The em dash peaked in usage in the 1860s.

Then it slowly faded from everyday writing.

Then AI trained on digitized 19th-century books and Medium posts (where two hyphens auto-converted to em dashes).

And suddenly the em dash was everywhere again — not because writers chose it, but because models learned it signals quality prose.

AI didn't just mimic human writing. It resurrected a 150-year-old stylistic tic and made it the hallmark of robot writing.

Punctuation history is weird.