Pervasive categorization (such an ugly word) is what humans like to do.
How about if they also have different coloured eyes? (via Dangerous Minds)
Journal of Personality and Social Psychology
Men and Women Are From Earth: Examining the Latent
Structure of Gender
Bobbi J. Carothers and Harry T. Reis
Online First Publication, October 22, 2012. doi: 10.1037/a0030437