Most smokers will tell you it takes a lot more than three weeks of mindful thinking to disrupt those synaptic pathways.

Photo courtesy of Iowa Department of Administrative Services

Habits: Make It and Break It

No one is entirely sure where the 21-day rule originates, but it seems to have first been set forth in a book called "Psycho-Cybernetics." It's a self-help book first published in the 1970s, and in it, you find out you can create or break a habit in just 21 days.

The problem is, the evidence supporting the theory is empirical, or based on experience, not clinical, or based on controlled experiments [source: Benefit]. The theory caught on, though, and has been backed up in other forums since then. In 1983, for instance, a woman chronicled her efforts to start flossing and stop criticizing in a piece for "Reader's Digest." The article was called "Three Weeks to a Better Me."

But does it really work for everyone, or are these just the experiences of a couple of individuals?

The reality is, habits are easier to make than they are to break. If you repeat a behavior often enough, those synaptic pathways are going to get worn in. The human brain is a very adaptive piece of machinery. But does that take 21 days? Who knows? Everyone's brain is different, and habit formation also relies on aspects of experience and personality.

Breaking a habit is a lot more complicated, because while parts of those worn-in pathways can weaken without use, they never go away [source: Rae-Dupree]. They can be reactivated with the slightest provocation [source: Delude]. If you've ever tried to quit smoking, you already know this. You can go a year without a cigarette, and then give in one time and BAM, the habit comes right back.

The best you can do, then, is to form a new, parallel pattern, like exercising when you feel stress, rather than indulge the old pattern, which triggers "cigarette" in response to stress.

So what about these 21 days?