Say you want to create a new habit, whether it's taking more exercise, eating more healthily or writing a blog post every day, how often does it need to be performed before it no longer requires Herculean self-control?

    Clearly it's going to depend on the type of habit you're trying to form and how single-minded you are in pursuing your goal. But are there any general guidelines for how long it takes before behaviours become automatic?

    Ask Google and you'll get a figure of somewhere between 21 and 28 days. In fact there's no solid evidence for this number at all. The 21 day myth may well come from a book published in 1960 by a plastic surgeon. Dr Maxwell Maltz noticed that amputees took, on average, 21 days to adjust to the loss of a limb and he argued that people take 21 days to adjust to any major life changes.

    Unless you're in the habit of sawing off your own arm, this is not particularly relevant.

    Doing without thinking

    Now, however, there is some psychological research on this question in a paper recently published in the European Journal of Social Psychology. Phillippa Lally and colleagues from University College London recruited 96 people who were interested in forming a new habit such as eating a piece of fruit with lunch or doing a 15 minute run each day Lally et al. （2009）. Participants were then asked daily how automatic their chosen behaviours felt. These questions included things like whether the behaviour was 'hard not to do' and could be done 'without thinking'.

    When the researchers examined the different habits, many of the participants showed a curved relationship between practice and automaticity of the form depicted below （solid line）. On average a plateau in automaticity was reached after 66 days. In other words it had become as much of a habit as it was ever going to become.

    This graph shows that early practice was rewarded with greater increases in automaticity and gains tailed off as participants reached their maximum automaticity for that behaviour.

    Although the average was 66 days, there was marked variation in how long habits took to form, anywhere from 18 days up to 254 days in the habits examined in this study. As you'd imagine, drinking a daily glass of water became automatic very quickly but doing 50 sit-ups before breakfast required more dedication （above, dotted lines）. The researchers also noted that:

    * Missing a single day did not reduce the chance of forming a habit.

    * A sub-group took much longer than the others to form their habits, perhaps suggesting some people are 'habit-resistant'.

    * Other types of habits may well take much longer.

    No small change

    What this study reveals is that when we want to develop a relatively simple habit like eating a piece of fruit each day or taking a 10 minute walk, it could take us over two months of daily repetitions before the behaviour becomes a habit. And, while this research suggests that skipping single days isn't detrimental in the long-term, it's those early repetitions that give us the greatest boost in automaticity.

    Unfortunately it seems there's no such thing as small change: the much-repeated 21 days to form a habit is a considerable underestimation unless your only goal in life is drinking glasses of water.

    研究顯示練習和自動節律性之間存在一個曲線性關系。

    假設你想要養成一種新習慣，不管是做更多的鍛煉，吃更健康的食品或是每天寫一篇博客文章，需要多久才不再用費勁的自我控制（而自動執行）?

    顯然，它將取決于你想要形成的習慣的種類，和你是多么一心一意地追求你的目標。但在行為成為自動性前有什么普遍的指南嗎？

    問google的話，你會得到一個介于21天到28天的數據。事實上對此根本沒有可靠的證明。21天的神話很可能來源于一個整形外科醫生在1960 年出版的一本書。麥克斯維爾。 莫茲Maxwell Maltz 注意到截肢者平均需要21天來適應斷肢的情況，因此他提出人們需要21天來適應任何大的人生改變。

    除非你處于鋸斷你自己胳膊的習慣養成期，這根本是不相干的。

    不假思索

    無論如何，現在有篇關于這個問題的心理學研究論文最近發表于《歐洲社會心理學雜志》。菲利普。蘭利Phillippa Lally和倫敦大學學院的同事招集了96個對養成一個新習慣（例如午飯吃一片水果或每天跑15分鐘）有興趣的人（見蘭利2009年的實驗）.參與者當時每天被問他們所選擇的習慣感覺有多自動性。諸如此類的問題包括行為是否"很難不去做"和可以"不假思索"地去做。

    當研究人員檢查這些不同的習慣是，許多參與者展現出下圖描繪（實線）出的練習及自動性間的一個曲線關系。平均來說，自動性的一個高峰在66天后到達。換句話說，要形成一個習慣就得需要這么長時間。

    這個圖表展示出早期的練習在自動性上的增長率更大，而到參與者達到他們這個行為的最大自動性后所獲效應會縮減。

    雖然平均數是66天，在多久養成習慣上有很顯著的不同，這項研究所檢測的習慣中從18天到254天都有。你可以想象得到，每天喝杯水形成自動性是很快的，而在早餐前做50個仰臥起坐則需要付出更多努力。研究者同樣注意到：

    少做一天不會降低養成一個習慣的幾率。一個子群比另外的組群養成習慣所耗的時間更長，也許這暗示了某些人是"抵制習慣（體質）的".其他種類的習慣極有可能耗時更長。

    不要小改變

    這個研究所顯示的事當我們想要形成一個相對簡單的習慣，例如每天吃一片水果或者走10分鐘的路，在形成一個習慣前都需要耗費我們超過2個月的時間來每天練習重復。而且，這項研究也顯示出，錯過一兩天長期來說并不算有害，正是那些早期的重復給我們自動性上的最大推進。

    不幸的是，似乎沒什么事像（這些）小改變：養成一個習慣的21天重復法是一個相當大的低估，除非你生活中的唯一目標是喝水。