While Mischel was beginning to dismantle the methods of his field, the Harvard psychology department was in tumult. In 1960, the personality psychologist Timothy Leary helped start the Harvard Psilocybin Project, which consisted mostly of self-experimentation. Mischel remembers graduate students’ desks giving way to mattresses, and large packages from Ciba chemicals, in Switzerland, arriving in the mail. Mischel had nothing against hippies, but he wanted modern psychology to be rigorous and empirical. And so, in 1962, Walter Mischel moved to Palo Alto and went to work at Stanford.

There is something deeply contradictory about Walter Mischel—a psychologist who spent decades critiquing the validity of personality tests—inventing the marshmallow task, a simple test with impressive predictive power. Mischel, however, insists there is no contradiction. “I’ve always believed there are consistencies in a person that can be looked at,” he says. “We just have to look in the right way.” One of Mischel’s classic studies documented the aggressive behavior of children in a variety of situations at a summer camp in New Hampshire. Most psychologists assumed that aggression was a stable trait, but Mischel found that children’s responses depended on the details of the interaction. The same child might consistently lash out when teased by a peer, but readily submit to adult punishment. Another might react badly to a warning from a counsellor, but play well with his bunkmates. Aggression was best assessed in terms of what Mischel called “if-then patterns.” If a certain child was teased by a peer, then he would be aggressive.

One of Mischel’s favorite metaphors for this model of personality, known as interactionism, concerns a car making a screeching noise. How does a mechanic solve the problem? He begins by trying to identify the specific conditions that trigger the noise. Is there a screech when the car is accelerating, or when it’s shifting gears, or turning at slow speeds? Unless the mechanic can give the screech a context, he’ll never find the broken part. Mischel wanted psychologists to think like mechanics, and look at people’s responses under particular conditions. The challenge was devising a test that accurately simulated something relevant to the behavior being predicted. The search for a meaningful test of personality led Mischel to revisit, in 1968, the protocol he’d used on young children in Trinidad nearly a decade earlier. The experiment seemed especially relevant now that he had three young daughters of his own. “Young kids are pure id,” Mischel says. “They start off unable to wait for anything—whatever they want they need. But then, as I watched my own kids, I marvelled at how they gradually learned how to delay and how that made so many other things possible.”

A few years earlier, in 1966, the Stanford psychology department had established the Bing Nursery School. The classrooms were designed as working laboratories, with large one-way mirrors that allowed researchers to observe the children. In February, Jennifer Winters, the assistant director of the school, showed me around the building. While the Bing is still an active center of research—the children quickly learn to ignore the students scribbling in notebooks—Winters isn’t sure that Mischel’s marshmallow task could be replicated today. “We recently tried to do a version of it, and the kids were very excited about having food in the game room,” she says. “There are so many allergies and peculiar diets today that we don’t do many things with food.”

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Mischel perfected his protocol by testing his daughters at the kitchen table. “When you’re investigating will power in a four-year-old, little things make a big difference,” he says. “How big should the marshmallows be? What kind of cookies work best?” After several months of patient tinkering, Mischel came up with an experimental design that closely simulated the difficulty of delayed gratification. In the spring of 1968, he conducted the first trials of his experiment at the Bing. “I knew we’d designed it well when a few kids wanted to quit as soon as we explained the conditions to them,” he says. “They knew this was going to be very difficult.”

At the time, psychologists assumed that children’s ability to wait depended on how badly they wanted the marshmallow. But it soon became obvious that every child craved the extra treat. What, then, determined self-control? Mischel’s conclusion, based on hundreds of hours of observation, was that the crucial skill was the “strategic allocation of attention.” Instead of getting obsessed with the marshmallow—the “hot stimulus”—the patient children distracted themselves by covering their eyes, pretending to play hide-and-seek underneath the desk, or singing songs from “Sesame Street.” Their desire wasn’t defeated—it was merely forgotten. “If you’re thinking about the marshmallow and how delicious it is, then you’re going to eat it,” Mischel says. “The key is to avoid thinking about it in the first place.”

In adults, this skill is often referred to as metacognition, or thinking about thinking, and it’s what allows people to outsmart their shortcomings. (When Odysseus had himself tied to the ship’s mast, he was using some of the skills of metacognition: knowing he wouldn’t be able to resist the Sirens’ song, he made it impossible to give in.) Mischel’s large data set from various studies allowed him to see that children with a more accurate understanding of the workings of self-control were better able to delay gratification. “What’s interesting about four-year-olds is that they’re just figuring out the rules of thinking,” Mischel says. “The kids who couldn’t delay would often have the rules backwards. They would think that the best way to resist the marshmallow is to stare right at it, to keep a close eye on the goal. But that’s a terrible idea. If you do that, you’re going to ring the bell before I leave the room.”

According to Mischel, this view of will power also helps explain why the marshmallow task is such a powerfully predictive test. “If you can deal with hot emotions, then you can study for the S.A.T. instead of watching television,” Mischel says. “And you can save more money for retirement. It’s not just about marshmallows.”

Subsequent work by Mischel and his colleagues found that these differences were observable in subjects as young as nineteen months. Looking at how toddlers responded when briefly separated from their mothers, they found that some immediately burst into tears, or clung to the door, but others were able to overcome their anxiety by distracting themselves, often by playing with toys. When the scientists set the same children the marshmallow task at the age of five, they found that the kids who had cried also struggled to resist the tempting treat.

The early appearance of the ability to delay suggests that it has a genetic origin, an example of personality at its most predetermined. Mischel resists such an easy conclusion. “In general, trying to separate nature and nurture makes about as much sense as trying to separate personality and situation,” he says. “The two influences are completely interrelated.” For instance, when Mischel gave delay-of-gratification tasks to children from low-income families in the Bronx, he noticed that their ability to delay was below average, at least compared with that of children in Palo Alto. “When you grow up poor, you might not practice delay as much,” he says. “And if you don’t practice then you’ll never figure out how to distract yourself. You won’t develop the best delay strategies, and those strategies won’t become second nature.” In other words, people learn how to use their mind just as they learn how to use a computer: through trial and error.

But Mischel has found a shortcut. When he and his colleagues taught children a simple set of mental tricks—such as pretending that the candy is only a picture, surrounded by an imaginary frame—he dramatically improved their self-control. The kids who hadn’t been able to wait sixty seconds could now wait fifteen minutes. “All I’ve done is given them some tips from their mental user manual,” Mischel says. “Once you realize that will power is just a matter of learning how to control your attention and thoughts, you can really begin to increase it.”

Marc Berman, a lanky graduate student with an easy grin, speaks about his research with the infectious enthusiasm of a freshman taking his first philosophy class. Berman works in the lab of John Jonides, a psychologist and neuroscientist at the University of Michigan, who is in charge of the brain-scanning experiments on the original Bing subjects. He knows that testing forty-year-olds for self-control isn’t a straightforward proposition. “We can’t give these people marshmallows,” Berman says. “They know they’re part of a long-term study that looks at delay of gratification, so if you give them an obvious delay task they’ll do their best to resist. You’ll get a bunch of people who refuse to touch their marshmallow.”

This meant that Jonides and his team had to find a way to measure will power indirectly. Operating on the premise that the ability to delay eating the marshmallow had depended on a child’s ability to banish thoughts of it, they decided on a series of tasks that measure the ability of subjects to control the contents of working memory—the relatively limited amount of information we’re able to consciously consider at any given moment. According to Jonides, this is how self-control “cashes out” in the real world: as an ability to direct the spotlight of attention so that our decisions aren’t determined by the wrong thoughts.

Last summer, the scientists chose fifty-five subjects, equally split between high delayers and low delayers, and sent each one a laptop computer loaded with working-memory experiments. Two of the experiments were of particular interest. The first is a straightforward exercise known as the “suppression task.” Subjects are given four random words, two printed in blue and two in red. After reading the words, they’re told to forget the blue words and remember the red words. Then the scientists provide a stream of “probe words” and ask the subjects whether the probes are the words they were asked to remember. Though the task doesn’t seem to involve delayed gratification, it tests the same basic mechanism. Interestingly, the scientists found that high delayers were significantly better at the suppression task: they were less likely to think that a word they’d been asked to forget was something they should remember.

In the second, known as the Go/No Go task, subjects are flashed a set of faces with various expressions. At first, they are told to press the space bar whenever they see a smile. This takes little effort, since smiling faces automatically trigger what’s known as “approach behavior.” After a few minutes, however, subjects are told to press the space bar when they see frowning faces. They are now being forced to act against an impulse. Results show that high delayers are more successful at not pressing the button in response to a smiling face.

When I first started talking to the scientists about these tasks last summer, they were clearly worried that they wouldn’t find any behavioral differences between high and low delayers. It wasn’t until early January that they had enough data to begin their analysis (not surprisingly, it took much longer to get the laptops back from the low delayers), but it soon became obvious that there were provocative differences between the two groups. A graph of the data shows that as the delay time of the four-year-olds decreases, the number of mistakes made by the adults sharply rises.

The big remaining question for the scientists is whether these behavioral differences are detectable in an fMRI machine. Although the scanning has just begun—Jonides and his team are still working out the kinks—the scientists sound confident. “These tasks have been studied so many times that we pretty much know where to look and what we’re going to find,” Jonides says. He rattles off a short list of relevant brain regions, which his lab has already identified as being responsible for working-memory exercises. For the most part, the regions are in the frontal cortex—the overhang of brain behind the eyes—and include the dorsolateral prefrontal cortex, the anterior prefrontal cortex, the anterior cingulate, and the right and left inferior frontal gyri. While these cortical folds have long been associated with self-control, they’re also essential for working memory and directed attention. According to the scientists, that’s not an accident. “These are powerful instincts telling us to reach for the marshmallow or press the space bar,” Jonides says. “The only way to defeat them is to avoid them, and that means paying attention to something else. We call that will power, but it’s got nothing to do with the will.”

The behavioral and genetic aspects of the project are overseen by Yuichi Shoda, a professor of psychology at the University of Washington, who was one of Mischel’s graduate students. He’s been following these “marshmallow subjects” for more than thirty years: he knows everything about them from their academic records and their social graces to their ability to deal with frustration and stress. The prognosis for the genetic research remains uncertain. Although many studies have searched for the underpinnings of personality since the completion of the Human Genome Project, in 2003, many of the relevant genes remain in question. “We’re incredibly complicated creatures,” Shoda says. “Even the simplest aspects of personality are driven by dozens and dozens of different genes.” The scientists have decided to focus on genes in the dopamine pathways, since those neurotransmitters are believed to regulate both motivation and attention. However, even if minor coding differences influence delay ability—and that’s a likely possibility—Shoda doesn’t expect to discover these differences: the sample size is simply too small.

In recent years, researchers have begun making house visits to many of the original subjects, including Carolyn Weisz, as they try to better understand the familial contexts that shape self-control. “They turned my kitchen into a lab,” Carolyn told me. “They set up a little tent where they tested my oldest daughter on the delay task with some cookies. I remember thinking, I really hope she can wait.”

While Mischel closely follows the steady accumulation of data from the laptops and the brain scans, he’s most excited by what comes next. “I’m not interested in looking at the brain just so we can use a fancy machine,” he says. “The real question is what can we do with this fMRI data that we couldn’t do before?” Mischel is applying for an N.I.H. grant to investigate various mental illnesses, like obsessive-compulsive disorder and attention-deficit disorder, in terms of the ability to control and direct attention. Mischel and his team hope to identify crucial neural circuits that cut across a wide variety of ailments. If there is such a circuit, then the same cognitive tricks that increase delay time in a four-year-old might help adults deal with their symptoms. Mischel is particularly excited by the example of the substantial subset of people who failed the marshmallow task as four-year-olds but ended up becoming high-delaying adults. “This is the group I’m most interested in,” he says. “They have substantially improved their lives.”

Mischel is also preparing a large-scale study involving hundreds of schoolchildren in Philadelphia, Seattle, and New York City to see if self-control skills can be taught. Although he previously showed that children did much better on the marshmallow task after being taught a few simple “mental transformations,” such as pretending the marshmallow was a cloud, it remains unclear if these new skills persist over the long term. In other words, do the tricks work only during the experiment or do the children learn to apply them at home, when deciding between homework and television?

Angela Lee Duckworth, an assistant professor of psychology at the University of Pennsylvania, is leading the program. She first grew interested in the subject after working as a high-school math teacher. “For the most part, it was an incredibly frustrating experience,” she says. “I gradually became convinced that trying to teach a teen-ager algebra when they don’t have self-control is a pretty futile exercise.” And so, at the age of thirty-two, Duckworth decided to become a psychologist. One of her main research projects looked at the relationship between self-control and grade-point average. She found that the ability to delay gratification—eighth graders were given a choice between a dollar right away or two dollars the following week—was a far better predictor of academic performance than I.Q. She said that her study shows that “intelligence is really important, but it’s still not as important as self-control.”

Last year, Duckworth and Mischel were approached by David Levin, the co-founder of KIPP, an organization of sixty-six public charter schools across the country. KIPP schools are known for their long workday—students are in class from 7:25 A.M. to 5 P.M.—and for dramatic improvement of inner-city students’ test scores. (More than eighty per cent of eighth graders at the KIPP academy in the South Bronx scored at or above grade level in reading and math, which was nearly twice the New York City average.) “The core feature of the KIPP approach is that character matters for success,” Levin says. “Educators like to talk about character skills when kids are in kindergarten—we send young kids home with a report card about ‘working well with others’ or ‘not talking out of turn.’ But then, just when these skills start to matter, we stop trying to improve them. We just throw up our hands and complain.”

Self-control is one of the fundamental “character strengths” emphasized by KIPP—the KIPP academy in Philadelphia, for instance, gives its students a shirt emblazoned with the slogan “Don’t Eat the Marshmallow.” Levin, however, remained unsure about how well the program was working—“We know how to teach math skills, but it’s harder to measure character strengths,” he says—so he contacted Duckworth and Mischel, promising them unfettered access to KIPP students. Levin also helped bring together additional schools willing to take part in the experiment, including Riverdale Country School, a private school in the Bronx; the Evergreen School for gifted children, in Shoreline, Washington; and the Mastery Charter Schools, in Philadelphia.