I agree that it's a learned behaviour, but probably disagree on how it's learned.

I consider it a natural form of learning. Animals that don't have predators are known to lack any cautious behavior towards other species. We even caused the extinction of such a bird by introducing predators into their habitat. Children who start out as blank slates are like that. They don't care until they accrue enough experience to start recognizing the differences.

We learn to distrust through experience. Whether this is directed by outside manipulation or self-realization, degrees of separation are gained over time through interactions or lack of. And the colour of one's skin is an easily identifiable attribute that can give rise to the simple thought that we're different. While to come to the conclusion that even if our skin colour is different, we are still the same on the inside is a lengthier thought process.

The bias you might fall to in this context is generalizing racists. I don't know if you do, so this is a hypothetical presentation.

You don't know how individuals have reached their conclusion to become racist and whether their personal experience has made them justified. You just assume that because racism is bad, they must be bad. Just like someone who becomes racist due to their bad experiences assumes everyone of a specific race is the same. It's the same form of bias - to generalize a group based on your own experience with a few individuals.

All we can say in support of our theories is "all the representatives of a group that I've met have been like this", but even so, on a planet of 8 billion people, all of those are only but a drop in a bucket.

Do you have documentation proving (beyond doubt) that racism is only a learned behavior? As an anthropology student I read so many textbooks that state otherwise, and I agree with their conclusions that racism is inherent in the "reptilian" human mind. If you don't know what that is, you need to do more reading.