Making the Connection: Strategies to Build Effective Personal Relationships (Collection) (62 page)

Critical Periods in Brain Development

A critical period is a window in time when certain brain circuits are open to essential environmental information. Arrival of this information shapes the circuits in a lasting way.
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Once this shaping is completed, the window is closed.

The most famous example of a critical period comes from Konrad Lorenz, who studied the behavior of baby
geese. Lorenz found that each baby is primed to pay special attention to the first moving creature it sees after hatching—generally, its mother. This information is immediately imprinted in its brain, which leads it to follow its mother in those cute little trails of goslings. But if the mother goose is removed during hatching and replaced by another moving creature—such as Lorenz himself—the babies may imprint on him instead. The result is recorded in pictures of goslings trailing the bearded scientist.

Another well-known example is the development of the vocalizations of male songbirds, and it, too, involves a social interaction. In this case, the critical period of brain development is not confined to the minutes after hatching, but lasts for a few months. During this time, each juvenile male bird shapes its simple innate song by progressively matching it to the complex song of an adult male.
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Without such instruction during this critical period, it will never be able to sing like an adult.

These critical periods in goslings and songbirds provide the opportunity to incorporate essential environmental information that is uniquely valuable to each species. For humans, a notable example is learning to speak, which develops during a critical period that lasts for more than a decade.
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During this period, children don’t only learn their native language. They also pick up the accent of the people they grow up with, especially their peers.
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As this critical period closes, it becomes very difficult to speak like a native. This is why immigrants such as Henry Kissinger, who learned English in his teens, speak with a foreign accent. Even natives who
migrate to a different region can be spotted in this way: Four decades in California have not erased the vestiges of my own linguistic imprinting in New York City.

Although researchers have studied these critical periods of brain development for many years, we still have limited information about their number and the ways they are closed. But the main message is clear: Certain brain circuits become established at particular times, and their properties tend to endure. A similar process appears to be at work in the development of many aspects of our personalities.

What Will My Child Be Like?

Although a baby is born with an immature brain, it immediately becomes a player in the world. At first, it can only cry to signal distress or coo to signal contentment. But its behavioral repertoire grows rapidly in its first few years of life as it builds new brain circuits and remodels others.

As brain development continues, parents begin wondering if their child’s early patterns of behavior can provide clues about his or her mature personality. Researchers have tried to answer this question by examining children repeatedly from infancy to adulthood. Because each research group uses its own system for describing behavioral patterns, it’s difficult to compare the results. Nevertheless, there is general agreement that early patterns persist in some children, whereas other children change a lot.

Evidence for some persistence of patterns comes from pioneering studies by Stella Chess and Alexander Thomas,
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a wife-and-husband team of child psychiatrists. From their observations of babies they identified three broad patterns of behavior, which they called temperaments. Forty percent of the babies were called “easy” because they approached new situations without difficulty, had high adaptability to change, accepted most frustration with little fuss, and were not very moody. In contrast, the 10% of the babies who were called “difficult” were much more inclined to be irritable, showed intense negative emotions, and had trouble adapting to change. Another 15%, called “slow to warm up,” were initially uncomfortable in new situations but adapted after repeated contact. The remaining 35% showed a mixed picture.

Follow-ups of the children as young adults indicated that there were “only modest levels of consistency in temperament over time for a group of subjects as a whole.” Their conclusion in 1986 fits well with what we know today: “Maturational factors, neurophysiologic changes, and a host of environmental influences—all these serve to produce continuity in some individuals and change in others.”
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A series of studies led by Jerome Kagan also found evidence for both continuity and change. Kagan identified subgroups of children that he called inhibited and uninhibited, based on their willingness to engage with unfamiliar people when they were 2 and 7 years old. When he re-examined them in adolescence, he found that the majority of the children in the inhibited group remained quiet and serious, while only 15% were as lively and talkative as the average teen from the uninhibited group. Of the children in the uninhibited group, 40% maintained that style as teens, and only 5% had become
subdued and quiet. As Kagan summed it up, “[A]bout one-half the adolescents retained their expectable demeanor, while only 15 percent had changed in a major way.”
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Some behavioral continuity of members of the two groups was also observed at age 22. In brain imaging studies, the inhibited group showed significantly more activation of the amygdala when shown pictures of unfamiliar faces.
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This sign of a stronger emotional response to new faces is reminiscent of their greater wariness of strangers as toddlers. Other researchers have also found that children retain many of their characteristics as adults.
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Evidence of continuity into adulthood is particularly strong for a subgroup of children who show signs of antisocial behavior in grade school. If they are sufficiently aggressive and impulsive to be singled out as having a conduct disorder before the age of 10, they tend to maintain this antisocial pattern when they have grown up.
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In contrast, children who don’t show signs of antisocial behavior until their teens are more easily reformed and have a better chance of becoming law-abiding adults.
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Other kinds of behavior that are prominent in childhood may change dramatically in adolescence, including some behaviors that are known to be heritable. For example, heritable childhood fears of heights, snakes, or blood frequently disappear by the time the children are in their teens.
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How these and other waxing and waning genetic effects eventually play out also depends, in part, on interactions with the person’s environment.

Gene–Environment Dialogues

Persuasive evidence of the combined effects of environment and genes comes from studies of people with antisocial personalities.
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Everyone who has watched
The Sopranos
knows that antisocial behavior runs in families, and studies show that 10% of a community’s families commit most of its crimes.
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So you won’t be surprised to learn that studies with twins show a 40% to 50% heritability of antisocial traits.
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But in this case, family environment also has a significant effect. Furthermore, adopted children raised in antisocial families have an increased risk of developing an antisocial personality pattern,
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even though they are genetically unrelated.

Added support for the importance of family environment comes from a study of a group of children in Dunedin, New Zealand. The researchers enrolled all of the 1,037 children born in this city from April 1972 through March 1973, assessed them at multiple intervals through the age of 26, and stored the data for subsequent analysis. This provided detailed information about child development in the entire community without preconceptions about what might show up.

One notable finding was that many of the children were abused: 8% had “severe” maltreatment, 28% had “probable” maltreatment, and only 64% had no maltreatment.
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But this should not be taken to mean that New Zealanders are particularly nasty. In carefully controlled interviews of 8,667 American adults, 22% reported sexual abuse during childhood, 21% reported physical abuse, and 14% reported witnessing their mother being beaten; many reported all three.
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A substantial portion also described repeated emotional abuse.

Having detected considerable child abuse in Dunedin, the researchers wondered whether it was correlated with the development of an antisocial personality pattern. To answer this question, they concentrated on boys because they are more likely than girls to develop this pattern. They found that the degree of maltreatment of the boys was, indeed, correlated with the degree of antisocial behavior. But there was considerable individual variation. Some of the severely maltreated boys developed a troublesome antisocial pattern, whereas others did not.
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Why?

One possibility is that the boys who became antisocial had a genetic predisposition to turn out this way. For example, they might have been innately defiant or aggressive, which might have called forth more abuse. Such interactions between a child’s innate tendencies and parental reactions are one reason children raised in the same family turn out to be so different,
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and this likely played some role in the Dunedin study. But in this case, the researchers decided to get more specific by looking for a single gene variant that influenced the antisocial outcome.

To get started, they examined a plausible suspect: the MAOA gene. This gene makes monoamine oxidase-A, an enzyme that degrades serotonin, norepinephrine, and dopamine, three neurotransmitters that control brain circuits involved in emotional behaviors. Two characteristics of the MAOA gene made it seem relevant: brain levels of monoamine oxidase-A influence many types of antisocial
behavior;
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and variants of the MAOA gene’s promoter control the manufacture of different amounts of the monoamine oxidase-A enzyme in the brain.
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Furthermore, the MAOA gene happens to be located on the X chromosome, which simplifies its study in boys because they have only one copy (girls have two). In the Dunedin study, 63% of the boys had the high-MAOA variant, which makes a lot of the enzyme in the brain, and 37% had the low-MAOA variant, which makes less of it.
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Is having the high- or low-MAOA gene variant correlated with antisocial behavior? The researchers found that, by itself, it is not. Boys who hadn’t been abused had little antisocial behavior, regardless of which variant they had. But among the abused children, there was a significant effect. Those abused children with the low-MAOA variant were more likely to become antisocial.
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Several subsequent studies of antisocial men support these findings.
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So does a study of women from an American Indian tribe who had experienced childhood sexual abuse.
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In this case, too, abuse was correlated with an antisocial pattern of behavior, and those abused women with two copies of the low-MAOA gene (one on each of their X chromosomes) had the highest rate of antisocial behavior. In contrast, those with two high-MAOA genes had the lowest rate of antisocial behavior. Furthermore, as with men, the MAOA gene didn’t matter in the absence of abuse.

This doesn’t mean that being born with the low-MAOA variant is bad news. Having more or less monoamine oxidase-A has multiple effects on brain functions,
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and these
may have desirable or undesirable consequences. The outcome depends on individual circumstances, other gene variants, and one’s taste in personalities. The big story from the studies of childhood abuse and MAOA is more general. It illustrates the principle that genetic differences can influence the effects of childhood environments on a personality.

Enduring Effects on Gene Expression

It also works the other way: Environment can have enduring effects on the expression of particular genes that affect behavior. The best example comes from studies in Michael Meaney’s laboratory of the effects of rat mothering on the personalities of their pups. The studies began by comparing the behavior of the offspring of two types of rat mothers: high-lickers who licked and groomed their pups vigorously, and low-lickers who were less enthusiastic.
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When these offspring were tested months later, those raised by the high-lickers were less fearful and less reactive to stress than those raised by the low-lickers. Furthermore, their greater emotional stability was apparent not only in behavioral tests, such as open field activity, but also in their blood levels of glucocorticoids, stress-related hormones released from the adrenal gland.

Was the greater emotional stability of the highly licked pups caused by the maternal behavior (nurture)? Or did the high-licking mothers also have genetic differences that were transmitted to their pups via their DNA (nature)? To find the answer, pups born to high-licking mothers were swapped immediately after birth with those born to low-licking mothers, the adoption tactic that Galton had proposed to
distinguish nurture from nature. The results of this cross-fostering pointed to nurture, the maternal behavior, rather than the maternal genes. High-licking foster mothers did just as good a job as high-licking biological mothers in producing stress-resistant pups, and vice versa.