not my downvote, my feelings were not hurt at all

The meat of the book is the references, not the extreme interpretation that catches most peoples' attention. No hallucinations please unless something like substances were involved.

The plasticity of the brain whether damaged or not is emphasized, and uneven recovery of language skills gives some clues.

Monkey see, monkey do was certainly a thing before humans arrived and it's not been left behind very completely at all.

In a world where almost everybody starts out in early childhood learning to read & write at the same time, and it is structurally taught largely to right-handed kids from right-handed examples, and this has been going on since like "written history"[0], this can not help but provide an overwhelming mainstream norm about what side of the brain is for what. Especially when it comes to language. You could almost think of it as a force acting to confine language, or more so the writing part of literacy to one side of the brain more so than the other. The bulk of the highest-educated and most respected vocabularies have been largely built through literacy rather than orally for quite some time, it wasn't always like that even with equivalent vocabularies, and that part lasted longer than the part since literacy got popular.

For those where it doesn't perfectly apply, like left-handed people, those who don't read and/or write, those who learned from left-handed people, those that taught themselves, etc, that all adds up to a small minority and of course there is great pressure to conform. But that doesn't mean the same language ends up in the same parts of the brain as the "normal" specimen in the jar.

The most recent development is the way more early writing is accomplished using both hands on a keyboard like never before when it was traditionally penmanship for generations back.

Now some people actually "think" in language more so than others, some not at all, plus often bilingual works too.

I'll go ahead and say it, if you were up there in the highlands where Dolly Parton was born back in the 19th century before they really got to building schools, you could find an "illiterate" culture often having a wider vocabulary than those in places like Boston where they had schools continuously since arriving on the continent. Unlike PNG the hillbillies of the Smokies descended directly from a literate culture, they just didn't bring many books that far up the mountains, and odds are they would be lost to mildew by the mid 1700s.

Sometimes you also get the idea that when they were feuding, they just didn't understand each other and everything was fine among the disagreeable factions as long as everyone stayed on their own mountain. People had lost the ability to read, and when they only had one book that was probably the Bible but they couldn't literally decipher it any more. It was still holy though for the faithful who would pretend to read from it while more or less "lip-syncing" the scriptural quotes that had been passed down orally ever since.

Maybe that's where the saying comes from; "If I had half a brain I'd <obviously do what the mainstream would do>." From those who have no history of using less than both halves even for language, so they naturally don't do what the mainstream does? While simultaneously recognizing that they don't even have the same first half that the formally educated (whom they often respect so highly) do possess.

With PNG you've got to figure all the highlanders have language and stuff well-imprinted in their brains like anybody else, "just" not with the pattern recognized as most common to outsiders.

That doesn't mean their superstitions were based on hallucinations of any kind, among different cultures you can expect different parts of different halves of the brain to be doing some of the same things differently. Especially when they have always been isolated from each other.

There might even be a tendency going far back into "modern" literate culture, where the "literate" half of the brain is generally respected most when it is brilliant enough to get by on its own, and that ends up predominating.

Anyone who comes along and has always used all their brain may still have an orthogonal advantage. That could be why professor Diamond attributed the PNG highlanders as perhaps more intelligent than most in general. Or with a cool million of them there's got to be an Einstein or two. What would an Einstein casually having twice as much brainpower than previously imagined be like?

[0] Recent written history, the kind that means the most to a literate culture. Note the way with some traditionally illiterate groups how oral history can easily go back many generations further.

I can't stop thinking about Papua New Guinea