A habit is a behaviour that a context without a decision being made.
The definition matters because it explains why is a poor instrument for changing one.
Motivation operates on decisions, and a habit has removed the decision.
Laboratory work suggests that in a stable context is the mechanism that matters.
The frequently quoted figure of twenty-one days has no basis in that literature.
Reported times to vary from eighteen days to well over two hundred, depending on the behaviour.
Drinking a glass of water becomes automatic quickly; a forty-minute run may never do so.
Changing a habit is easier when the context changes at the same time.
Moving house, changing job or returning from a long trip all the that maintained the old behaviour.
Public health campaigns that target people at those moments outperform the same campaigns sent at random.
The practical instruction that follows is narrow and unexciting.
Attach the new behaviour to a cue that already exists, keep it small enough to complete on a bad day, and repeat it in the same place.
Most published advice adds a , and the evidence for s in habit formation is weaker than for .
A may even become the cue, so that the behaviour stops when the does.
What survives without attention is the only reasonable definition of a habit that has formed.