I've been using em-dashes in my own writing for years and it's annoying when I get accused of using AI in my posts. I've since switched to using commas, even though it's not the same.

Another reason I think attributes to it at least partially is that other languages use em-dashes. Most people use LLMs in English, but that's not the only language they know and many other languages have pretty specific rules and uses for em-dashes. For example, I see em-dashes regularly in local European newspapers, and I would expect those to be written by a human for most part simply because LLM output is not good enough in smaller languages.

I wonder what happens to all that 18 century books scanning data. I imagine it stays proprietary and I've heard a lot of the books they scan are destroyed afterwards.

I’m now reading Pride and Prejudice (first edition released in 1813) and indeed there are many em dashes. It also includes language patterns the models didn’t pick up (vocabulary, to morrow instead of tomorrow)

In Russian written languages, the quotes for the people speaking are prefixed with em-dash, instead of double-quoted like it would be in typical English book:

Instead of

"The time has come," the Walrus said,

"To talk of many things:"

... it would be spelled as

— The time has come, — the Walrus said,

— To talk of many things:

I wonder how much of russian language content was in training model.

Are people surprised that training biases a distinct style? I'd think it's kind of expected

Because Sam Altman said so

I always figured it was because of training on Wikipedia. I used to hate the style zealots (MOStafarians in humorous wiki-jargon) who obsessively enforced typographic conventions like that. Well I still hate them, but I'm sort of thankful that they inadvertently created an AI-detection marker. I've been expecting the AI slop generators to catch on and revert to hyphens though.

Robert A. Heinlein used a lot of em-dashes and much of the Internet was created by Heinlein fanboys?

The conclusion is really a guess unfortunately.