. I'm not surprised by the results in this regard. I think it has to do with our society generally considering the women (and children) as more vulnerable.

Regarding the helmet, I'm a bit puzzled because I know the guy did the study while always wearing the same clothes and using the same bike. I know that, personally, when I see a cyclist with no helmet on a city bike, big-box-store bike, etc. I will probably treat that 'vehicle' more like I would treat a pedestrian, give more space, etc. If it's a person on a road bike with a helmet, I sort of expect him or her to be able to hold his or her line a bit more, although this is probably a bad assumption to have. I do give a lot of space anyway. If I see a roadie without a helmet going really fast, I think this guy (usually it's a guy) is nuts and I do everything I can to avoid him, and expect any random move from him, so I keep as far as I can.