It is sometimes claimed that racism could be the result of a natural instinct, something we overcame due to civilization.
For example, Charles Miller made that claim in 1999 in the Anomalist.
"But my contention that "racism" may also be a human instinct is the one that usually draws a storm of criticism from all corners--probably because we in 20th Century Western Culture prefer to believe that racism is a bad habit or something that we can extinguish with a little social discipline. If we were to accept that racism is a survival instinct--left over, perhaps, from hundreds of thousands of years ago when several different species of human were competing against one another for survival--then we would also have to accept that racism as an instinct may defy the social engineering of one or two generations (thus lending to the angst of many civil rights activists, I can imagine)."
(Taken from the comments)
Richard Dawkins seems to propose the theory that racism could be the result of an evolutionary adaptation. From The God Delusion:
[racism] could be interpreted as an irrational generalization of a kin-selected tendency to identify with individuals physically resembling oneself, and to be nasty to individuals different in appearance
Is there any evidence for racism being a human instinct, as opposed to a learned behaviour.