Your brain reads 9.99 and hears 9

In 2003, researchers from Chicago and MIT ran a test with a real clothing catalogue: the same dress was offered at $34, $39 and $44. The $39 version won. Raising the price boosted sales — as long as it ended in nine.

The explanation is called the left-digit effect. We read from left to right, and the brain anchors value to the first figure: 9.99 gets filed away as "nine-something", even though it sits one cent from ten. We process the price before we finish reading it.

That's why prices ending in 9 have ruled shop windows, petrol stations and supermarkets for over a century. It isn't tradition: it works, and companies have measured it a thousand times.

Next time you see 9.99, make the correction yourself: say "ten" out loud. It's remarkable what that does to a purchase.