This link has a good explanation of the guy's theories: [link]
Jaynes adduces evidence for this astonishing hypothesis from several sources. One is the "voices" heard by schizophrenic patients, which Jaynes interprets as a throwback to the bicameral mind of ancient times. Another is evidence from neurosurgery, where patients hear "voices" upon having their brains electrically stimulated. Another is the polytheistic gods of ancient civilizations, which spoke directly and intimately to individuals:
"Who then were these gods who pushed men about like robots and sang epics through their lips? They were voices whose speech and directions could be as distinctly heard by the Iliadic heroes as voices are heard by certain epileptic and schizophrenic patients...The gods were organizations of the central nervous system"(73-4).
Jaynes suggests that each person had his own individual "god", which always told them what to do. The theory further accounts for why the gods were so naturalistic and anthropomorphic, rather than supernatural and otherworldly.
Where did the gods go, then? Jaynes proposes that a series of unprecedented environmental stresses in the second millennium B.C. forced the two halves of the brain to merge into unicamerality. (This was a cultural, rather than a biological, transformation, Jaynes notes.) The stresses might have included natural disasters (the story of the Flood comes to mind), population growth, forced migrations, warfare, trade, and the development of writing. A common denominator among all these is the introduction of complexity and difference, things the bicameral mind deals with only with difficulty. Jaynes suggests, among other things, that traders in contact with other cultures might have been forced to develop a "protosubjective consciousness" to cope with the gods of unfamiliar people.
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To establish the gods' disappearance, Jaynes cites a number of illustrations and cuneiform tablets dating from Sumerian times. He shows a stone-carven image of the King of Assyria kneeling in supplication before an empty throne, from which his god is conspicuously absent. The accompanying cuneiform script reads, "One who has no god, as he walks along the street,/ Headache envelopes him like a garment." Another tablet reads,
My god has forsaken me and disappeared,
My goddess has failed me and keeps at a distance.
The good angel who walked beside me has departed.
Jaynes interprets this as evidence of a new subjectivity in Mesopotamia. The bicameral mind has begun to collapse into the modern unicameral mind of the self-willed, self-aware "I", and as a consequence the gods no longer speak to people, as they did in the days of old (223).
Interesting. But unprovable (I think, without having read the book).