An unconscious crack of the knuckles, an automatic grab for the salt shaker, a seemingly innocuous eye roll in front of the boss. Bad habits are way too easy to come by and, despite whatever quick-remedy self-help cure-all is blowing up the Internet this week, often way too hard to break.

Whether it's that innocent flip of the hair or something much more insidious, behaviors learned over time — and reinforced time and time again — mostly can't be changed in a couple weeks. That was the major takeaway of a study done in 2009, the results of which appeared in the European Journal of Social Psychology.

Certainly, bad habits can be changed. That's the good news. People stop smoking, give up chocolate, get off the couch to start exercising and stop torturing their poor knuckles all the time. Good habits can be formed to replace bad ones, too. In fact, switching out nasty for nice is something scientists and psychologists have been preaching for years.

But changing a lifetime of cola-guzzling, for just an example, in a couple weeks? In 21 days?

Well, it's possible. Maybe. But you'd better plan for it to take a lot longer than that.

"I think that's one of the biggest problems, when people think they can do anything in three weeks," Amy Morin, a psychotherapist, clinical social worker and author of "13 Things Mentally Strong People Don't Do," told MNN. "I think that can set us up for failure."

Number 13 on Morin's list — which first appeared as a blog post on, then went practically viral in other places, with more than 10 million total views — is especially pertinent when it comes to breaking bad habits. Mentally strong people, Morin insists at the bottom of her list, don't expect immediate results.

"When you think about it, even from a logical level, it makes no sense," Morin said of the 21 days to a miracle movement. "We really like our habits. [Breaking them] requires a lot of hard work. I think we underestimate how hard it's going to be. And we overestimate our abilities [to break the habits]."

The paper in the European Journal of Social Psychology studied the length of time it took participants in a study to replace bad habits with good ones. The fastest was an astonishing 18 days. But the average time to change, among the participants who self-reported their results, was not three weeks but closer to three months (66 days). The high end of the spectrum, for replacing bad with good, was a whopping 254 days.

So someone expecting to change a life habit in 21 days — no matter how motivated that someone might be, or who that someone might be — is probably expecting a little too much. Still, huge numbers of books (just check out this Amazon list) all but promise that it can be done in 21 days or so by following a few easy steps. And, of course, by shelling out a few bucks for the book. Plus shipping and handling.

"The more you do it," Patricia A. Farrell, PhD, a clinical psychologist in Englewood, New Jersey, and author of "How to Be your Own Therapist," told WebMD, "the more difficult it is to get rid if it."

It's hard to pinpoint where the idea that it takes just 21 days to kick a bad habit began. Many cite a 1960 work from a plastic surgeon, Dr. Maxwell Maltz, who is regarded as a pioneer in the self-help book industry. His book, "Psycho-Cybernetics: A New Way to Get More Living Out of Life," centers on improving a person's self-image. Some of his advice for breaking bad habits: "To change a habit, make a conscious decision," he said, "then act out the new behavior."

Ahhh, if only it were that simple.

A lot of psychological and biological reasons exist to explain why it's so difficult to lose a bad habit. One important one: Some of our most enjoyable or satisfying actions (say, reaching for the salt or pulling off that oh-so-sweet crack of the knuckles) trigger the production of dopamine in the brain. Dopamine is present when we do it over and over again, creating the habit, Dr. Russell Poldrack, a neurobiologist at the University of Texas at Austin, says in this National Institute of Health feature. And dopamine creates a craving to repeat the behavior. Again and again and again.

So even when the behavior itself does not provide the satisfaction we crave — that salt just isn't making those fries taste any better, and that knuckle-crack was just okay — the body is telling you to keep going for it anyway.

It's important to keep in mind, as experts everywhere will tell you, that kicking a habit, even the nastiest of habits, is doable. Whole books — heck, whole libraries — are available to explain the steps that must be taken to break a bad habit and keep it broken. First off: acknowledging that there is a bad habit. Writing down your goals.  And "put your gym shoes where the remote is," Morin suggests for a start to kicking that TV addiction and getting in shape.

The better news is that trashing those bad habits and replacing them with good ones can be absolutely transformational. Even if it might take a little more than three weeks to do it.

Related on MNN: