Popular culture is full of numbers that sound believable, but which are actually false. You’ve probably heard these repeated all over the place. Perhaps you’ve even believed some of them - after all, they tend to have some intuitive plausibility. For example:
10,000 hours of practice are required to achieve world-class performance in anything
Humans need 8 hours of sleep per night
It takes 21 days to form a habit
We should walk 10,000 steps per day
This week, we’re diving into these four numbers. Read on to learn what the truth behind these popular falsehoods really is.
1️⃣ 10,000 hours of practice are required to achieve world-class performance in anything
This false idea was popularized by Malcolm Gladwell in his book Outliers. The scientist whose work it's based on (Anders Ericsson) has explained it's a misinterpretation of his work. So, what's true?
Ericsson explains that there's nothing special about 10,000 hours. Gladwell could have mentioned the amount of time the best violin students practiced by the time they're 18 (7400 hours). Alternatively, he could have drawn on the fact that pianists who win international competitions tend to do so around age 30 with 22,000 hours.
To Gladwell's credit, he eventually admitted he was wrong (after having his view criticized): "This is David Epstein. He devoted several pages (of his book) to attack my work and I read the attack and realized that actually he was correct, so we became friends."
A meta-analysis on the link between the number of hours of deliberate practice and sports performance found a correlation of 0.43. That's not nothing, but it still leaves quite a lot unexplained. Intriguingly, among just elite athletes, practice hours explain almost none of the variance!
Click here to see the source of this image.
But there's a kernel of truth in the 10,000-hour "rule." A large amount of practice (where reliable, rapid feedback is received, whether from a coach or just from self-observation of mistakes) is absolutely essential for developing expertise in most fields.
2️⃣ Humans need 8 hours of sleep per night
While this common claim isn't true, it is also not totally false, and the claim exists for some good reasons. So what's actually true?
In three studies we analyzed (where people had been asked what their ideal number of hours of sleep they need to feel their best), the average responses were 7.5, 7.7 and 7.7 hours. And 8 was the most common answer. These numbers come very close to matching the often-cited figure that, on average, sleep cycles are 90 minutes long, and people have 5 of them on a typical night, leading to 90 minutes x 5 cycles = 7.5 hours of sleep.
However, in all three studies, at least half as many people chose 7 hours per night as chose 8 per night, with 9 hours or 6 hours being the third most popular choices. In fact, in each of the three studies, more people chose a time other than 8 hours than chose 8 hours.
Data from PersonalityMap.io, which can be seen here.
The reality is that different people have different sleep needs. It's generally believed that genetics influence how much sleep we need - some lucky people need less. Environmental factors (like chronic health issues, which can impact sleep quality) and age also play a role. And the heuristic that people sleep for 5 cycles of 90 minutes each is also just an approximation - in practice, the duration of sleep cycles varies.
To give a specific example of how genetics may impact sleep, some research suggests that a mutation in the DEC2 gene in both mice and humans may affect sleep duration and speed of recovery from sleep deprivation.
3️⃣ It takes 21 days to form a habit
This false claim seems to somehow have come out of an observation from a plastic surgeon (Maxwell Maltz) who reported that it took 21 days for a patient to get used to their new face (post nose job) or absent limb (post amputation).
So, how long does it actually take before a repeated (previously consciously done action) becomes automatic and effortless? In a neat but small study on this topic, 82 people who were asked to choose a new habit reported (on average) roughly every other day how "automatic" it had become.
Researchers fit a curve to model the increase in this self-reported level of automaticity over time to predict when each participant would reach 95% of their own automaticity "asymptote". The average was 66 days, with a range of 18 to 254.
In reality, how long it takes to form a habit likely depends on a variety of factors:
the complexity of the behavior (simpler behaviors become habits faster)
the traits and skills of the person (martial arts experts can likely build a counter-attack habit faster than beginners)
how often the behavior is repeated (which is different than the "number of days" the habit is practiced for - you may be able to make a many-times-per-day habit automatic in just a few days)
consistency (if you sometimes miss the habit, it will likely become automatic slower than if you do it every single time)
What's more, many things we call "habits" never become automatic, and that's okay.
If you go to the gym 3 times per week without fail, that might not be an automatic behavior, but if it's a consistent routine (rather than a true habit) that you do without fail, that's pretty much just as good!
4️⃣ We should walk 10,000 steps per day
There is a fascinating theory about this misconception. According to epidemiologist I-Min Lee, this likely started as a "marketing tool". The "manpo-kei" was an early commercial pedometer, which apparently translates roughly to "10,000 steps meter". Lee says, "Not only is 10,000 an easy number to remember, the character for 10,000 in Kanji - a script used in Japanese, looks a little like a person walking": 万
Either way, the 10,000 steps figure has “limited scientific basis”.
That said, numerous studies link greater exercise to improved health and lower mortality, and having a goal (even an arbitrary one) like 10,000 steps (roughly 5 miles) per day can be great for people who tend not to move around much.
If you find that using the number 10,000 as a step goal (e.g., to track on a smartwatch) motivates you to walk more, that's a good thing! Stick with it. It's just not some kind of magic number. But it may be a helpful goal.
What lessons can you learn from this?
When a specific number is quoted it can make a recommendation sound credible. But some popular health and behavior change advice is based on fairly arbitrary numbers that lack a strong scientific foundation.
We should have some skepticism when it's claimed that there is a magic number for human health or behavior change. Human traits, needs and behaviors tend to vary substantially from person to person. This means that while many people will exhibit an average amount of a trait or need (such as requiring an average number of hours of sleep to feel their best), a significant number will need slightly more or less, and a few will fall quite far from the average. A simple rule of thumb can be useful at times, even if it ignores some of this variation, but then it's worth asking where this rule of thumb came from, and whether there is actually science behind it in the first place.
However, that doesn’t mean that such numbers are of no use at all. Studies consistently show that human health is complicated and multi-faceted, but many of the proposed numbers discussed above have an aspect of truth to them: expertise requires the dedication of a lot of time (even if it's not 10,000 hours like claimed); it’s possible to have too much or too little sleep, so finding a number that works for you can be healthy, and 8 is not a bad place to start from; and so on.
The numbers in popular discourse can be a helpful starting point for some people, but they should be treated with care. It can help to know what kind of range the science actually says is relevant and remember that even if a number is based on the average, it may not be the right number for you.
If you’re interested in going further with examining what's true and what's B.S. when it comes to popular ideas, why not try our Common Misconceptions Quiz, which asks you to identify the misconceptions among 30 common beliefs.
Thanks to Hashem Elassad for alerting us to the fact that Gladwell had admitted he was wrong.