ONCE UPON A TIME, by some estimates about 1.8 million years ago, our evolutionary ancestors were emerging from the trees of the river forest environment of the great apes. They were becoming Homo erectus, walking around on two feet in the grassy areas and struggling to live and reproduce. In these prehistoric adventures, the human species probably survived and multiplied thanks to the hot emotional system of the brain, the limbic system.
The limbic system consists of primitive brain structures located under the cortex on top of the brain stem, which developed early in our evolution. These structures regulate basic drives and emotions essential for survival, from fear and anger to hunger and sex. This system helped our ancestors cope with the hyenas, lions, and other wild beasts that were both their food supply and their daily mortal danger. Within the limbic system, the amygdala, a small almond-shaped structure (amygdala means “almond” in Latin), is especially important. It plays a key role in fear responses and in sexual and appetitive behavior. The amygdala rapidly mobilizes the body for action. It does not pause to think and reflect or worry about long-term consequences.
We still have a limbic system that works much as it did for our evolutionary ancestors. It remains our emotionally hot Go! system, specialized for quick reactions to strong, emotion-arousing stimuli that automatically trigger pleasure, pain, and fear. At birth it is already fully functional, making the infant cry when hungry or in pain. Although these days we rarely need it later in life for dealing with angry lions, it’s still invaluable for avoiding menacing strangers in dark alleys or a swerving vehicle on an icy road. The hot system gives life its emotional zest. It motivates preschoolers to want two marshmallows, but it also makes it hard for them to endure the wait.
Activation of the hot system triggers instantaneous action: hunger for food and desire for other alluring stimuli elicit rapid hot Go! behaviors; threats and danger signals elicit fear and automatic defensive and flight reactions. The hot system is somewhat similar to what Freud called the id; he saw this as the unconscious structure of the mind, which contained sexual and aggressive biological impulses to seek immediate gratification and tension reduction, impervious to the consequences. Like Freud’s id, the hot system operates automatically and mostly unconsciously, but it is in the service of much more than the sexual and aggressive impulses of Freud’s concern. Reflexive, simple, and emotional, it automatically and quickly triggers consumptive behavior, arousal, and impulsive action. It makes the preschooler ring the bell and eat the marshmallow, the dieter bite into the pizza, the cigarette addict inhale the smoke, the angry abuser strike the partner, and the sexually out-of-control male grab the cleaning lady.
A focus on the hot features of a temptation easily triggers the Go! response. In the marshmallow experiments, I’ve watched a preschooler’s hand suddenly lurch out and hit the bell hard, as the surprised child looks down in distress to see what his hand has done. For four-year-olds, the trigger can be anticipating the chewy, sweet taste of the marshmallows; for dieters, alcoholics, and smokers, each of the hot features has its own distinctive pull that can make its victims helpless. Even the sight or thought of the candy bar, or the whiskey, or the cigarette can elicit the action automatically. And the more often that happens, the more difficult it becomes to change the mental representation and avert the automatic Go! response. Learning and practicing some strategies for enabling self-control early in life is a lot easier than changing hot, self-destructive, automatic-response patterns established and ingrained over a lifetime.
High stress activates the hot system. This response was adaptive in evolutionary history for dealing with oncoming lions because it produces amazingly rapid (in milliseconds), automatic, self-protective reactions, and it is still useful in many emergencies in which survival requires instant action. But this hot response is not useful when success in a given situation depends on staying cool, planning ahead, and problem-solving rationally. And the hot system is predominant in the first few years of life, which makes it especially difficult for the young preschooler to exert self-control.
Closely interconnected with the brain’s hot system is its cool system, which is cognitive, complex, reflective, and slower to activate. It is centered primarily in the prefrontal cortex (PFC). This cool, controlled system is crucial for future-oriented decisions and self-control efforts of the kind identified in the Marshmallow Test. It’s important to note that high stress attenuates the cool system and accentuates the hot system. The hot and cool systems continuously and seamlessly interact in a reciprocal relationship: as one becomes more active the other becomes less active. Although we rarely deal with lions, we daily face the endless stresses of the modern world, in which the hot system is often up, leaving us with our cool system down just when we need it most.
The PFC is the most evolved region of the brain. It enables and supports the highest-order cognitive abilities that make us distinctively human. It regulates our thoughts, actions, and emotions, is the source of creativity and imagination, and is crucial for inhibiting inappropriate actions that interfere with the pursuit of goals. It allows us to redirect our attention and to change strategies flexibly as the requirements of the situation shift. Self-control ability is rooted in the PFC.
The cool system develops slowly and becomes gradually more active in the preschool years and the first few years of elementary school. It does not fully mature until the early twenties, leaving the young child as well as the adolescent greatly vulnerable to the vicissitudes of the hot system. Unlike the hot system, the cool system is attuned to the informational aspects of stimuli and enables rational, reflective, and strategic behavior.
As I described in earlier pages, successful delayers in the Marshmallow Test invented ways to strategically distract themselves from the tempting treats and the bell. They also focused on the cool, abstract, informational features of the temptations as they imagined them (the marshmallows are like puffy clouds, or cotton balls), and avoided or transformed their hot features to cool them down (make believe it’s just a picture; it’s got a frame around it; you can’t eat a picture). The diverse cognitive skills they used to wait for their treats are prototypes for those they needed years later to study for high school exams rather than heading out to the movies with friends, or countless other immediate temptations that awaited them in life.
Age matters. Most children younger than four are unable to sustain delay of gratification on the Marshmallow Test. When faced with the temptations, they ring the bell or start nibbling on the treats within about 30 seconds. Their cool system is not yet sufficiently developed. In contrast, by age 12 almost 60 percent of children in some studies have been able to wait even as long as 25 minutes, a very long time to be sitting facing a few cookies and a bell in a barren little room.
Gender also matters. Boys and girls develop different preferences at different phases of their development, and their willingness to wait will be influenced by the available rewards: what’s rewarding to boys may be undesired by girls, and vice versa (fire engines, dolls, swords, makeup kits). But even if the reward values are equated and the motivation is the same, girls usually wait longer than boys, and their cooling strategies may differ. I have not measured it, but preschool boys seem to use more physical strategies, like tilting and rocking back and forth on the chair or pushing the temptations away, while girls seem to sing to themselves more or try to simply tune out. But that’s my impression only, not a finding.
The greater willingness and ability of girls to wait longer is consistent with the finding that throughout the school years, at least in the United States, girls are usually rated higher on self-discipline measures than boys by their teachers, their parents, and themselves. Even in the first four years of life, girls are generally more compliant than boys. In later childhood, girls, on average, are usually seen as more self-disciplined in their schoolwork and they often get better grades than boys. However, the raters, including the children themselves, share cultural stereotypes about gender differences. “Good girls” are expected to be conscientious and careful, and “real boys” are supposed to be more impulsive, harder to control, and even rowdy, rehearsing their football tackles more than their times tables. On hypothetical choices about delayed rewards, like “Would you prefer $55 today or $75 in 61 days?,” girls choose delayed rewards more often than boys. But when the choice becomes real, rather than hypothetical (keep an envelope containing a one-dollar bill today, or return it exactly one week later and get two dollars), the sex difference evaporates.
In short, we keep looking for sex differences on the Marshmallow Test and other measures of self-control. We don’t always find them, but on the whole girls seem to have an advantage in the cognitive self-control skills and motivations that enable delay of gratification, at least in the populations and age groups studied so far.
When dealing with temptations, one way to momentarily escape the hot system is to imagine how someone else would behave. It’s easier to use the cool system when making hot choices for others rather than for oneself. A researcher whose name I can’t remember but whose story I can’t forget asked preschoolers to consider a choice between a small piece of chocolate right now and a very large piece in ten minutes (he showed both pieces of chocolate to the children). When he asked a young boy, “What would an intelligent child choose?,” the child responded that he would wait; when the researcher asked, “What will you do?,” the child said, “I’ll take it now!” The same point was made in an experiment with three-year-olds. They were given the choice between an immediate small reward and a delayed larger reward. When asked which one the experimenter would choose, they were able to use their cool system and were more likely to choose the delayed reward. But when they were choosing for themselves, the choice became hot and most of them took the smaller reward right away.
The experience of short-term stress can be adaptive, mobilizing you into action. Stress can become harmful, however, even toxic if it is intense and persists—for example, in people who become enraged at every frustration, from traffic jams to checkout lines, or who feel overwhelmed under extreme and enduring conditions of danger, turbulence, or poverty. Prolonged stress impairs the PFC, which is essential not only for waiting for marshmallows but also for things like surviving high school, holding down a job, pursuing an advanced degree, navigating office politics, avoiding depression, preserving relationships, and refraining from decisions that seem intuitively right but on closer examination are really stupid.
After reviewing research on the effects of stress, neuroscientist Amy Arnsten at Yale University concluded that “even quite mild acute uncontrollable stress can cause a rapid and dramatic loss of prefrontal cognitive abilities.” The longer stress persists, the more those cognitive abilities are hurt and the more permanent the damage, ultimately leading to mental as well as physical illness. Thus, the part of the brain that enables creative problem solving becomes less available the more we need it. Remember Hamlet: as his stress escalated, he became increasingly trapped and tortured, paralyzed in his angry ruminations and fragmented feelings, and unable to think or act effectively, thus wreaking havoc all around him and further accelerating his undoing.
More than four hundred years after Shakespeare so eloquently dramatized Hamlet’s mental anguish, we can reconstruct what must have been going on in his brain—not with the Bard’s magical language, but with a model of the brain under chronic stress. The architecture of the brain is literally remodeled under chronic stress. Hamlet didn’t stand a chance. When his stress persisted, his cool system, specifically the prefrontal cortex, crucial for problem solving, and the hippocampus, important for memory, began to atrophy. Concurrently, his amygdala, at the core of the hot system, increased excessively in size. This combination of brain changes made self-control and cool thinking impossible. Further, as his stress continued long term, his amygdala went from hypertrophy to atrophy, ultimately preventing normal emotional reactions. No wonder Hamlet was a tragedy.