Some of the action times listed in Table 4.1, “Timings for various interface actions” are so long that it seems fair to wonder how human beings ever get anything done at all! The answer is implicit in the remark on long-term memory retrieval: habituation. As humans repeat tasks, they form habits — a change that pushes the task from conscious cognition to unconscious cognition. This change observably cuts way down on task times and increases quality of performance.

This is where it helps to remember that consciousness seems to be a recently-evolved subsystem for coping with novelty, layered over mind/brain circuitry intended to deal with routines in a more habitual and much less self-aware way. Even physical skills that are learned consciously often cannot be practiced at a level of mastery when one is being conscious about them; the very act of attention gets in the way. The same is true of many cognitive skills, including those involved in using GUIs.

Raskin makes the empirical point that human beings cannot be prevented from forming habits about routine tasks, and elegantly defines good interfaces as those which encourage benevolent habituation. We can go a bit further and observe that “benevolent habituation” — the ability to do the right thing by unconscious or half-conscious reflex — is precisely what we normally mean by expertise.

Thus, our Rule of Modelessness. Users will form gestural habits, and it is a bad thing when gestural habits they form in one part of the system become inappropriate or even destructive in another part. When this leads to problems, the fault lies not with the user's habits but with system designers' failure to maintain consistency.

Mediocre interfaces don't support the development of expertise. Actively bad interfaces create dangerous habits; they promote anti-expertise. Raskin observes “Any confirmation prompt that elicits a fixed response soon become useless” but the actual situation is worse than the word “useless” implies. Potentially destructive operations, for which racing through a confirmation prompt becomes reflexive, are traps that turn habituation into a weapon against the user's interests.

Thus, our Rule of Confirmation: Every confirmation prompt should be a surprise. It is better still to eliminate confirmation prompts and irreversible operations entirely, as suggested in our Rule of Reversibility: Every operation without an undo is a horror story waiting to happen.

The usual outline of that horror story is that a habituated user running through a sequence of actions that he/she has learned to treat as a unit realized just too late that he is in the unusual circumstances where those reflexes are inappropriate, and has stepped on something critical. It is no use trying to discourage users from forming habits; that would prevent them from gaining expertise. The correct adaptation is to support undo so users can always back out of the jams their habits occasionally land them in.

In these latter days of abundant computation and inexpensive memory, the only good excuse an interface designer has for not supporting undo is when the software is triggering some intrinsically irreversible action in the physical world, or some financial/legal move like a stock trade in which non-repudiation is part of the definition. In the world of information that most computer programs inhabit, full undo with a reversible transaction history should be considered mandatory.