Oooh, this paper is a lot of fun! http://webh01.ua.ac.be/funmorph/kris...t_al_2009b.pdf

You can order it through a journal service ($$) if you have trouble getting the pdf above. Footwear Science vol 1 No 2 June 2009, 81-94

A fun stat mentioned in passing in that paper: 88% of healthy women in a USA study were wearing shoes smaller than their feet! (Frey, 1993)