I started reading this thinking that it would be a speculative but fun theory. I gave up after the first third of the book.

The core theory explains far too much. Pretty much any religious or cultural behavior in the historical record can be explained by “the voices told them to do it”. People venerate the dead because they are still hallucinating their voices. Villages are built around a central religious or civic building to give the voices somewhere to reside. People carry carved idols to prompt hallucinations. Cultures that don’t behave like this have moved to a modern mind. Later cultures that do behave like this have reverted to a bicameral state.

And yet the wording used when discussing this slightly suggestive evidence is bizarrely strong:

… it shows a boy seemingly tearing his hair with one hand as with the other he stuffs food into the mouth of a corpse, probably his mother’s. This is difficult to appreciate unless the feeder was hallucinating something from the dead at the time.

I’m also not totally clear on what the core theory even is. Reading between the lines, I might suggest that it’s that premodern peoples had no introspective access to their planning and decision facilities, and instead received the results directly as hallucinated voices. But this doesn’t explain all the religious trappings like building houses for dead kings. Either the voices told them to do it, and we have to explain why the voices (aka the persons planning facilities) decided to do that, or the receiving mind decided to do it, in which case we have to explain how a pre-conscious mind managed to build a house without planning. I have a very hard time making any predictions from this theory.

This is perhaps unfair, since I did only read the first third of the book, but the fuzzy nature of the argument so far doesn’t lead to me to expect satisfaction from continuing.