Stress can be toxic at any age. It rattles us when it strikes, shaking up our relationships and narrowing our focus. When it becomes chronic, it ravages our health. Physically, emotionally and intellectually, stress can drag us down.

An even more insidious effect is the assault it can launch on a child's brain, impeding the development of critical cognitive skills. A number of researchers, including myself, have discovered that psychological stress affects the thinking skills and brain development of even very young children, likely beginning prenatally. It is no mystery that stress thrives in difficult situations, but research is now showing that a disadvantaged upbringing may set back children in profound, lasting ways. In fact, stress may be one important mechanism through which poverty adversely affects children's ability to perform well in school.

Although children differ in their susceptibility to the problems of poverty, data show that youngsters from lower-income homes are very likely to start school behind their more affluent peers. This socioeconomic gap persists throughout the school years and is difficult to close. People have long argued that disadvantaged homes tend to offer an impoverished learning environment that does not sufficiently prepare children for the rigors of school. This theory is, at best, only half of the story. My work suggests that the stresses that accompany low income—such as crowded conditions, noise, financial worries and an inability to provide adequate child care—directly impair specific learning abilities in children.

A stressful childhood may emerge from conditions other than poverty, whether from challenging family circumstances such as a divorce or death, overbearing or distracted parents, or learning disabilities that create anxiety about school. A focus on reducing stress through changes in home or in school could improve the well-being of large numbers of schoolchildren and set them up for greater success throughout life.

Flooding the Engines

Stress hormones can shape the developing circuitry of the brain. Most notably, they influence the neural connections in the prefrontal cortex, located behind the forehead, that buttress what are known as executive functions. These include the ability to hold information in mind (working memory) and to inhibit automatic or impulsive responses to stimulation. Executive functions are critical for reasoning, planning and problem solving and for regulating emotions and attention. They are essential to academic success. [For more on training executive function, see “The Education of Character,” on page 48, and “Building Better Brains,” on page 59.]

The effects of stress on the brain depend on how much of it is present. A little stress heightens alertness; it improves people's performance on complex tasks. But as the dose exceeds a certain level, stress starts to erode performance. This relation between arousal and performance can be expressed as an inverted U-shaped curve, first identified by psychologists Robert Yerkes and John Dodson in 1908. In the brain, moderate amounts of stress hormones such as cortisol and noradrenaline boost activity in prefrontal areas that underlie executive functions. At high levels, however, they flood this engine of self-regulation, shutting it down. Over time the brain circuits that control stress hormone levels are shaped by experience toward a tendency to unleash either very large or very small amounts of these hormones onto the prefrontal cortex in response to stress or to maintain a more optimal level of arousal.

In 2001 I began to wonder how this physiology played out in the brains of young children and whether it might explain how poverty “gets under the skin.” I set out to explore whether the chronic stress of poverty might be impairing the developing executive function of children enough to set them back at school.

My graduate student at the time, Rachel Peters, and I, then at Pennsylvania State University, gave two tests to 170 four-year-olds in central Pennsylvania enrolled in Head Start, the federal preschool program for children in poverty. We measured mental flexibility by asking children to identify different ways in which small groups of objects were similar. We examined working memory and inhibitory control by asking them to tap a peg twice when the experimenter tapped it once, and vice versa. Here the child has to remember the rule and control the impulse to copy the experimenter. We also asked teachers to rate each child's behavior and academic abilities. And in collaboration with neuroendocrinologist Douglas Granger, also then at Penn State, we took samples of the children's saliva to determine levels of cortisol at the beginning, middle and end of our experimental session.

The children with better executive function and behavior had low cortisol at the beginning of the session that rose and then returned to baseline, as expected, in response to the mild stress of meeting one of us and participating in our tasks. Those who showed either a sustained high level of cortisol or a blunted response—high initial levels, which then dropped, indicating a shutting down of the process—tended to have low executive function; their teachers also rated them as more aggressive and lacking in self-control. We published these results in 2005.

As we followed these children into kindergarten, we observed that executive function matters for achievement: this suite of mental skills was the main determinant of math proficiency, far outweighing other aspects of intelligence. And in an analysis published this year our team, led by postdoctoral researcher Daniel Berry, found that elevated cortisol in children directly predicts academic difficulties, as indicated by their knowledge of math, letters and words. A statistical analysis also backs up our hypothesis that detriments to executive function—as opposed to, say, low general mental ability—are the critical link tying high cortisol to low academic ability.

Positive Parenting

Meanwhile my colleagues and I also set out to determine what aspects of poverty might contribute most to children's stress. We focused on parenting style. Impoverished parents tend to elicit obedience through discipline rather than encouraging exploration and learning by doing. The latter approach, known as scaffolding, is essential to sensitive parenting. In this type of parenting, mothers and fathers interact with their children during play and create opportunities for them to accomplish small tasks, such as stacking blocks. Although poor parents can and do provide sensitive care, they are less likely to do so, given the realities of their situation and, potentially, their own high stress levels.

To investigate further, we have been following 1,292 children, starting at birth, and their families, most of whom live in poverty in rural communities in Appalachia and the Deep South. For about seven years now our team has been visiting these homes annually to collect data on family and economic conditions as well as on executive function and cortisol levels. My colleagues Martha Cox and Roger Mills-Koonce of the University of North Carolina at Chapel Hill analyzed video recordings of mothers interacting with their children in free play. In our first analysis, published in 2008, we found that infants whose mothers displayed the sensitive, scaffolding parenting style had lower cortisol levels and were calmer and more attentive than those whose mothers either completed the activity for them or restricted their attempts to do so.

Furthermore, at age seven months the children whose parents displayed the positive parenting style were more likely to exhibit a healthy cortisol response—a rise and fall—to fear (triggered by an experimenter in a mask) and frustration from a toy placed just out of reach. At age 15 months these children again had lower cortisol levels and were more likely to respond appropriately to the emotional challenges. We now had evidence that parenting style shapes the developing stress response system.

We next sought to sketch the complete path from poverty to parenting to increased stress and diminished executive function in the same group of children. Most recently, we found that the more severely impoverished the family, the less likely parents were to be sensitive and responsive. As expected, the children in such homes had elevated cortisol, which was, in turn, associated with lower executive function. We also saw that less positive parenting went hand in hand with poorer executive function in children, indicating that mothers and fathers can directly stimulate the development of important mental skills.

Creating Capable Kids

Research indicates that stress from a variety of sources—chaotic and poorly run classrooms, for example, or problems with family or peers—impedes learning. The potential good news: knowing that stress is a malevolent force means that finding ways to thwart it could boost children's learning capacity.

In that vein, my collaborators and I are testing a program that teaches parents how to be more sensitive and how to structure opportunities for their children to learn while providing warm and loving care. We are also trying out a new curriculum that gives kindergarteners and preschoolers more control over their learning activities. After a year we will see whether the children's stress regulation and executive function improve. Although this work is in its early stages, we are encouraged by the possibility that informed changes to environments can boost children's self-control and academic competence, giving many of our youth a far greater chance of succeeding in life.