​The '85% Rule': Why a dose of failure optimizes learning

If you're always succeeding, you're probably not learning much.

Photo by Minas Panagiotakis/Getty Images
  • A recent study examined the rates at which machine-learning algorithms learned to recognize images of tumors.
  • The results showed that learning was optimized when the algorithms guessed incorrectly about 15 percent of the time.
  • The researchers suggested that their findings apply to human and animal learning, too.


In learning, most people intuitively recognize that a bit of a challenge is a good thing. The task shouldn't be too hard, nor too easy. This conventional wisdom explains, for example, why the levels of a video game become incrementally more difficult, or why a piano instructor would choose to teach a beginning student "Twinkle Twinkle Little Star" instead of a Chopin Étude.

But exactly how difficult should learning be? Is there a "sweet spot"?

The answer seems to be yes, according to a recent study which found learning is optimized when the learner gets it right about 85 percent of the time. To get that number, scientists trained machine-learning algorithms to recognize images of tumors at various levels of difficulty. They found that the algorithms learned most efficiently when the failure rate was about 15 percent.

The 85% rule for machines and humans

"These ideas that were out there in the education field — that there is this 'zone of proximal difficulty,' in which you ought to be maximizing your learning – we've put that on a mathematical footing," Robert Wilson, an assistant professor of psychology and cognitive science at the University of Arizona, and lead author of the study, told UA News. "If you have an error rate of 15% or accuracy of 85%, you are always maximizing your rate of learning in these two-choice tasks."

Of course, the study involved algorithms, not humans. However, the researchers wrote that their findings also describe optimal learning in humans and animals, "from perception, to motor control to reinforcement learning." In the study, the researchers tweaked their model to reflect the ways in which monkeys learn a task over time. The results showed that, in all scenarios, learning was optimized with an accuracy rate of about 85 percent.

Lung cancer, MRI

Photo by: BSIP/Universal Images Group via Getty Images

Wilson said the 85 percent rule would be particularly applicable in perceptual learning, in which we gradually learn tasks by interacting with the environment, such as learning to identify tumors in images.

"You get better at figuring out there's a tumor in an image over time, and you need experience and you need examples to get better," Wilson said. "I can imagine giving easy examples and giving difficult examples and giving intermediate examples. If I give really easy examples, you get 100% right all the time and there's nothing left to learn. If I give really hard examples, you'll be 50% correct and still not learning anything new, whereas if I give you something in between, you can be at this sweet spot where you are getting the most information from each particular example."

Grit and flow states

But there's another reason why it's important for us to incorporate a healthy dose of failure into learning: it prepares people for the inevitable challenges of life. Tom Hoerr, former leader of the New City School in St. Louis, Mo., said students need to learn not only curriculum, but also the emotional tools necessary to withstand challenges.

"If our kids have graduated from here with nothing but success, then we have failed them, because they haven't learned how to respond to frustration and failure,"Hoerr told KQED.

There's also reason to think following the 85 percent rule could help people enter a flow state — a feeling of being "in the zone" that occurs when you're fully immersed in a task that's appropriately challenging.

"Boredom is where you're not learning, and your accuracy is at 100 percent," Wilson told Psychology Today. "And anxiety is where you're not learning, and your accuracy is at 50 percent or chance. This is pure speculation, but that's something we're excited to think about going forward."

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The surprise reason sleep-deprivation kills lies in the gut

New research establishes an unexpected connection.

Reactive oxygen species (ROS) accumulate in the gut of sleep-deprived fruit flies, one (left), seven (center) and ten (right) days without sleep.

Image source: Vaccaro et al, 2020/Harvard Medical School
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  • A study provides further confirmation that a prolonged lack of sleep can result in early mortality.
  • Surprisingly, the direct cause seems to be a buildup of Reactive Oxygen Species in the gut produced by sleeplessness.
  • When the buildup is neutralized, a normal lifespan is restored.

We don't have to tell you what it feels like when you don't get enough sleep. A night or two of that can be miserable; long-term sleeplessness is out-and-out debilitating. Though we know from personal experience that we need sleep — our cognitive, metabolic, cardiovascular, and immune functioning depend on it — a lack of it does more than just make you feel like you want to die. It can actually kill you, according to study of rats published in 1989. But why?

A new study answers that question, and in an unexpected way. It appears that the sleeplessness/death connection has nothing to do with the brain or nervous system as many have assumed — it happens in your gut. Equally amazing, the study's authors were able to reverse the ill effects with antioxidants.

The study, from researchers at Harvard Medical School (HMS), is published in the journal Cell.

An unexpected culprit

The new research examines the mechanisms at play in sleep-deprived fruit flies and in mice — long-term sleep-deprivation experiments with humans are considered ethically iffy.

What the scientists found is that death from sleep deprivation is always preceded by a buildup of Reactive Oxygen Species (ROS) in the gut. These are not, as their name implies, living organisms. ROS are reactive molecules that are part of the immune system's response to invading microbes, and recent research suggests they're paradoxically key players in normal cell signal transduction and cell cycling as well. However, having an excess of ROS leads to oxidative stress, which is linked to "macromolecular damage and is implicated in various disease states such as atherosclerosis, diabetes, cancer, neurodegeneration, and aging." To prevent this, cellular defenses typically maintain a balance between ROS production and removal.

"We took an unbiased approach and searched throughout the body for indicators of damage from sleep deprivation," says senior study author Dragana Rogulja, admitting, "We were surprised to find it was the gut that plays a key role in causing death." The accumulation occurred in both sleep-deprived fruit flies and mice.

"Even more surprising," Rogulja recalls, "we found that premature death could be prevented. Each morning, we would all gather around to look at the flies, with disbelief to be honest. What we saw is that every time we could neutralize ROS in the gut, we could rescue the flies." Fruit flies given any of 11 antioxidant compounds — including melatonin, lipoic acid and NAD — that neutralize ROS buildups remained active and lived a normal length of time in spite of sleep deprivation. (The researchers note that these antioxidants did not extend the lifespans of non-sleep deprived control subjects.)

fly with thought bubble that says "What? I'm awake!"

Image source: Tomasz Klejdysz/Shutterstock/Big Think

The experiments

The study's tests were managed by co-first authors Alexandra Vaccaro and Yosef Kaplan Dor, both research fellows at HMS.

You may wonder how you compel a fruit fly to sleep, or for that matter, how you keep one awake. The researchers ascertained that fruit flies doze off in response to being shaken, and thus were the control subjects induced to snooze in their individual, warmed tubes. Each subject occupied its own 29 °C (84F) tube.

For their sleepless cohort, fruit flies were genetically manipulated to express a heat-sensitive protein in specific neurons. These neurons are known to suppress sleep, and did so — the fruit flies' activity levels, or lack thereof, were tracked using infrared beams.

Starting at Day 10 of sleep deprivation, fruit flies began dying, with all of them dead by Day 20. Control flies lived up to 40 days.

The scientists sought out markers that would indicate cell damage in their sleepless subjects. They saw no difference in brain tissue and elsewhere between the well-rested and sleep-deprived fruit flies, with the exception of one fruit fly.

However, in the guts of sleep-deprived fruit flies was a massive accumulation of ROS, which peaked around Day 10. Says Vaccaro, "We found that sleep-deprived flies were dying at the same pace, every time, and when we looked at markers of cell damage and death, the one tissue that really stood out was the gut." She adds, "I remember when we did the first experiment, you could immediately tell under the microscope that there was a striking difference. That almost never happens in lab research."

The experiments were repeated with mice who were gently kept awake for five days. Again, ROS built up over time in their small and large intestines but nowhere else.

As noted above, the administering of antioxidants alleviated the effect of the ROS buildup. In addition, flies that were modified to overproduce gut antioxidant enzymes were found to be immune to the damaging effects of sleep deprivation.

The research leaves some important questions unanswered. Says Kaplan Dor, "We still don't know why sleep loss causes ROS accumulation in the gut, and why this is lethal." He hypothesizes, "Sleep deprivation could directly affect the gut, but the trigger may also originate in the brain. Similarly, death could be due to damage in the gut or because high levels of ROS have systemic effects, or some combination of these."

The HMS researchers are now investigating the chemical pathways by which sleep-deprivation triggers the ROS buildup, and the means by which the ROS wreak cell havoc.

"We need to understand the biology of how sleep deprivation damages the body so that we can find ways to prevent this harm," says Rogulja.

Referring to the value of this study to humans, she notes,"So many of us are chronically sleep deprived. Even if we know staying up late every night is bad, we still do it. We believe we've identified a central issue that, when eliminated, allows for survival without sleep, at least in fruit flies."

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