Research into how cars overtake cyclists shows that if you don't wear a
helmet, cars give you more room (about 8cm).  If you wear a wig
(ie if you look like a woman from behind) they also give you more
room. 

In the morning cars give you less room.  Presumably to do with how busy it is.

Worryingly, the further away from the kerb you ride, the less room you
get.  Almost as if you only get as much as the car driver thinks
you should get, try to get a bit more and they'll give you less. 

From elsevier.nl