It's the combination of being instinctively attracted to someone genetically similar to yourself and not growing up with that person.
It's sick but that seems to be how it works. It's happened to siblings who were separated at birth and didn't find out they were related until after they'd married and had a child. Egyptian Kings and Queens were often brother and sister but raised separately. There is a story of a Japanese (i think) King and Queen that did not want to marry, they were not blood related but they were raised together and considered each other brother and sister.
It is freaky but it's even freakier to think that we'd all do it if it wasn't for the fact most of us are raised with our parents and siblings.
http://en.wikipedia.org/wiki/Genetic_sexual_attraction
Nov 2016 - NES outsells PS1 (JP)
Don't Play Stationary 4 ever. Switch!











