You do not swallow spiders in your sleep, and the debunk is also made up

There is a short answer about swallowing spiders while asleep and a useful one, and they are not the same. What follows is the useful one.

The claim

A specific number of spiders is said to be swallowed by the average person during sleep over a lifetime, or in some versions each year. The number varies between tellings, which is the first sign that nobody is quoting a measurement. It circulates in lists of surprising facts and is repeated with the confidence that a precise figure always attracts.

No study, survey or observation has ever been produced in support of it, and it is difficult to imagine the methodology. Counting swallowed spiders would require watching sleeping people continuously for years, which nobody has funded.

Why it is biologically unlikely

A sleeping human breathes, snores, radiates heat and produces constant low-level vibration through the bed and the floor. Spiders detect vibration acutely and treat it as the primary signal of something large and dangerous, or something worth avoiding.

Web-building species stay near their webs and have no reason to cross an open expanse of bedding towards a source of noise. Hunting species are looking for insects, and a human mouth offers neither prey nor shelter. The occasional spider does end up in the wrong place, but the routine and repeated event described by the claim has no mechanism.

The famous debunking

A widely circulated account holds that the claim was planted deliberately in a 1990s magazine column by a writer demonstrating that people repeat anything they read online. The story is usually told with a name, a publication and a year, which gives it the appearance of documentation. Nobody has produced the article, the issue, or evidence that the named person existed, despite considerable effort by people who tried.

Trace it back and the debunking is therefore in exactly the same evidential position as the claim it debunks, which is a pleasing symmetry. It also demonstrates its own supposed point, since it spread on the strength of being a good story about credulity.

Why the debunk was so attractive

It supplies a motive, a villain and a moral, which the bare correction of an unsourced statistic cannot offer. It flatters the teller, who is now not merely correcting a myth but revealing a clever hoax that fooled everyone else. It also arrived at a moment when anxiety about misinformation online was becoming a popular topic in its own right.

A story about how easily false things spread is one of the false things most likely to spread.

Checking it would have required looking for a magazine issue, which is exactly the work nobody does.

What actually happens with spiders indoors

Most spiders found in houses are species adapted to indoor conditions, and many never live outdoors at all. Wandering males looking for mates account for a large share of the ones people encounter, which is why sightings cluster seasonally. They are attracted to quiet undisturbed spaces, which describes the corners of a room rather than an occupied bed.

Insects, including some that people would much rather not think about, are far more likely to end up in a mouth than spiders are. None of this is a reason to worry, and the household spider is doing useful work on the insect population.

Printing a correction has a poor record of taking a myth out of circulation.

How to handle a fact like this

Two questions dispose of most such claims: who counted, and where was it published. If the answer to either is a list of surprising facts, the claim has no source and should be treated accordingly. The same questions should be asked of the correction, which is the step almost everyone skips.

Checked against the record, a debunking that is more entertaining than the myth deserves more scrutiny than the myth, not less. This particular pair is a useful exercise, because both halves fail the same test for the same reason.

The takeaway

A story about how easily false things spread turned out to be one of the false things that spread most easily.

Ask who gained from the claim being repeated. That usually explains the rest.