I believe I understand the distinction you’re making: sometimes we form biases from our direct experience with the people/objects in question, and other times from indirect information that is transmitted about those people/objects.

I’ve never personally considered whether the majority of human biases fall into one category or the other, nor do I recall that this author discussed it. My article was really just an 800-word summary of someone else’s book, and I don’t see where in the article I discussed or made any assumptions about that particular question you’re raising now.

I imagine it would be difficult to quantify all human biases, as well as to cleanly delineate what information is received through direct experience and what is culturally taught and then to quantify that distinction, so I can’t say I either agree or disagree with your claim that the “vast majority” of biases are generated by firsthand experience. We receive neverending cycles of feedback from our firsthand experiences and also from the world’s secondhand information. All of it is informing all the rest of it all the time. I don’t feel equipped to quantify or otherwise evaluate that.