What Science Is Telling Us

How Neurobiology and Developmental Psychology are Changing the Way Policymakers and Communities Think About the Developing Child

Early brain architecture ultimately gets refined as the connections that are used or stimulated consistently in those first few years grow stronger, while those that aren’t die off in a process known as synaptic pruning. By the time we reach adolescence, our brain has discarded probably half of these neural links. The thought of losing so many connections in the brain seems unsettling, to be sure, but science tells us it’s a perfectly normal process that is necessary to make the remaining circuits work more efficiently.

Levitt likens the brain’s activity to his native Tennessee monkey grass, a hardy groundcover that sprouts uncontrollably until “it has to be hacked back at the beginning of every summer so it grows in lush and pretty.” Council member Charles Nelson—a developmental psychologist and cognitive neuroscientist at Harvard Medical School—prefers the metaphor of building a house to demonstrate that brain development doesn’t simply cease in early childhood. “In the first few years of life, most of the major milestones are reached and the basic architecture is complete,” he explains. “But the detail work”—the décor, the wallpaper and carpets—“has a long developmental trajectory and really goes on for maybe 20 more years.” As the father of a 17-year-old, he adds, “I hope it’s not over at age three!”

The Role of Early Experiences

So what do these complex physical changes have to do with a baby’s earliest experiences? Almost everything, as it turns out. The wide range of early stimuli referred to above helps determine which synapses grow strong and which are pruned or eliminated. More and more, new brain imaging technology allows us to see just how dramatic this is, as pictures of the brains of children exposed to a well-balanced palate of experiences, from music to books and building blocks, show they can actually form 25 percent more neural connections. Conversely, children who are starved for stimulation or harmed by too much stress pay a very real price, as their brains can remain, in severe cases, shockingly underdeveloped.

At the forefront of this area of research are several other scientists on the Council. For veteran neuroscientist William Greenough and his colleagues at the University of Illinois Urbana-Champaign, young rats—not babies—are the study subjects. Greenough’s team investigated what happens to the brain when mammals are exposed early in life to environments that vary in the type and amount of stimulation they offer. In this case, laboratory rats raised in cages with challenging toys and other animals were found to have larger brain regions and more synaptic connections than those raised in bare cages.

The first group also performed better on various learning and problem-solving tests. “In essence, our work shows that the normal brain development process is sensitive to and dependent upon the experiences the animal—and, presumably, the human—has,” Greenough explains. And when it comes to human children, he says, this has obvious implications for the quality of the environment—within the family and beyond.

Fellow Council member Judy Cameron studies a variation of that question: What happens to the brain when something goes seriously wrong? Specifically, Cameron, of the Oregon Health and Science University and the University of Pittsburgh, and her colleagues study the effects of everyday stresses on long-term health and behavior. Their subjects are young monkeys who are separated from their mothers or face other social disruptions early in life. The findings confirm Cameron’s hypothesis that responses to stress are dictated partly by one’s genetic background and partly by environmental influences, especially during the formative years. “We have found that individuals can get set on trajectories that make them more stress-sensitive. But these trajectories are not etched in stone and they can be changed,” she adds. The challenge is to learn how to change them early to avert serious problems, ranging from anxiety and depression to obesity and infertility in adulthood.

Sadly, the implications of this area of research for humans have been tested in many reallife “laboratories,” as scientists have examined the developmental consequences for children who are abused and neglected or otherwise exposed to debilitating trauma or stress. Several Council members, for example, have been among a large group of researchers studying the wave of Romanian children adopted from state orphanages in the early 1990s. This research tells a compelling story about the severe developmental harm that comes from the kind of neglect these children suffered as babies. It also reveals the remarkable recovery possible for many such children once they are adopted into loving homes—but also the troubling impairments that continue to plague some of them years later.

Children coming from the worst circumstances show signs of early neglect not only in their brain structure but also in their stress “set points.” For some of these children, the stress system seems to have been permanently ramped up in ways that will increase their risk for stress-related physical and mental disorders throughout life. Megan Gunnar, a developmental psychologist at the University of Minnesota, studies the emotional and social processes that regulate the body’s response to childhood stress. These phenomena can be tested in something as common as a baby’s visit to the doctor for immunizations. Gunnar’s research shows that the way children react to this frightening situation varies greatly based on their circumstances. Children who feel loved and secure—typically those from stable, nurturing families—have a built-in protection against the threat. “That secure attachment literally blocks the baby’s hormonal response to stress,” she says, explaining that chemicals in the brain react differently when the perceived danger approaches. Conversely, children who don’t derive a sense of security from their relationships with loving adults often overreact to the fear of the shot—or other sources of perceived harm.

The longer children have lived under damaging conditions, the greater the risk that they have suffered permanent effects. When it comes to more severe and chronic forms of stress—like that experienced by the Romanian orphans—the same principle applies, and the effects are still being studied. Gunnar summarizes key findings from the literature.

“Behaviorally, we know these kids fell behind in their physical, cognitive, and social development—maybe by a month for every two to three months they spent in the orphanage.” (Other research on orphans has shown even more striking delays—with some babies rendered alarmingly unresponsive, unable to babble or even cry.) Apart from malnutrition or physical dangers like parasites, these delays were largely the result of a lack of attention and love, says Gunnar, and the consequent insults to their developing brains. But once the orphans were adopted into nurturing homes, they typically showed dramatic rebounds—growing physically even faster than the “normal” rate, and soon catching up in their cognitive and social development. However, children who spent more time in institutions have shown slower recoveries and more serious long-term problems.

For information about commonly used terms in Council publications, see Definitions.