Guesses have a strange way of disguising themselves as facts, and taking root in popular knowledge. Consider the claim that the human nose can distinguish between 10,000 different smells. The statement crops up in all manner of websites, along with textbooks and scientific publications.

The figure came from a paper published in 1927, which suggested that people could tell the difference between odours according to four different qualities—fragrant, acidic, caprylic, and burnt—along a nine-point scale. That gives us 6,561 distinguishable smells, which was later rounded up to 10,000!

And that’s it.

There wasn’t any evidence for any of these assumptions, but that didn’t stop an uneducated guess from becoming enshrined as fact.

When Andreas Keller at Rockefeller University learned about this,