In Horizon: Forbidden West, I think it's pretty well implied that when members of the GAIA Gang learn to "use their Focuses" then they're also learning to read. Yes, there are scrolls and parchments and other datapoints you can collected that signal literacy among the tribes, but I'd argue it's a specialised skill that most soldiers/warriors don't need.
Like, Varl mutters "That's a lot of glyphs" when he first puts on his Focus in HFW's intro. He, Erend, and even Zo talk about having difficulty getting through Focus documents on multiple occasions. In HZD, Helis records his personal thoughts and manifesto on thousand-year-old voice recorders, which has to be less efficient than a written journal for your average warlord-on-the-go. Not that they all start from scratch, necessarily, but there's a big difference between understanding CAUTION signs and reading summaries of historical events.
So we can say that a large chunk of the GAIA Gang learned to read as adults, yes? But did they learn to write?
That's not as well established in the Horizon series. So - headcanon time!
Writing is an entirely different skill to reading, as it's a productive skill while reading is receptive. The same goes for speaking and listening, respectively. We don't see the gang actively learning writing, but some of them do leave written datapoints behind. My headcanon is that they *do* learn to write, but at different times and with different skill levels.
(This is not a reflection on mental ability, btw. Oral history is a thing. Reading maths and diagrams is separate from reading text. This is just my personal HC because Guerrilla maybe didn't separate writing as a distinct skill from reading.)
Here's my list of the HFW characters' writing backgrounds, on a scale of "writing is as natural as breathing" to "getting along fine without it."
Beta: Taught to write very early in life, because she was created as a tool for scientific discovery. Received no praise for learning "basic" skills, even as a child. Exclusively trained on reports, essays, and proofs. Zero skill at creative or fiction writing, save for mental challenges and paradoxes she wrote for herself out of sheer desperation for entertainment. Effortlessly types on a full keyboard one-handed, Eureka Seven style.
Aloy: Angrily taught herself how to write after finding her Focus at age 8. No writing models, so she became extremely perturbed by the differences between standard examples on her Focus and snippets of adverts found in the wilds. Writing skills further separated her from the rest of the Nora; but it's a skill so she's gonna master it, damnit. She can write very well, but doesn't.
Alva: As a Diviner, she embodies one of the specialised roles that would require reading and writing skills. She learned writing when she began her Diviner training, which I assume would begin around age 11 or so - old enough to start thinking about the future, but young enough to manipulate into Ceo's vision of the future. Yes, this is an allegory for religion. Since all Diviners are taught writing in their tween years, abbreviations and slang developed during that time are not cringe but a mark of pride for your generation.
Zo: Easily the best student among the adult learners in the GAIA Gang. She may have had some experience in writing earlier, but whatever training she had was completed at the Base. Since she's repeatedly pulled in to help train other members of the gang, her grammar and spelling are excellent. Her sentences are straightforward - nothing too complicated, but still effective.
Varl: One of the few creative writers in the gang. Being part of the Nora, he would have experience with their songs and stories, both in evidence at the Proving ceremony. He learned to write while at the Base, and during that time he was especially interested in turning those songs into written works. At first, the interest was just to make nice things for Zo, but he developed a passion for it as he practiced. He gained the most out of the DEMETER files, just because he found that poetry didn't have to follow a specific pattern or even a specific grammar. His prose writing skills aren't great, but his poems are thoughtful and earnest.
Kotallo: An example of learning exactly what he needs to hone his skills, and no more. Like the rest of the warriors, he learned writing at the Base. It was effective to learn how to write clear instructions, so he did. But after that, there were more important things to focus on. Questions, dialogue, clauses and phrases - none of it. Kotallo writes exclusively in statements.
Erend: He had a hard enough time learning to read at the Base, so there wasn't a great deal of time to learn to write as well. He's excellent with numbers and diagrams - he's the captain of specialised royal guards, which means good organisation - but text proved frustrating. And, frankly, Erend knows that you need to take what enjoyment out of life that you can get. So, he has the barest of basics only. Rather than beating himself up over a skill he never needed before, he'd much rather beat you at a game of Machine Strike.
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