Chapter Introduction

CHAPTER 5
The First Two Years: Biosocial Development

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What Will You Know?

  1. What part of an infant grows most in the first two years?

    From two weeks after conception to two years after birth, the brain grows more rapidly than any other organ, reaching about 25 percent of its adult weight at birth and almost 75 percent of its adult weight by age 2.

  2. Does brain wiring in the first two years depend on genes or experience?

    The specifics of brain structure and growth depend not only on genes and maturation. Experience may be even more crucial. Infant brain organization itself depends partly on input, and some dendrites wither away because they are never used—that is, no experiences have caused them to send a message to other neurons. Expansion and pruning of dendrites occur for every aspect of early experience, from noticing musical rhythms to understanding emotions. Experience—both expected and varied—shapes the infant brain, as pruning eliminates unused connections.

  3. Which of the five senses develops last: seeing, hearing, tasting, touching, or smelling?

    Vision is the least mature of the newborn's senses.

  4. What happens if a baby does not get his or her vaccinations?

    Immunization protects babies not only from many illnesses but also from complications related to those illnesses, including deafness, blindness, sterility, and meningitis. Not getting immunized puts babies at risk for contracting diseases that are easily preventable through vaccines. Moreover, each vaccinated child stops transmission of the disease and thus protects others, a phenomenon called herd immunity.

  1. Body Changes

    Body Size

    Sleep

    opposing perspectives: Where Should Babies Sleep?

  2. Brain Development

    Connections in the Brain

    Experience Shapes the Brain

    a view from science: Face Recognition

    Harming the Infant Brain

    Implications for Caregivers

  3. Perceiving and Moving

    The Senses

    Motor Skills

    Cultural Variations

  4. Surviving in Good Health

    Better Days Ahead

    a case to study: Scientist at Work

    Immunization

    Nutrition

Our first child, Bethany, was born when I was in graduate school. I memorized developmental norms, including walking and talking at 12 months. In her first year, she babbled and said “mama,” but at 14 months, Bethany had not yet taken her first step.

To reassure myself, I told my husband that genes were more influential than anything we did. I had read that babies in Paris are among the latest walkers in the world, and my grandmother was French. To my relief, Bethany soon began walking, and by age 5 she was the fastest runner in her kindergarten. My genetic explanation was bolstered when our next two children, Rachel and Elissa, were also slow to walk. My students with ancestors from Guatemala and Ghana bragged about their infants who walked before a year; those from China and France had later walkers. Genetic, I thought.

Fourteen years after Bethany, Sarah was born. I could afford a full-time caregiver, Mrs. Todd, from Jamaica. She thought Sarah was the most advanced baby she had ever known, except for her own daughter, Gillian. I told her that Berger children walk late.

“She’ll be walking by a year,” Mrs. Todd told me. “Gillian walked at 10 months.”

“We’ll see,” I graciously replied, confident of my genetic explanation.

I underestimated Mrs. Todd. She bounced my delighted baby on her lap, day after day, and spent hours giving her “walking practice.” Sarah took her first step at 12 months—late for a Todd, early for a Berger, and a humbling lesson for me.

As a scientist, I know that a single case proves nothing. My genetic explanation might be valid, especially since Sarah shares only half her genes with Bethany and since my daughters are only one-eighth French, a fraction I had ignored when they were infants.

Nonetheless, I now notice how caretakers influence every aspect of biosocial growth. As you read this chapter, you also will see that caregiving enables babies to grow, move, and learn. Development is not as straightforward and automatic, nor as genetically determined, as it once seemed. Genes provide the outline, but every moment of life after birth shapes and guides the young person to become a distinct, and special, human being.

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