SO IT'S ENTIRELY LEARNED (via The Other Brother)

Study finds out why it's gross to kiss your sister (Maggie Fox, February 14, 2007, Reuters)

Researchers who wanted to find out why it is not only taboo to kiss your sister, but also disgusting, said on Wednesday they have discovered why in a discovery that challenges some basic tenets of Freudian theory.

The instinct evolved naturally and cannot be taught, John Tooby and Leda Cosmides of the University of California Santa Barbara wrote in their report in the journal Nature.

Spending time in the same household and watching your mother care for your brother or sister is all it takes.