The em dash question
The em dash became the internet's favourite proof that a machine wrote something. Of all the tells it is the weakest. It has a false-positive problem that rarely gets mentioned. The best account of why models overuse it opens by admitting the cause is unknown.
This page collects what is established, which is less than you would expect, and one useful thing: the spacing version of the tell is wrong about an entire national publishing tradition.
It sits under the AI writing tells, and if you want a punctuation rule enforced rather than argued about, the questionnaire's optional rounds put yours in the file.
Is an em dash proof of anything?
No and the catalogue everybody cites for it says so directly. Wikipedia's guidance on machine-writing signs states that the em dash is most useful taken in combination with other indicators, not by itself, and notes it is far more common on discussion pages than in article text, where people write more loosely.
It also describes what the models do with the mark, which is more specific than the mark itself: they tend to use it in a formulaic way, punching up clauses to give sentences an emphatic, sales-like rhythm. That habit is the tell. The character is incidental.
Why does the spacing version of the tell misfire?
This is why British writers get accused unfairly.
The refined version of the tell is not the em dash itself but a spaced one. Wikipedia's guidance says machine-written em dashes are usually surrounded by spaces, contrary to common typographic guidelines. That is true of American house style: Chicago and the APA manual both set the em dash closed, with no spaces around it.
It is not true generally, and two large populations write spaced dashes on purpose.
| Style | Practice | Consequence for the tell |
|---|---|---|
| Chicago Manual of Style | Em dash with no spaces | The tell works here |
| APA Publication Manual | Em dash, closed up | The tell works here |
| Oxford Guide to Style | Em dash, closed up, like most US publishers | The tell works here |
| AP Stylebook | Em dash set open, with spaces around it | The tell misfires on anyone following AP |
| Penguin, Cambridge University Press, Routledge | Spaced en dash as house style in the United Kingdom | The tell misfires on a national publishing tradition |
So a spaced dash identifies a machine, an AP-trained journalist, or somebody who learned to write from British books. The Oxford Guide to Style acknowledges the split itself: it records that the spaced en dash is used by other British publishers while Oxford University Press, like most US publishers, does not.
If you are being told your writing looks machine-made because of spaced dashes, point to this. You can check it in ten seconds.
Why do models use so many of them?
The cause is unknown. The most careful published attempt says so in its first paragraph. Sean Goedecke's account, from October 2025, opens by conceding it is strange that we do not really know, and labels his own explanation as still largely based on speculation.
His hypothesis is that the high-quality book data these models were trained on skews towards late-nineteenth and early-twentieth-century print, which uses around thirty per cent more em dashes than contemporary English prose. He also reports a version step-change: GPT-3.5 did not use them much, GPT-4o used roughly ten times more, and GPT-4.1 more again.
Read it for the theory he rejects rather than the one he proposes. The popular alternative is that the habit comes from the dialect of the people doing the reinforcement work. He tested it and found em dashes made up 0.022 per cent of words in Nigerian English data against 0.25 to 0.275 per cent in general English, which points the opposite way.
A second account, from Maria Sukhareva in July 2025, proposes token economy: an em dash is a single token where the alternatives cost more. She ran a hundred and fifty story prompts across three model versions and found the same roughly tenfold jump. Treat the two as one line of thinking rather than independent confirmation, because the first cites the second.
Can you tell a model to stop?
Historically this was unusually hard and the difficulty is documented rather than folklore. An OpenAI developer forum thread opened in July 2025 collects users failing to suppress it, with reports that the same instruction worked through the API and not in the chat interface.
Whether that still holds on the model you use today is not something this site will tell you, because we publish no measurements of model behaviour. Claims in this area go stale within a release and a dated table would be wrong by the time you read it.
So decide where to put the instruction. A punctuation prohibition survives because it is short and mechanical: a model cannot half-satisfy it. Which instruction shapes survive covers why that shape works when an adjective does not.
What if you want to keep using them?
Then keep using them. The em dash is a legitimate mark with a long history and no amount of model output changes that. Readers now notice, so the practical questions are consistency and density rather than permission.
Pick a convention and hold it: em dash closed up, or spaced en dash, but not both in one document. Inconsistent dash style is a genuine sign of assembled text, and unlike the mark itself it is something a checker can catch.
This site takes the other option and forbids them outright, which is a house rule rather than a recommendation. It is stated in the file the tool generates, because a rule you write down is enforceable and a rule you remember is not.
What this page will not claim
- That the em dash tell has inverted. That claim traces to an article behind a paywall that nobody here has read, so it is not repeated.
- That any particular model now honours a no-em-dash instruction. We run no tests, and reports of this go stale within a release.
- That two independent researchers confirmed a tenfold rise. Two accounts report it, and one cites the other.
- That an em dash means anything about a specific piece of writing. It is a weak signal about a population, which is not the same thing.
Where these come from
Every claim above is quoted from one of these, and each was read on the date beside it. If one of them has changed since, the page is wrong and we would like to know.
- Wikipedia: Dash, on spaced and closed practice checked 2026-08-17
- Wikipedia: Signs of AI writing, on the em dash sign checked 2026-08-17
- Sean Goedecke: why do AI models use so many em dashes checked 2026-08-17
- Maria Sukhareva: the mystery of em dashes, part two checked 2026-08-17
- OpenAI developer forum: instructions to avoid dashes checked 2026-08-17
A punctuation rule is worth more written down than argued about: state yours once and hand it to every assistant.
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