Earlier quoted context omitted.
There are people today who hear voices, but they are a minority and we label them as mentally ill. The Jaynes hypothesis, as I understand it, is that in early civilisations the majority of the population heard voices and it was considered normal. If you see a reference in literature to a god speaking to someone, you can't easily tell whether that's fiction, metaphor, an exaggeration, a lie, a narrative style, or a li…
I actually don't reject the theory that accounts of Gods appearing and speaking to people could be actual mental processes, guided by culture. I find this more believable than it is all fiction or lies (or that Gods have physical existence outside the human mind). But I do reject the bicameral hypothesis that we either have Gods speaking or we have consciousness, and that shift happened a particular point in time aro…
Also, the breakdown is not instantaneous, but happens over time. Once it starts, you get religion and oracles (the Oracle at Delphi for example) as the populous attempts to hear the gods' voices for direction. Oracles lasted for about a millennium after the initial breakdown in Greece, and over time, the gods' voices were head less and less.
He also states that modern remnants of the bicameral mind exist in hypnosis and schizophrenia (but in the case of hypnosis, how it works is very culturally dependent).