In terms of behavioral development, i was something of a late bloomer. My mother reports that I slept away most of my infancy and toddlerhood, and even my adolescence was unremarkable. I didn’t enter my angst-and-experimentation phase until my mid-20s, when, like a tortured teen, I blamed my parents for everything. Several years and several thousand dollars of psychotherapy later, I let my parents off the hook. I realized it couldn’t all be their doing--my faults, my fears, my penchant for salty, cheese-flavored snack foods. I am not, after all, the simple product of my upbringing.
This healthy outlook threatened to come undone one recent afternoon as I stood outside the cages at the National Institutes of Health Animal Center in Poolesville, Maryland, watching Stephen Suomi’s monkeys. Suomi, a primatologist at the National Institute of Child Health and Human Development, studies the effects of rearing environments on the behavior of young rhesus macaques. Fifty graduates of his program live in the center’s five-acre enclosure; at the moment, they are gathered in a large chain-link cell with sawdust on the floor and monkey toys dangling from the ceiling. The arrival of human visitors stirs this cohort like dry leaves in a whirlwind, and its members quickly segregate into three factions. The boldest rush to get a cage-front view of the newcomers; a second phalanx hovers behind them, cautious but curious; and at the far end of the cage a third group forms a simian huddle of abject fear.
These monkeys are most definitely the products of their respective upbringings. The three groups were raised in three distinct settings. The bold monkeys spent the first six months of life being shuttled between monkey play groups and individual cages (and so were used to human handling); the sensibly cautious ones were reared by their natural families, with mothers, fathers, and siblings; and the fearful monkeys grew up parentless among same-age peers, to whom they retain an abnormally strong attachment.
Suomi is keenly interested in the spectrum of behavior among his macaques--from bold and aggressive to anxious and withdrawn--for it parallels the human trait known as temperament, the fundamental cast of personality that governs our propensity for hobnobbing, taking risks, or seeking thrills. He and other researchers have found that temperament is reflected in biology as well as behavior: heart rate, immune response, stress-hormone levels, and other physiological measures can be correlated with temperamental styles in humans and monkeys alike. And despite some investigators’ assertions to the contrary, Suomi’s experiments imply that temperament may be largely the result of a young monkey’s home life.
The patterns have some genetic heritability, says Suomi, jangling his car keys in front of the cage to get an even more exaggerated response. But our work shows that you can modify these tendencies quite dramatically with certain types of early experiences.
Suomi belongs to the league of scientists who are studying the role that early childhood environment plays in determining adult behavior. He and his colleagues are working a bit beyond the pale, as late twentieth- century science seems to savor the notion of genetic determinism. But the effects of childhood environment--specifically, the environment supplied by parenting--are coming under renewed scrutiny now, in large part because recent neurological studies have revealed that the structure of a child’s brain remains surprisingly malleable months and even years after birth. The number of connections between nerve cells in an infant’s brain grows more than twentyfold in the first few months of life, for example; a two-year- old’s brain contains twice as many of these connections, called synapses, as an adult’s brain. Throughout early childhood, synapses multiply and are pruned away at a furious pace. Something directs this dynamic rewiring, and researchers have concluded that that something is experience.
Of course, experience can come in all shapes and sizes. Childhood illness and diet, for example, count as experiences, too. But there’s reason to believe that a child’s experience of his parents is an especially potent sculptor of the parts of the brain involved in emotion, personality, and behavior. Some studies indicate that the strength of a child’s bonding with his caregivers may increase his ability to learn and to cope with stress. Others show that childhood abuse and neglect can prime the brain for a lifetime of inappropriate aggression and scattered attention.
As the twentieth century draws to a close, more than half of America’s one-year-olds are spending their days with someone other than their mothers. This historic surge in day care has coincided with a rush of reports showing that early experiences may be more critical to brain development than anyone had previously imagined. Naturally, each new bulletin tweaks the guilty fears of working parents. So far, however, the news about kids and day care is pretty good. Children in day care appear to do just fine--provided the quality of the interactions between caregiver and child is high--and good day care may even enhance their social skills and performance in school. Low-quality day care, on the other hand, may compromise a child’s adjustment and academic performance.
These results are not surprising to behavioral researchers, who have long appreciated the importance of bonds between caregivers and children. We know that little kids don’t hop up and run away from lions-- they don’t deal directly with the world much, says Megan Gunnar, a developmental psychologist at the University of Minnesota in Minneapolis. Their survival depends on their relationships. Hence, children are keenly attuned to the cues they receive from parents, says Gunnar, and they are especially sensitive to signs of indifference. Responsive, sensitive parents inspire trust in their children, giving rise to what behavioral scientists call secure attachment; insensitive or withdrawn parents can foster insecure attachment.
Nearly four decades of behavioral research has painted a dramatic picture of how important this attachment is to a child’s emotional health. University of Wisconsin psychologist Harry Harlow’s studies in the late fifties and early sixties found that monkeys reared in total isolation developed aberrant feeding, mating, parenting, and socializing behaviors. Developmental psychologists now believe that bonding with a parent or other caregiver is as essential to a normal childhood as learning to walk and talk. In the absence of a good mother, a child will attach as best he can to whatever figure presents itself--just as Harlow’s infant monkeys became virtually inseparable from the cloth-and-wire surrogates in their cages.
Stephen Suomi’s simian charges are another example of how behavior can be warped by bonding with a maladroit mom. The timid peer- reared monkeys at the nih center are the victims of insecure attachment; their peers didn’t provide the stability and sensitivity that make for a secure bond. (Imagine what a wreck you’d be if you were raised by a twin sister.) These monkeys are anxious and inhibited, and their temperaments are reflected in their reluctance to explore strange objects, their shyness with unfamiliar peers, their low status in monkey communities, and their distress on being separated from their companions. Some peer-reared monkeys, mostly males, also have tendencies toward impulsive behavior and aggression. They’re the playground bullies, and they’re often shunned by, or even kicked out of, their play groups.
Clearly, peer-rearing has unhappy consequences for an individual’s social skills and ability to cope with stress. It has at least one other embarrassing side effect as well. Every animal that’s reared without a mother, no matter what its other social experience may be, turns out to be hyperoral, says Suomi. They all suck their thumbs a lot.
Peer-rearing also leaves a distinctive stamp on the monkey’s physiology. Samples of cerebrospinal fluid from Suomi’s impulsive monkeys show that they grow up with lower levels of serotonin, a mood-regulating biochemical that has been linked with aggression, antisocial behavior, and depression in human beings. At the same time, turnover of norepinephrine, a chemical messenger associated with fearfulness, is unusually rapid in peer- reared monkeys. The monkeys’ immune systems tend to be suppressed, while their levels of stress hormones are higher and their heart rates faster than those of mother-reared monkeys. Might these be the fruits of insecure attachment?
Megan Gunnar thinks so. Gunnar studies the relationship between attachment security and reactions to stress in human infants and toddlers. She’s found that stressful circumstances such as vaccinations, the presence of strangers, and separation from mom produce elevations of the stress hormone cortisol in infants. By age two, however, children with secure attachments to their mothers don’t get these cortisol rushes, even when they act stressed out. Children with insecure attachments, on the other hand, continue to show elevations of cortisol. It’s as if secure attachment comforts the body more than the mind.
In the animal literature, the contact with adult conspecifics-- it doesn’t have to be the mom, but it needs to be somebody who acts like a mom and that the baby is familiar with--has powerful effects at blocking the activity of stress-response systems, says Gunnar. If the attachment figure is present, and the relationship has been reliable, then some aspect of the stress response just doesn’t happen.
That’s a good thing, says Gunnar, because a hyperactive stress- response system can wreak havoc on the body. The racing heartbeats and suppressed immune systems that Suomi sees in peer-reared monkeys, for example, are responses that would normally occur to help the young animal cope with a transient stress--such as being left alone while mom goes out and mates. But in peer-reared monkeys, the stress response is cranked up day in, day out, and that super-responsiveness persists into adolescence-- long past the age of primate attachment. Gunnar proposes that such a skewed stress-response system can promote lasting behavioral changes by interfering with brain development. In rat pups, she points out, chronic stress is known to disturb the development of the limbic system, frontal lobes, and hippocampus, parts of the brain that are involved in fearfulness and vigilance, attention focusing, learning, and memory. Gunnar suggests that secure attachment serves as a buffer against these disturbances, while insecure attachment leaves the brain open to insults that can result in lifelong anxiety, timidity, and learning difficulties.
Of course, anxious, inhibited, or impulsive behavior isn’t necessarily the result of early attachment problems. The extensive work of Harvard psychologist Jerome Kagan certainly suggests that such traits can be inborn. Kagan finds that 20 percent of human infants have the behavioral and physiological signs of an inhibited temperament at just four months of age--presumably, in Kagan’s view, before a child’s environment would have exerted its effects. He has also found that up to 40 percent of four-month- olds have signs of a bold or fearless nature. These tendencies often mellow with time, however, so that by age four only 10 percent of children are either fearful or reckless.
Suomi finds virtually the same proportions of bold and fearful monkeys in his mother-reared troops--a fact that seems to argue for the genetic conservation of temperament. But Kagan contends that the rich inner life of the child may limit the relevance of animal studies, despite the seeming parallels in primate personalities. It’s not just what happens to you that counts--it’s what you think happens to you, says Kagan. And it is inordinately difficult to figure out what a child is thinking. Until we devise ways to measure what is in a child’s head, we’re not going to understand the child’s environment.
The inner life of the child may help explain the phenomenon of so-called resilient children, those who somehow manage to rise above difficult home environments and live normal, even accomplished, lives. But these children are exceptional; it’s clear that abusive or negligent parenting can have devastating effects on a child’s emotional development. All the evidence suggests that physical abuse in childhood, for example, leads to a higher risk of drug use, mood disorders, violence, and criminality in adulthood. Girls who are sexually abused are more prone to depression, panic attacks, eating disorders, drug use, and suicide. And children reared in orphanages, without any parenting at all, often develop a disturbing array of social and behavioral problems. Researchers are beginning to explore the biological mechanisms for these associations, but it’s not hard to imagine the psychological ones.
I think there are people who, for genetic reasons, are more susceptible to certain kinds of stressful stimulation, says Bruce Perry, who studies the physiology of abused and neglected children at the Baylor College of Medicine in Houston. But even with the optimum genetic organization, trauma will create the problems we’re talking about.
Many of the kids Perry sees have been exposed to domestic violence, and their unpredictable and threatening home lives, he says, can be read in both their physiology and their behavior. They seem to be in a perpetual state of arousal: their fight or flight response has somehow been permanently activated, and they have tense muscles, rapid heart rates, and trouble sleeping. Their stress-response systems may be irreparably altered. These kids grow up with a neurophysiology that is perfectly adapted to survive in a chaotic, distressing environment, says Perry. They develop this extreme hypervigilance because they never know what is going to happen next.
But the children of domestic violence are poorly adapted to life in a nonviolent world. Their vigilance can lead them to radically misinterpret other people’s behavior and intentions, says Perry. Boys, for example, will perceive hostility and aggression in a look or an offhand remark and respond too readily in kind (think Robert DeNiro in Taxi Driver). Girls are more likely to shut down or withdraw completely from even mildly threatening circumstances. In school, both boys and girls tend to tune out verbal information and become hypersensitive to nonverbal cues. They might focus more on a teacher’s hand gestures, for example, than the subject he’s lecturing on.
Perhaps it’s not surprising that severe stress in childhood leaves both biological and behavioral scars. But researchers are learning that even less extreme emotional stressors, such as parental conflict or depression, can also have an impact on kids’ behavior and biology. The children of depressed mothers, for example, are at increased risk for depression themselves, and most psychologists think the risk cannot be ascribed entirely to genetics. eeg studies by psychologist Geraldine Dawson of the University of Washington in Seattle show that babies whose mothers are depressed have reduced activity in the left frontal region of the brain--the area implicated in joy, interest, and other positive emotions.
Growing up in even a mildly bad environment appears to affect your biology. The question, of course, is whether those changes can be reversed. Several lines of research suggest that they can be. Suomi, for example, has shown that even monkeys who are born anxious and inhibited can overcome their temperamental handicap--and even rise to the top of the dominance hierarchy in their troop--if they are raised by ultranurturing supermoms. Kagan’s work confirms that mothering can alter the course of an inhibited child’s development. A pioneering day-care program at the University of North Carolina in Chapel Hill has cut the incidence of mental retardation by as much as 80 percent among kids whose unstimulating home environment put them at high risk for low iq. Dawson, too, has found that psychologically depressed mothers who manage to stay positive and engaged in caregiving can minimize the impact of their depression on their babies’ brain waves. And her follow-up work revealed that, at age three, children’s eegs will return to normal if their mothers’ depression lifts.
So I wonder, how plastic is the brain? says Dawson. At what point in development do we start to see enduring effects as opposed to transient effects?
The answer may be never. The new model of neural development holds that the primitive areas of the brain mature first: in the first three years of life, the regions in the cortex that govern our sensory and motor skills undergo the most dramatic restructuring, and these perceptual centers, along with instinctual ones such as the limbic system, will be strongly affected by early childhood experiences. This vulnerability is nothing to scoff at, says Robert Thatcher, a neuroscientist at the University of South Florida College of Medicine in Tampa. The limbic system is where we live, and the cortex is basically a slave to that, he explains.
But the frontal cortex, which governs planning and decision making, and the cerebellum, a center for motor skills, are also involved in emotional development. And those parts of the brain don’t get rewired until a person is five to seven years old. What’s more, another major restructuring of the brain occurs between age nine and eleven, says Thatcher. Suddenly, the brain is looking less like a sculpture in stone, and more like a work in progress.
In fact, Thatcher’s readings of the eegs of adolescents and adults have revealed that some reorganization of the brain may occur about every two years from birth to death. He proposes that these reorganizations happen in response to waves of nerve growth factor that sweep across the cerebral hemispheres in two-year cycles, revamping up to one-fifth of the brain’s synaptic connections at the leading edge of the wave. The idea of the traveling waves is just a theory now--but it’s a theory that’s making more sense to more scientists.
The brain doesn’t stop changing after three years, says Megan Gunnar. For some things, the windows of influence are only beginning to close at that age, and for others they’re only beginning to open. If Thatcher is right, the brain is, in fact, under lifelong renovation. Long- term studies are just now beginning to demonstrate that experiences later in life can redirect emotional and behavioral development, even in adulthood. Some of us--and our parents--are greatly relieved by the news.
