Science Interviews


Tue, 14th Jan 2014

How long does it take to form a habit?

Ben Gardner, University College London

Listen Now    Download as mp3 from the show Are old habits hard to break?

Ginny -  Let’s say that I can use my willpower or pre-commitment strategy to resist chocolate this year and I try to eat say, my healthy greens instead.  How long do I need to keep doing that before it becomes a habit to eat healthily and I stop having to think about it all the time?  To try and answer that, we’re joined by Ben Gardner from the Health Behaviour Research Centre, University College London.  Hi, Ben.

Ben -  Hello.

Ginny -   Hi, there.  So, I want my healthy eating become a habit this year.  What I mean by that is that I want to do a lot and I don't want to have to keep thinking, “I really should eat my greens every day.”  But what does the word ‘habit’ mean to you as a psychologist?

Ben -   Well, it’s an interesting question actually.  I mean, in the colloquial sense, ‘habit’ is used to refer to anything that we do repeatedly that's stable and persistent over time.  But within psychology, we use the term in a slightly different way.  As a couple of your previous speakers mentioned, we refer to habitual behaviour as behaviour that's elicited automatically when we encounter a particular situation.  So, if you do want to get a new behaviour up and running then this perspective would suggest that you repeat it so often that the behaviour becomes automatic.  The way that it becomes automatic is through repeatedly doing the behaviour in a given situation so that an association develops between the situation and the behaviour.  That association becomes so strong that subsequently encountering the situation is sufficient to prompt your behaviour without you really needing to think about it.  That's when we say it’s become an automatic behaviour.

Ginny -   So, if I want to get into the habit of eating an apple in the afternoon, what I have to do is every time I make my cup of tea at 3 o’clock, I eat an apple and then after awhile, that will become automatic and I’ll pick up the apple without thinking of it.

Ben -   Well, yes.  I mean, that's what we’d suggest.  The more specific your plan, the better.  It is best to think about for example, when I'm going to make my cup of tea, I will say, eat an apple or to tie it to something that you already do rather than to say for example, “In the afternoon, I will eat an apple.”  So yes, if you want to do the behaviour repeatedly then it’s a kind of generic principle of behaviour change.  Set yourself a plan and be as specific as possible, and realistic as possible within the context of that plan.

Ginny -   Now, I've read in quite a few places that it takes 21 days to do something which become a habit.  So, I’d have to spend 21 days remembering to eat an apple with my tea and then it would become automatic.  Is that true and where did that idea come from?

Ben -   That's a very interesting concept.  I mean, given how kind of well-known and pervasive its notion of it requiring 21 days to make a habit is.  I mean, you'd expect that there's a solid body of evidence behind that.  In fact, as part of my work, I actually looked into the kind of evidence base.  So, this actually comes from a book that was written by a plastic surgeon in 1960 – it was a self-help book.  It wasn’t a kind of scientific text, though.  It didn’t kind of undergo peer review or anything.  It was just based on his observations that among his patients, it tended to take them 21 days to get used to their new appearance.  He also then kind of extrapolated from that and said, well actually, it seems to take about 3 weeks to become used to being in a new house for example.  From that, this entire thing has kind of snowballed and we’re now in a situation where it’s kind of assumed that it takes 21 days to form a habit.  There's an intuitive appeal to it, but unfortunately, it’s not baseless but it’s based on very weak evidence indeed.

Ginny -   So, is there any evidence as to how long it does take for these habits to form?

Ben -   Yes.  Within our department, we did a study recently that suggested that it takes on average, 66 days for a habit to form, but that comes with a number of caveats.  Within this study, this was a study that was done where 96 postgraduate students were participants in the study.  They were asked to choose a behaviour that they wanted to do, a new behaviour.  So, that could be a dietary behaviour or a physical activity.  And they were asked to do that behaviour once a day every day for a period of 12 weeks.  They have to choose a particular context in which they do it.  So, for example, after breakfast, I will go for a walk and so on.  Now, what the participants had to do was, they had – not only do the behaviour but each day, they have to log on to a study website to log whether they'd done the behaviour and also, how automatic the behaviour felt.  So, that gives us that measure of habit as we as psychologists define it.  What we actually found was, there was massive variation in how long it took for people to reach their plateau of habit strength.  In other words, as automatic as this behaviour was going to become.  In fact, the median value is 66 days.  So, that's really a kind of headline finding.  If you dig a little deeper, something that I needed to put across is that actually, there was a massive variation in how long it took for people’s automaticity or habit strength to peak.  There were some people that reached the plateau of their automaticity within just 18 days.  There was one person who didn’t reach their peak at all within the 12-week period, but it was forecast that if they carried on doing the behaviour, it would take them 254 days so as you can see, a massive range.  And there's also variation in the level at which the habit peaked.  So, for some people, it would peak around the midpoint of the scale – a 0 to 42 scale that was used to measure habit.  For some people, it peaked there, whereas for others, it peaked to the very top of the scale suggesting that some people could actually form stronger habits than others.  But why there should be that difference, we’re not really sure.



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