An experiment found that the brain uses one set of neural circuits to identify the numbers 1–4; these circuits are very specific to their own numbers. A separate set of circuits respond to the numbers 5–9; these are less precise, and are activated by adjacent numbers.
For this reason, it is easier to determine when there are four things than to determine there are five things.
I bet this is also why old-timey phone numbers encoded the first digits with letters. The US had the famous ABC2/DEF3/… system that’s still displayed on most keypads, and the former Soviet Union mapped the first letters of the Russian alphabet (skipping З to avoid confusion with 3)…
I don’t think that’s why, not least because this is a recent discovery. The reason is, presumably, because 26+ keys are difficult to fit on a small but still usable keyboard.
Think you misunderstand me. Long before texting was a thing, landline phones (with rotary dials!) also had letters associated with digits. This layout was later transferred to keypads, which in turn became the SMS layout.
