A fine line

As you probably know, there are four sorts of horizontal line separating symbols in text: the minus sign, −; the hyphen, -; the en-dash, –; and the em-dash, —.  Some people use just hyphens — or perhaps paired hyphens to indicate an em-dash — because that’s what a standard English keyboard provides.

Recently, the em-dash has been touted as an outward and visible sign of ChatGPT output.  This annoys people who deliberately use the full variety of English punctuation marks. The em-dash is in LLM output, they retort, because LLM output is trained on English writing and so will extrude em-dashes and semicolons, just as it will extrude metaphor and metonymy, zeugma and syllepsis.

On the other hand, there do seem to be a lot of em-dashes in ChatGPT output nowadays.

An analysis by Maria Sukhareva suggests a compromise explanation. Yes, the stretch hyphens come from the training data, and yes, they are somewhat new, but there are also too many of them.  We’re seeing a combination of two factors: addition of older books— with more em-dashes— to the training materials, and the fact that an em-dash is fewer tokens than other ways of setting off parenthetical comments.