Welcome to Your Child's Brain: How the Mind Grows From Conception to College (8 page)

Do babies who walk earlier end up with better motor skills than those who walk later? Probably not, unless they continue to practice these skills more than other children. In some cultures, adults routinely run long distances or carry huge weights, but those skills require many years of training.

Similarly, experience with language influences the development of concepts. The Korean language includes a complex system of verb endings to carry information, whereas English relies heavily on nouns to convey meaning. Korean baby talk is full of verbs that contain implied prepositions (
moving into
) and often omits nouns entirely, while English baby talk contains a lot of nouns (
a doggy
). Perhaps because of this experience, American toddlers begin to categorize objects, for instance, by sorting them into piles by type, at a younger age than Korean children. In contrast, Korean children learn to use a rake to retrieve a toy that’s out of reach earlier than they learn to categorize, suggesting that they find it easier to think about actions than about objects.

From birth, babies can imitate other people and seem to enjoy doing it, which not only is a powerful tool of social bonding but also provides direct examples of behavior for babies to copy. Infants imitate the goals of actions, rather than their exact form, and other people’s movements appear to be coded in their brains in terms of goals as well. For example, if a fourteen-month-old baby watches a person tap her head on a box, which then lights up, a week later he will tap his head on the box to make it light up. But if the demonstrator’s hands were wrapped up in a blanket when she used her head, most babies will instead touch the machine with their hands, apparently assuming that the demonstrator used her head because her hands weren’t available. You might amuse yourself by cooking up a similar game to play with your own baby.

During this period of intense learning, a huge number of connections between neurons are added to the baby’s brain. Just before and after birth, as many as forty thousand new synapses are added every second. A baby’s brain reaches 70 percent of adult size by the first birthday and 80 percent of adult size by the second birthday. This growth is pronounced in the cerebellum, a region that integrates sensory information to help guide movement, as babies are learning how to control their bodies. The cerebral cortex also has a lot of growing left to do at
birth. It doubles in size over the first two years of life, with most of that growth happening before age one. Though a small part of the growth is due to birth of new neurons, most of it is caused by the formation of new connections. The elaboration of axons, dendrites, spines, and synapses, all parts of a neuron that allow it to talk to other neurons, occurs rapidly throughout the first year. Myelination of axons is also intense during this time, as glial cells wrap themselves around axons to form an insulating layer that increases the speed and efficiency with which signals are carried from one neuron to another.

Babies are extremely good at getting what they need from their adult caregivers.

You might imagine that a baby’s experiences would determine where new synapses are formed, but that doesn’t seem to be what happens. Instead the brain produces a huge number of relatively nonselective connections between neurons in early development and then gradually removes the ones that aren’t used often enough (see
chapter 5
). If the brain were a rosebush, life experience would be the pruning system, not the fertilizer.

Motor development occurs in a sequence that is determined by brain maturation. Because the primary motor cortex contains a map of the body that develops in sequence, babies learn to control their head and face movements before they learn to reach, and only later do they learn to walk. By the third month of life, the infant’s brain has developed enough to produce significant advances in behavioral control. At this age, babies start to be able to inhibit reflexes and eye movements. Their motor abilities allow them to react to maintain equilibrium when their posture is disturbed. They also develop clearly goal-directed behaviors, including head–eye coordination and reaching for objects. This transition also reduces the amount of time that babies spend crying. Fortunately for the parents of fussy newborns, behavior in the first three months of life does not predict future temperament very well.

By four months of age, the eye movements of babies show that they can predict when an object will emerge from behind a screen, the earliest exercise of a skill that becomes increasingly important with age. Learning to anticipate future events, such as correcting your posture to offset a threat to balance before it occurs, is a key aspect of adult motor function. Predictive motor control is another
function of the cerebellum, so its maturation is likely to be important for the development of this ability.

Even when they’re very young, babies know something about objects, but they still have much more to learn. The fact that space is three-dimensional seems to be apparent even to young babies. Newborns will duck away from objects that are heading toward them, and as soon as they can control their arms, babies will try to reach in the direction of objects that they desire.

The idea that objects have fixed properties, on the other hand, seems to dawn slowly. In early life, motion seems to be a key to object perception. Adults use this cue too—things that move together are seen to be parts of the same object—but babies take the idea to the extreme. At five months, babies who are shown a stuffed animal going behind a screen and a toy car emerging on the same trajectory do not appear surprised. At that age they can certainly tell the difference between the two toys, but the object’s motion appears to be more important to them. By the age of one, changes in most object properties (such as shape or color) will elicit a reaction, suggesting that the brain’s representation of occluded objects has become much richer.

Babies do all this work without needing any special classes or equipment. Any baby with a normal brain and environment can develop the skills that are important during this period of life. They are driven to practice these skills, and parents are well suited to teach them, just by interacting with their children in everyday life. Most parents of infants can simply do what comes naturally and enjoy watching and helping their babies make discoveries about the world.

Chapter 4
BEYOND NATURE VERSUS NURTURE

AGES: CONCEPTION TO COLLEGE

When scientists say that the “nature versus nurture” controversy is outdated because both forces work together, that’s not a case of fatigued combatants pleading, “Can’t we all just get along?” It’s a biological fact—and we understand quite a bit about how the process works.

So far, we have told you that your child’s brain builds itself through largely automatic programs, and that it adapts to its environment. These two statements might seem to contradict each other if you think of
automatic programs
as
genes
, as some people may be tempted to do. That’s not quite right, though; these programs control the interplay between genes and the environment during your child’s development.

One reason that people get so worked up over this debate is the widespread assumption that genetic contributions to development are deterministic, while environmental contributions are flexible. That’s why it’s seen as conservative to argue that boys and girls are biologically distinct, and as liberal to argue that socialization is responsible for their behavioral differences. Such discussions lead nowhere because both assumptions are incorrect.

Genes establish a program to build a brain (see
chapter 2
), but then that brain reacts to the world, extensively tuning itself to local conditions as your child grows. The human ability to live in a wide variety of circumstances has resulted from natural selection favoring genes that contribute to behavioral flexibility (see
Did you know? Culture can drive evolution
). Nearly all genes that influence behavior act by changing the odds of a particular developmental outcome, not by specifying it exactly—so your child’s heredity is not destiny.

On the other hand, some environmental effects on development cannot be undone. For instance, the surrounding culture completely determines which native language your child will speak, but once the learning process is finished, there’s no possibility of substituting a different native language in its place.

Indeed, from an individual neuron’s perspective, it would be hard to distinguish between “genetic” and “environmental” influences. Signals that enter your brain through your eyes or ears (that is, via experience) influence development by causing chemical signals to modify genes or proteins—just as genetic influences do. Some of these changes are reversible, and some are not, but whether they originated inside or outside the body is not the determining factor.

Later in this book we will talk about how experience can change the connections and chemistry of neurons. Here we want to explain just how entangled genes and environment are in your child’s development.

DID YOU KNOW? FOOTPRINTS ON THE GENOME

How can your child’s experiences cause permanent changes to her genes? This idea may seem to fly in the face of what you learned in science class, but it relies on cellular processes that are familiar to biologists. In response to a variety of signals, so-called epigenetic modifications can silence a region of DNA by making chemical changes that affect its shape, so that the protein encoded by that gene cannot be made (see figure opposite). When DNA is copied during cell division, the pattern of epigenetic modification is copied as well, so that all descendants of the cell maintain the information.

Researchers have long known that this process explains why various cell types (such as neurons versus kidney cells) look and act very different, even though they contain the same DNA. More recent work reveals that environmental events can cause similar long-lasting changes to DNA, providing a way for transient experiences to permanently modify gene expression. The accumulation of epigenetic modifications also explains why identical twins, who share all their genes, do not look exactly the same.

When epigenetic modifications occur in sperm or eggs, they can affect future generations. This process is best understood in laboratory animals. For example, female mice that spent a particular two weeks of their youth in an “enriched environment” (with many toys) learned more easily as adults. And so did their pups—even when those pups were raised by a foster mother and did not receive any enrichment themselves. The pups instead benefited from their mother’s experience, passed down through epigenetic modifications to her DNA.

This research is in an early stage, so the list of known epigenetic effects continues to grow. Early social experience can modify later behavior, including stress responsiveness (see
chapter 26
), due to epigenetic modification of particular genes. Prenatal and early postnatal nutrition can influence the adult risk of heart disease, type 2 diabetes, obesity, and cancer in people. Experiments in animals support the idea that these effects may be due to epigenetic modification of DNA. Cocaine addiction also seems to involve epigenetic changes, perhaps explaining why it is so difficult to reverse. Epigenetic modification is a simple chemical process, but it can encode life’s experiences, quite literally, into who you are, and even who your children will become.