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.

    研究显示练习和自动节律性之间存在一个曲线性关系。

    假设你想要养成一种新习惯，不管是做更多的锻炼，吃更健康的食品或是每天写一篇博客文章，需要多久才不再用费劲的自我控制（而自动执行）?

    显然，它将取决于你想要形成的习惯的种类，和你是多么一心一意地追求你的目标。但在行为成为自动性前有什么普遍的指南吗？

    问google的话，你会得到一个介于21天到28天的数据。事实上对此根本没有可靠的证明。21天的神话很可能来源于一个整形外科医生在1960 年出版的一本书。麦克斯维尔。 莫兹Maxwell Maltz 注意到截肢者平均需要21天来适应断肢的情况，因此他提出人们需要21天来适应任何大的人生改变。

    除非你处于锯断你自己胳膊的习惯养成期，这根本是不相干的。

    不假思索

    无论如何，现在有篇关于这个问题的心理学研究论文最近发表于《欧洲社会心理学杂志》。菲利普。兰利Phillippa Lally和伦敦大学学院的同事招集了96个对养成一个新习惯（例如午饭吃一片水果或每天跑15分钟）有兴趣的人（见兰利2009年的实验）.参与者当时每天被问他们所选择的习惯感觉有多自动性。诸如此类的问题包括行为是否"很难不去做"和可以"不假思索"地去做。

    当研究人员检查这些不同的习惯是，许多参与者展现出下图描绘（实线）出的练习及自动性间的一个曲线关系。平均来说，自动性的一个高峰在66天后到达。换句话说，要形成一个习惯就得需要这么长时间。

    这个图表展示出早期的练习在自动性上的增长率更大，而到参与者达到他们这个行为的最大自动性后所获效应会缩减。

    虽然平均数是66天，在多久养成习惯上有很显着的不同，这项研究所检测的习惯中从18天到254天都有。你可以想象得到，每天喝杯水形成自动性是很快的，而在早餐前做50个仰卧起坐则需要付出更多努力。研究者同样注意到：

    少做一天不会降低养成一个习惯的几率。一个子群比另外的组群养成习惯所耗的时间更长，也许这暗示了某些人是"抵制习惯（体质）的".其他种类的习惯极有可能耗时更长。

    不要小改变

    这个研究所显示的事当我们想要形成一个相对简单的习惯，例如每天吃一片水果或者走10分钟的路，在形成一个习惯前都需要耗费我们超过2个月的时间来每天练习重复。而且，这项研究也显示出，错过一两天长期来说并不算有害，正是那些早期的重复给我们自动性上的最大推进。

    不幸的是，似乎没什么事像（这些）小改变：养成一个习惯的21天重复法是一个相当大的低估，除非你生活中的唯一目标是喝水。