How habits work

How much of daily life runs this way? In two diary studies published in 2002, the psychologist Wendy Wood and her colleagues gave American college students a watch that chimed every hour. At each chime the students wrote down what they were doing and what they were thinking. An action counted as a habit if they did it just about every day in a stable context, usually the same place.

In the larger study, 43 per cent of the actions they recorded were habits. Washing and dressing were habits almost nine times in ten, schoolwork only about a third of the time. The diaries also caught how a habit feels from the inside. During habits, the students' minds were more often on something unrelated to the action. During other actions, their thoughts were usually on what they were doing.

How does a context gain this power? Mostly through repetition. Each time you do the same thing in the same context, the link between them grows a little stronger, until the context can call up the response by itself. Rewards help: an action that pays off strengthens the link it has just used.

Popular books picture this as a habit loop. A cue sets off a routine, the routine earns a reward, and the reward makes you want to run the loop again. It is a useful picture, but the research adds a twist. Reward matters most while a habit is still forming. Once the habit has formed, the cue alone can set it off, even when you no longer want the reward. A habit can outlive the reason it began.

How can you tell whether an action is a habit or aimed at a goal? Animal researchers use a test with rats. In a 1982 study by the psychologist Christopher Adams, rats first learned to press a lever for sugar. Then the sugar was spoiled for them: it was paired with a bout of sickness, so they no longer ate it. Finally the lever came back, with no sugar at all.

A rat pursuing a goal should now lose interest, since the goal is worthless. Rats trained briefly, for about a hundred rewarded presses, did press less. But rats that had earned about five hundred rewards kept pressing as if nothing had changed. Spoiling a reward like this is called devaluation, and a response that carries on regardless has become a habit: it no longer checks what it is for.

Do people behave the same way? In a 2011 study, David Neal, Wendy Wood and their colleagues gave moviegoers at a campus cinema a box of popcorn while they rated film trailers. Some boxes were fresh; others had been popped a week earlier and were stale, and people liked them less. The researchers also measured habit strength: how often each person usually ate popcorn at the cinema.

People with weak habits ate less of the stale popcorn. People with strong habits ate about as much stale popcorn as fresh, around 63 per cent of the box, even though they disliked it. But the habit belonged to the cinema. When strong-habit eaters watched music videos in a meeting room, where the context was different, they ate about half as much stale popcorn as fresh. Away from its cues, the habit let their taste decide.

What changes in the brain as an action turns into a habit? Much of the work happens in the basal ganglia, a cluster of structures deep beneath the brain's wrinkled outer layer, and especially in their largest part, the striatum.

In the 1990s the neuroscientist Ann Graybiel and her team at MIT recorded neurons in the striatum of rats learning a maze. A tone told each rat whether to turn left or right for a reward. While the rats were still learning, the neurons fired all through the run. Once the run was automatic, the firing gathered at its start and its finish, as if the whole run had been bundled into one unit that starts, runs and stops by itself. Graybiel calls this bundling of a sequence of steps into a single unit chunking.

A popular claim says a new habit takes 21 days to form. In 2010 Phillippa Lally and her colleagues at University College London put this to the test. Each of 96 volunteers chose a small healthy behaviour, such as eating a piece of fruit with lunch, and did it every day in the same context for 12 weeks, rating each day how automatic it felt.

Automaticity typically climbed quickly at first, then more slowly, until it levelled off at a plateau. Among the 39 volunteers whose results fitted this curve well, the median time to reach it was 66 days: half took longer, half less. The spread was huge, from 18 days to an estimated 254. Missing a single day did no lasting harm. When a volunteer resumed, automaticity soon picked up again.

If you truly decide to change a habit, why does the old habit so often win? In 2006 Thomas Webb and Paschal Sheeran pooled 47 experiments that changed what people intended to do. A medium-to-large change in intentions produced only a small-to-medium change in behaviour. This shortfall is known as the intention–behaviour gap, and changing intentions did least for actions repeated often in a stable context, the ones most likely to be habits.

The reason lies in how a habit runs. The habit is fast and automatic; the intention is slow and deliberate, and has to be remembered at the right moment. Deciding to change leaves the cue in place. When the cue appears, the habit fires first, often before the new intention even comes to mind.

If a habit is a response handed over to a cue, you can hand a new response to a cue on purpose. The psychologist Peter Gollwitzer called such a plan an implementation intention, or if-then plan: "If situation X arises, then I will do Y." For example: "If I pour my morning coffee, then I will take my tablet." Because the decision is made in advance, the planned action starts almost automatically when the moment arrives, with little need for willpower.

A 2006 review of 94 tests found that such plans helped people reach their goals, with a medium-to-large effect. A much larger 2024 review of 642 tests put the typical benefit at about half that size: modest, but real. The if-then form itself matters: in that review, plans that merely scheduled when and what to do helped less.

Because a habit depends on its cues, a change of setting can break it. In a 2005 study, Wendy Wood and her colleagues followed students who transferred to a new university. Their habits of exercising, reading the newspaper and watching television survived the move only where the setting stayed similar, for example if they still read the paper with other people. Where the old cues were gone, what some students did followed their current intentions instead.

Researchers call this habit discontinuity: a life change such as moving house or starting a new job opens a brief window for change. In a 2016 field experiment, Bas Verplanken and Deborah Roy encouraged households to adopt greener everyday habits. It changed the behaviour of people who had moved house within the previous three months, but not of those who had moved longer ago or not at all.

If habits live in contexts, changing the context is often a more reliable way to change one than trying harder. There are two main levers. The first is taking away the cue. When several British supermarkets took sweets and crisps away from their checkouts, purchases of small packs of those snacks to take home fell by about 17 per cent straight away.

The second lever is friction: the small effort that stands between you and an action. A 2019 review of twelve studies found that people ate less of a snack when it was placed farther from them, though the evidence was of low certainty. Friction works in both directions. Make an unwanted habit a little harder to start, such as keeping biscuits on a high shelf, and a wanted one a little easier, such as leaving your running shoes by the door.

People with strong self-control tend to do better in many areas of life. The usual explanation is that they are better at resisting temptation. The evidence suggests another story. In 2015 Brian Galla and Angela Duckworth reported six studies of more than 2,200 people. Those higher in self-control had stronger good habits, such as regular exercise, healthy snacking and a steady sleep schedule, and they reported less effortful resisting, not more. Their habits accounted for part of the link between self-control and outcomes such as finishing homework and getting good grades.

An earlier study that sampled people's desires through the day, for a week, found a similar pattern: people high in self-control reported fewer attempts to resist temptation. Much of what looks like willpower is a life arranged so that the right action is the automatic one.

Islamic ethics has long held that character is built by practice. Ibn Miskawayh, who died in 1030 and built on Greek ethics, defined a trait of character (khuluq) as a state of the soul that moves a person to act without thought or deliberation. Some traits, he wrote, are inborn; others are acquired by repeating the actions of a trait until it becomes a settled disposition.

Al-Ghazālī drew on him in his Iḥyāʾ ʿulūm al-dīn, in the book on disciplining the soul. If character could not change, he argued, advice and discipline would be pointless; even a falcon or a horse can be trained. A miser who wants to become generous must make himself give, again and again, until giving comes easily and even brings pleasure. In a hadith in al-Bukhārī and Muslim, the Prophet said the deeds most beloved to Allah are the most regular, even if small.