The mechanism behind the habit
- Older calculators carried more than one clearing control, typically a clear-entry alongside a full clear, and they did different things.
- Clear-entry removes the number you just typed but leaves the pending operation intact, so after entering a sum and pressing it you see zero while the machine still holds the first operand and the operator.
- On some models one press cleared the entry and two cleared everything, with a third sometimes reaching the memory.
- None of this was visible on the display, which is the whole problem.
Why it persists
The highest-voted answer put it simply: the device has state because it performs multi-step operations, but it does not clearly show that state, so people press reset several times to be sure. The habit outlived the hardware that caused it. Someone who once used a calculator requiring two presses learned it there and carried the behaviour to every calculator since, including ones where a single press has always been enough.
Why nobody unlearns it
One answer inverted the question usefully. Pressing clear an extra time costs nothing and has no bad outcome, so the interesting question is not why people do it but why they would ever stop. A habit that is occasionally necessary and never harmful has no pressure acting against it, which is exactly the condition for it to survive indefinitely.
The lesson that transfers
- If a control has more than one meaning depending on context, the context has to be visible or people will guess by repeating the action.
- A display that looks the same in two different states is not neutral — it actively teaches distrust of the control.
- Repeated pressing is a signal worth watching for in any interface. It usually means the person cannot tell whether the first press worked.