Welcome to Your Child's Brain: How the Mind Grows From Conception to College (9 page)

To start with a basic principle, your child’s genes can influence his environment—and vice versa. His personal characteristics lead him to seek out certain experiences in life (see
Did you know? Why you’re turning into your mother
), and his tendencies to react to other people in certain ways affect how they behave toward him. A fussy infant who is difficult to soothe cannot be treated exactly like a calm and happy infant, no matter who the caretaker is. At the same time, the environment that your child encounters (before or after birth) can cause permanent changes to his genes. Chemical modifications in response to experience, such as methylation, can turn off certain genes in particular cells—often for a lifetime (see
Did you know? Footprints on the genome
).

Because the influences run in both directions, many developmental processes are feedback loops, in which our genes influence our environment, which then influences our genes (or at least the way they are expressed), and so on. The idea of reciprocal influence is challenging to grasp. When we think about genetic inheritance, our minds usually reach for a familiar example, such as our schoolhouse lessons on Gregor Mendel’s wrinkled and smooth peas, or in our lives, genes for eye color. These examples are taught in school because they act in simple ways that can be drawn on a chalkboard, but the ways that most genes influence the brain and behavior are much more complicated.

There is no doubt that both genes and environment strongly influence individual differences in behavior. The genes that a baby inherits from her parents do not determine exactly what kind of person she will become, but they do define the range of possible developmental outcomes that are open to her. Even so, the same genetic tendencies can play out very differently in different cultures (see
chapter 20
). With all this interaction, it is nearly impossible to figure out how much of a particular behavior is caused by genes and how much by environment.

The first problem is that these kinds of estimates apply only to one particular environment that was studied and may change (a lot) under other circumstances.

DID YOU KNOW? CULTURE CAN DRIVE EVOLUTION

Our life experiences can also modify the human genome over evolutionary time scales—through the effects of cultural changes on natural selection. Geneticists think of culture as any learned information that influences behavior, which can include beliefs, values, skills, and knowledge.

When people first learned to domesticate cows around eight to nine thousand years ago in Egypt, milk became available as food for adults, not just infants. Before this change in the environment, which was entirely due to a cultural innovation, adults were lactose intolerant and could not digest milk. As herding spread, the genes that lead to production of the lactase enzyme in adults became more and more common because adults who could digest milk had access to a better food supply than those who couldn’t. Different genetic mutations led to lactose tolerance in European and East African populations, in both cases around the time that herding was introduced. Today, lactose intolerance remains common in people of Asian and West African descent, whose ancestors did not adopt cattle herding.

Researchers estimate that between several hundred and two thousand human genes show signs of recent rapid evolution, many of which may have been driven by cultural changes. Genes that help our bodies respond to pathogens, for instance, have changed quickly. Evolution of new immune-system defenses may have been driven by the development of farming, which led to many new human diseases by bringing people into close contact with animals and their germs.

Other groups of genes are rapidly changing, too. One is related to digestion of various types of food and alcohol, which may be shaped by dietary practices. The invention of cooking was correlated with changes in digestion, bitter taste receptors, tooth enamel, and jaw musculature. Another example is genes that affect brain function, as we would expect from the substantial advantages provided by brain talents like language and learning ability. Somewhat curiously, a third category is genes that drive physical appearance, such as skin color, hair color and thickness, and eye color. Selection for these genes may be a product of culturally driven sexual preferences as well as environmental drivers such as sun exposure.

Cultural changes can also protect human populations from the selection pressures imposed by new environments. When people migrated to cold places, they learned to build fires and dress in fur coats, rather than developing the fur and insulating layers of fat that protect other animals from freezing temperatures. Some researchers have speculated that the ability to adapt to new environments through learned behaviors could free our species from some of the constraints imposed on other animals by natural selection, thus allowing us to maintain an unusually large variety of genetic traits in our population—another possible contributor to our famous behavioral flexibility.

For example, in middle-class populations, about 60 percent of the individual variability in IQ is attributed to genetic differences and almost none to the environmental circumstances shared by children within a family. In contrast, among people living in poverty (see
chapter 30
), about 60 percent of individual variability in IQ is due to shared environment, while genes account for less than 10 percent. In other words, what genes tell you about a child’s potential for intelligence is limited to the particular circumstances in which he or she is growing up.

The second problem is even more serious. A developmental outcome that occurs only when a child with a particular set of genes encounters a certain environment is known as a
gene-environment interaction
. Another way to put this idea is that certain genetic characteristics can make a child sensitive to aspects of her experience that wouldn’t have any effect on a child of a different genetic background, a theme we will return to later in the book. Such interactions explain the otherwise paradoxical findings that many highly heritable characteristics have increased in the population much faster than biological evolution could explain over the past few decades. Examples range from obesity to intelligence (see
chapter 22
) to nearsightedness (see
Practical tip: Outdoor play improves vision
).

Gene-environment interactions are a problem in this context because researchers assume that the two factors act independently when they calculate those percentages of genetic versus environmental influence that you’ve seen in the newspaper. But, as we’ve said, that’s rarely the case. Worse still, any interactions that do occur are included in the “genetic” percentage—making the effects of the environment look less important than they really are.

To illustrate these points, let’s look at a study of petty criminality in 862 adopted Swedish boys. In this study, either genetics (a criminal parent) or a bad environment (unstable early placement or poor adoptive family) increased the risk of criminality in a child. We would not expect geneticists to identify a “lawbreaking gene,” but traits like impulsiveness and aggression are influenced by heredity and can substantially affect a person’s odds of breaking the law. Compared to the baseline crime incidence of 2.9 percent for children born to noncriminals and raised in a good environment, the incidence was 12.1 percent for biological children of criminals raised in a good environment and 6.7 percent for biological children of noncriminals raised in a bad environment.

From an individual neuron’s perspective, it would be hard to distinguish between “genetic” and “environmental” influences.

Imagine that genetic and environmental influences were independent of one another. In that case, you could guess the likelihood that a child born to criminal parents
and
raised in a bad environment would commit a crime, simply by adding the two percentages to get 18.8 percent. But the study found something very different. Children with both risk factors, the biological children of criminals raised in a bad environment, had a much higher rate of criminality, 40 percent—more than twice the risk that would have been expected.

At the same time, neither factor determined the children’s fates. Even under the worst conditions, more than half the children with multiple risk factors turned out to be law-abiding citizens. None of these factors absolutely determines a child’s outcome—but they do change the odds.

So the next time you read that intelligence is 60 percent genetic or that researchers have discovered the gene for homosexuality or that children are aggressive only because they’ve learned the behavior from role models, keep in mind that biology doesn’t work that way. Genes and environment are irrevocably entangled throughout your child’s life.

PART TWO
GROWING THROUGH A STAGE

ONCE IN A LIFETIME: SENSITIVE
PERIODS

BORN LINGUISTS

BEAUTIFUL DREAMER

IT’S A GIRL! GENDER DIFFERENCES

ADOLESCENCE:
IT’S NOT JUST ABOUT SEX

Chapter 5
ONCE IN A LIFETIME: SENSITIVE PERIODS

AGES: BIRTH TO FIFTEEN YEARS

Your child’s brain is a bit like IKEA furniture: built through a series of steps that normally occur in order. Failing to complete certain steps on schedule—as we certainly have done when assembling a table—can interfere with later steps in the process, usually delaying them but sometimes preventing them from happening at all.

In this chapter, we discuss a special type of development that is central to matching your child’s brain to the environment.
Sensitive periods
are times in development when experience has a particularly strong or long-lasting effect on the construction of brain circuitry. Receiving the correct sort of experience during a sensitive period is essential for the maturation of the particular behaviors that rely on that circuitry.

Not all aspects of early development are so demanding. Much of brain maturation occurs without special help. For example, neural circuits in the retina and spinal cord mature according to a set program that is not responsive to experience at all. Other regions—such as the hippocampus and certain parts of the cerebral cortex—are modifiable by experience not during a short period of time but instead throughout life. These brain regions are always able to acquire new information, which helps us continue to adapt to our environment during adulthood.

Sensitive periods for particular functions are special because during these times, the quality of a child’s experience can have permanent effects. For example, the brain areas that are specialized for understanding language end up with different connections between neurons depending on whether your baby hears English or Mandarin during the first few years of life (see
chapter 6
). The brain changes that occur in response to this experience make your child an expert at understanding and producing the sounds of his native language.