Week 13 · Chapter

Chapter 13 — Adolescence: Physical and Cognitive Development

Child Development Free sample edition

The module's primary reading · ~35–45 minutes · Objective 8 (analyze development in adolescence: puberty, the adolescent brain, formal operational thought, adolescent egocentrism, identity statuses, and relationships with parents and peers) · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the workshop all build on it.

In plain language

This chapter is about the first half of the teen years, when the body, the brain, and the way a person thinks all change at once. You will learn about puberty (the hormonal and physical changes that lead to reproductive maturity) and why it starts at such different times for different teens. You will see a model of the changing teenage brain and what it can and cannot tell you. You will meet Piaget's last stage, when teens begin to reason about possibilities, and two feelings that come with new thinking: being watched by everyone, and being one of a kind. Every age in this chapter is a typical range, never a deadline.

Why you already care about this

Picture a youth program's showcase on a warm evening. A 15-year-old walks toward the microphone, catches a shoe on a cable, steadies, and sings the whole song. The audience claps. Later, in an interview given with permission, the teen says one thing about the night: they were sure "everybody saw" the stumble.

Then the teen mentions something ordinary. School starts at 7:30 a.m. In those two sentences sit three ideas from this week. One is a feeling about an audience. One is a school schedule set by adults. One is the new kind of thinking that lets a teen imagine what everyone in the seats was thinking. You will read that interview three ways before this chapter ends.

By the end of this chapter you will be able to:

describe puberty and the wide range in its timing in words, including the secular trend;
explain the adolescent brain model and name its limits;
state the American Academy of Pediatrics' 2014 school start position and compare a school's start time with it;
explain Piaget's formal operational stage and place a response on the pendulum problem;
tell the imaginary audience apart from the personal fable;
read record R8 of the running case with two theories and end in one plain sentence.

Carry this sentence with you: a teen's brain is changing, not broken.

Where we are

Already covered: Week 1 set the periods, with adolescence at about 11 to 18, and Bronfenbrenner's nested systems. Week 5 showed the infant brain building and pruning connections. Week 9 taught Piaget's preoperational stage and the egocentrism of early childhood. Week 11 taught the concrete operational stage, logic about real things. Week 12 turned to friendships, bullying, and moral reasoning in middle childhood.
This week: the physical and cognitive domains in early adolescence — puberty and its timing, the adolescent brain model, sleep and the school day, Piaget's formal operational stage, and adolescent egocentrism.
How they connect: the Piaget tasks you met in Weeks 9 and 11 reach their last row this week, and record R8 shows the running case as a teen.
Where we're going next: Week 14 stays in adolescence and turns to the social-emotional domain: identity, relationships with parents and peers, and risk-taking.

Two earlier ideas carry this week. From Week 11, a child in the concrete operational stage, at about 7 to 11, reasons well about real things in front of them. From Week 9, Piaget's egocentrism is a young child's difficulty taking another person's point of view, as in the three mountains task. You will use both on the running case, the illustrative, unnamed child whose eight records follow them from birth to 15.

1. Change on a wide schedule: puberty and its timing

Picture two 12-year-olds waiting at a pediatric clinic for their yearly checkups. One is already taller than a parent. The other is still the shortest in the class. Both families ask the same question at the desk: is this normal?

Here is the plain idea first. You can picture puberty as the body's change from a child's body to an adult's body. Hormones, chemical messengers in the blood, signal the change. A teen goes through a growth spurt, the body's shape changes, and the sex characteristics develop.

Puberty is the set of hormonal and physical changes that lead to reproductive maturity, including a growth spurt, changes in body shape, and the development of the sex characteristics; its timing varies widely from one healthy teen to the next.

The key words are "varies widely". Two classmates of the same age can sit at very different points in puberty, and both can be developing typically. That is why this course pins no single age for any part of it.

The timing has also shifted across generations. Over the past century, the average age at which puberty begins has moved earlier.

The secular trend is a change in a population's average over generations; in puberty, the shift over the past century toward an earlier average age at which puberty begins.

A trend in the average tells you nothing about one teen. It describes a whole population, the way a change in average height describes a country, not a child.

Now the question both families were really asking. Pubertal timing is whether a teen begins puberty earlier, later, or at about the same time as peers.

Pubertal timing is whether a teen begins puberty earlier, later, or at about the same time as peers; research describes its effects as associations that depend on context, not fixed outcomes.

Being early or late compared with friends can shape how a teen feels about the change and how other people respond to it. A teen who looks older may be treated as older. A teen who is still waiting may feel left behind. Research describes these patterns as associations, and they depend on context: the family, the school, the friends, and the culture around the teen. They are not destinies, and they do not show that timing causes any one outcome.

One caveat matters most here. This course gives no medical advice. A question about one real teen's growth belongs with that family and its health professional, never with this course or a student observer.

Misconception check — the single start date. Students often think puberty starts at one age for everyone, so a teen who is "late" or "early" must have a problem. It is tempting, because school groups everyone by age. The cure: the timing of puberty varies widely among healthy teens, and a family with a question talks with a health professional. A classmate who is early and a classmate who is late can both be developing typically.

Pause and think. Choose one option and answer it in two or three sentences:

(a) Think of a time you or someone you know was the first or last in a group to reach some change. How did other people respond?
(b) Why might a teacher of 12-year-olds avoid comparing students' growth out loud? Explain in your own words.

2. Model the teenage brain: reward early, planning later

Picture a high school robotics team the night before a competition. Someone suggests a risky last-minute change to the robot's arm, and half the team cheers. One teen says, "Let's test the old arm one more time first." Both reactions come from teenage brains, and the brain model in this section helps explain why the first one is so tempting.

Here is the plain idea you need: two parts of the brain mature on different schedules.

The limbic system is a set of brain regions involved in reward and emotion that, in the course's model of the adolescent brain, becomes more sensitive to reward early in adolescence.
The prefrontal cortex is the front part of the brain involved in planning and impulse control, which keeps maturing into the twenties.

Put the two together and you have the model.

The adolescent brain model is the account in which reward and emotion regions mature earlier in adolescence than the prefrontal regions for planning and impulse control; it describes average change in groups, not a reason or an excuse for one teen's behavior.

You can think of it as an accelerator that comes in early and a steering system that keeps improving for years. The cheer for the risky change is the reward side. The teen who says "test it first" is using planning skills that are growing, not missing.

Call back Week 5. There, the infant brain built connections in great numbers and pruned the ones it did not use. Brain development did not end in infancy. The planning regions are still maturing in the twenties.

Every model has limits, and you should know this one's biggest. It describes average change across groups of teens. It cannot predict what one teen will do on one night, and it is not an excuse for any choice. Many teens plan carefully, and many adults act on impulse.

Misconception check — the broken brain. Students often think the teenage brain is "broken", or that brain science excuses whatever a teen does. It is tempting, because headlines love a simple story. The cure: the model describes a brain in uneven change, with reward sensitivity rising early and planning regions maturing later. It is a model of groups, not a verdict on a person, and it does not excuse any choice.

3. Read the school day: sleep and the 2014 start time position

Picture yourself at a bus stop on a dark, early morning. A few teens stand with hoods up, one leaning on a backpack with eyes closed. Scenes like this one raise a question about the school clock, and a pediatric organization has taken a position on it.

The American Academy of Pediatrics' 2014 policy statement on school start times for adolescents recommends that middle and high schools aim for a start time of no earlier than 8:30 a.m. That is the issuing body's position, with its year. It is a recommendation for schools, and this course states it the way that body states it.

Here is the running case. The running case (Dataset A) is illustrative, constructed for this course, and describes no real teen. Record R8 was written at 15 years, in a youth program, as an interview given with permission.

Record R8 of the running case (illustrative), with behavior and interpretation in separate columns
What the teen did and said (behavior only)	What it might show (interpretation)
Says they were sure "everybody saw" a stumble on stage	the imaginary audience
Says the school day starts at 7:30 a.m.	a school start earlier than the AAP's 2014 position

Worked example — comparing R8's start time with the position. Work it step by step.

Write the two times: R8's school starts at 7:30 a.m., and the AAP's 2014 position is no earlier than 8:30 a.m.
Subtract: 8:30 − 7:30 = 1 hour.
Turn hours into minutes: 1 × 60 = 60 minutes.
Say it in words: R8's school starts 60 minutes earlier than the AAP's position.

In words: a 7:30 a.m. start is 60 minutes earlier than the AAP's 2014 position of no earlier than 8:30 a.m.

In practice (your tools). In a free spreadsheet, type 7:30 AM in cell B1 and 8:30 AM in cell B2. In B3, type =(B2-B1)*24*60. The spreadsheet stores times as parts of a day, so multiplying by 24 and then by 60 turns the gap into minutes, and B3 shows 60. Write the result out in words beside it.

Notice what your worked example does and does not say. It compares one school's schedule with an organization's recommendation. It says nothing about how much this teen sleeps, and it gives no advice about any teen's sleep.

Whether a district changes its schedule is a separate question. Districts weighing a later start talk about bus routes, after-school sports and jobs, and family work hours. That is a local decision with costs on several sides, and this course names the debate without settling it.

Misconception check — the position as advice. Students often treat the AAP's school start position as a personal rule, as in "a teen who starts at 7:30 a.m. is being harmed". It is tempting, because the position is about sleep. The cure: the position is the AAP's recommendation for when schools aim to start, stated with its year. It is not a diagnosis of any teen and not medical advice; a question about one teen's sleep belongs with that family's health professional.

Pause and think. Choose one option and answer it in two or three sentences:

(a) Think about the earliest start time you have had for school or work. What did the first hour feel like, and what did you notice about the people around you?
(b) A district is weighing a later start. Name one group that might welcome the change and one that might find it hard, and say why for each.

4. Name the formal operational stage: reasoning about possibilities

Picture two students arguing at a lunch table about a question with no object in sight: "If everyone in the world told the truth for one day, would things get better or worse?" They trade reasons, imagine cases, and test each other's ideas. Neither one can point at anything. They are reasoning about a possibility.

Piaget described 4 stages of cognitive development, and you have now met three: the sensorimotor stage in Week 6, the preoperational stage in Week 9, and the concrete operational stage in Week 11. This week brings the fourth.

The formal operational stage is Piaget's fourth stage of cognitive development, placed at about 11 onward, in which adolescents can reason about abstract ideas and possibilities, not only about concrete things.

You can compare it with the stage before. In the concrete operational stage, at about 7 to 11, logic stays tied to real things a child can see or picture. In the formal operational stage, logic can work on ideas, such as fairness, the future, or a "what if". The ages are approximate, never a timetable you can set a clock by.

The core skill of the stage has a long name and a short meaning.

Hypothetical-deductive reasoning is reasoning that forms a hypothesis, works out what should happen if it is true, and tests it in an orderly way, one variable at a time.

Break the name apart, and you will see it is less scary than it sounds. "Hypothetical" means starting from an idea that might be true. "Deductive" means working out what must follow from it. A teen who says, "If the bus is late because of the bridge work, then the other route should be on time, so let's check that one tomorrow" is using it.

Misconception check — the birthday switch. Students often think a child turns into a formal thinker on an 11th or 12th birthday. It is tempting, because Piaget's stages come with ages. The cure: the ages are approximate, and the stage names and their order are what you hold on to. A stage is a model, not a calendar.

5. Test one variable at a time: the pendulum problem

Picture a teen at a kitchen table planning a science-fair test. The question is how a paper airplane's wing shape affects how far it flies. The first plan changes the wings, the paper, and the throw all at once. A friend asks, "So if it flies farther, which change did it?" The teen has no answer, and that is the moment this section is about.

Inhelder and Piaget studied this kind of reasoning with a classic task, the pendulum problem in Dataset D, the standard tasks.

The pendulum problem is Inhelder and Piaget's task asking what controls how fast a pendulum swings; the typical concrete operational response changes several things at once, and the typical formal operational response tests one variable at a time.

The teen can change things such as the length of the string, the weight on the end, and how hard the pendulum is pushed.

Dataset D, the pendulum problem: the question asked and the typical earlier and later responses
What the teen is asked	Typical response earlier	Typical response later
What controls how fast a pendulum swings?	concrete operational: changes several things at once	formal operational: tests one variable at a time

Worked example — fixing the science-fair plan. Use the paper airplane test and walk it in the formal operational way.

Name the hypothesis: "A wider wing makes the plane fly farther."
Work out what should follow: if the hypothesis is true, then the wide-wing plane should fly farther when nothing else changes.
Hold the other things the same: the same paper, the same fold for the body, the same person throwing the same way.
Change only the wing shape, and compare the distances.

In words: changing one variable while holding the others the same is the typical formal operational response, and it is what lets the teen say which change made the difference.

Now the finding that keeps this stage honest, and it may match your own experience. Many adolescents and adults do not use formal reasoning consistently. A person may test one variable at a time on a science-fair project and still change several things at once when fixing a slow laptop. You may see it in one task and not the next.

Misconception check — the universal stage. Students often think every adolescent reaches formal operations and uses it all the time. It is tempting, because it is the last stage in the model. The cure: many adolescents and adults do not use formal reasoning consistently. Reaching the stage in Piaget's model does not mean using it on every problem.

6. Tell two feelings apart: the imaginary audience and the personal fable

Picture a group project in a classroom. A 14-year-old has one slide to present and rehearses it for an hour, sure that "the whole class will notice" if one word comes out wrong. In the same week, another teen with a new learner's permit tells a parent, "Nobody understands how ready I am."

Both teens are thinking about themselves in a way you may remember. Elkind (1967), "Egocentrism in Adolescence", in Child Development, gave the pattern its name.

Adolescent egocentrism is Elkind's 1967 term for a teen's difficulty telling their own concerns apart from what other people are thinking about, shown in two forms, the imaginary audience and the personal fable.
The imaginary audience is the feeling that others are constantly watching and judging you; it is a form of adolescent egocentrism, not vanity.
The personal fable is the feeling that your experiences are unique and that you are invulnerable; it is a form of adolescent egocentrism about being special or safe rather than about being seen.

Here is the test you can use to tell them apart: the imaginary audience is about being seen; the personal fable is about being special or safe. The teen rehearsing the slide is feeling watched, so that is the imaginary audience. The teen with the permit is feeling unlike anyone else, so that is the personal fable.

Worked example — sorting four sentences. Say which form each sentence shows.

"Everyone in the cafeteria noticed my haircut." Being seen: the imaginary audience.
"No one has ever been as heartbroken as I am." Being unique: the personal fable.
"I can't wear this jacket; people will stare." Being seen: the imaginary audience.
"Bad things happen to other people, not to me." Being invulnerable: the personal fable.

In words: two sentences are about being watched and two are about being special or safe, so two show the imaginary audience and two show the personal fable.

When you write about these ideas, credit the terms to the right people. Adolescent egocentrism is Elkind's term. The egocentrism of early childhood, shown when a preschooler in the three mountains task picks their own view, is Piaget's. The preschooler cannot yet take another person's view; the teen can think about other people's thoughts and then over-reads what those people are thinking about.

Misconception check — the vanity reading. Students often read the imaginary audience as vanity or self-absorption. It is tempting, because the teen seems focused on themselves. The cure: Elkind linked adolescent egocentrism to new thinking, the ability to think about what other people are thinking. The teen is not admiring themselves; they are over-reading what an audience noticed.

Pause and think. Choose one option and answer it in two or three sentences:

(a) Describe a moment when you felt everyone was watching you. Looking back, how much did other people notice?
(b) A coach wants to help a teen who freezes before speaking at a team meeting. Using the imaginary audience, what could the coach say that respects the teen?

7. Read record R8 with two lenses

Go back to the showcase. You now have the words for the interview, so you can put the week together on record R8 and walk the course's order.

Worked example — reading R8 with two theories. You walk four steps, in order.

What the teen did and said: at 15 years, in a youth program interview given with permission, the teen says they were sure "everybody saw" a stumble on stage, and says the school day starts at 7:30 a.m.
What it shows: "everybody saw" is the imaginary audience, the feeling that others are constantly watching and judging, from Elkind (1967). The sentence is about being seen, so it is not the personal fable; a personal fable would sound like "nothing bad can happen to me" or "no one has ever felt this".
Which theory explains it: Piaget's cognitive-developmental theory with Elkind's extension. The new power to think about other people's thoughts makes it easy to over-read what an audience noticed, which is why the imaginary audience appears in adolescence.
What a second theory adds: Bronfenbrenner's ecological systems. The 7:30 a.m. start is set by a district decision, an exosystem setting the teen does not take part in, and it shapes the microsystem of the school day. The start is 8:30 − 7:30 = 1 hour, or 60 minutes, earlier than the AAP's 2014 position.

You can place the stage, too. Piaget described 4 stages, and formal operational is the fourth, at about 11 onward. R8, at 15 years, falls in that range, and the ages are approximate.

One limitation keeps you honest. R8 carries no pendulum task, so the record cannot tell you whether this teen reasons formally, and a single stumble says nothing about the teen's overall self-image. R8 also holds a sentence about trying robotics and art, which Week 14 reads.

The reading in one plain sentence: being sure "everybody saw" one stumble shows the imaginary audience, which Elkind (1967) described as a form of adolescent egocentrism, and the 7:30 a.m. start, 60 minutes earlier than the AAP's 2014 position of no earlier than 8:30 a.m., is a decision made in a system around the teen.

The comparison shows something worth keeping: Elkind's lens versus Bronfenbrenner's is a look inside the teen's thinking versus a look at the systems around the teen, and the two lenses describe the same record without either being the one right theory.

Misconception check — the label. Students often turn a reading into a label, as in "this teen is insecure". It is tempting, because one vivid sentence feels like a window on the whole person. The cure: the record shows one feeling about one stumble, read as the imaginary audience. You name the evidence, and you never label the teen.

Key terms

Puberty — The set of hormonal and physical changes that lead to reproductive maturity, including a growth spurt, changes in body shape, and the development of the sex characteristics; its timing varies widely from one healthy teen to the next. (first used: 1. Change on a wide schedule: puberty and its timing)
Secular trend — A change in a population's average over generations; in puberty, the shift over the past century toward an earlier average age at which puberty begins. (first used: 1. Change on a wide schedule: puberty and its timing)
Pubertal timing — Whether a teen begins puberty earlier, later, or at about the same time as peers; research describes its effects as associations that depend on context, not fixed outcomes. (first used: 1. Change on a wide schedule: puberty and its timing)
Limbic system — A set of brain regions involved in reward and emotion that, in the course's model of the adolescent brain, becomes more sensitive to reward early in adolescence. (first used: 2. Model the teenage brain: reward early, planning later)
Prefrontal cortex — The front part of the brain involved in planning and impulse control, which keeps maturing into the twenties. (first used: 2. Model the teenage brain: reward early, planning later)
Adolescent brain model — The account in which reward and emotion regions mature earlier in adolescence than the prefrontal regions for planning and impulse control; it describes average change in groups, not a reason or an excuse for one teen's behavior. (first used: 2. Model the teenage brain: reward early, planning later)
Formal operational stage — Piaget's fourth stage of cognitive development, placed at about 11 onward, in which adolescents can reason about abstract ideas and possibilities, not only about concrete things. (first used: 4. Name the formal operational stage: reasoning about possibilities)
Hypothetical-deductive reasoning — Reasoning that forms a hypothesis, works out what should happen if it is true, and tests it in an orderly way, one variable at a time. (first used: 4. Name the formal operational stage: reasoning about possibilities)
Pendulum problem — Inhelder and Piaget's task asking what controls how fast a pendulum swings; the typical concrete operational response changes several things at once, and the typical formal operational response tests one variable at a time. (first used: 5. Test one variable at a time: the pendulum problem)
Adolescent egocentrism — Elkind's 1967 term for a teen's difficulty telling their own concerns apart from what other people are thinking about, shown in two forms, the imaginary audience and the personal fable. (first used: 6. Tell two feelings apart: the imaginary audience and the personal fable)
Imaginary audience — The feeling that others are constantly watching and judging you; it is a form of adolescent egocentrism, not vanity. (first used: 6. Tell two feelings apart: the imaginary audience and the personal fable)
Personal fable — The feeling that your experiences are unique and that you are invulnerable; it is a form of adolescent egocentrism about being special or safe rather than about being seen. (first used: 6. Tell two feelings apart: the imaginary audience and the personal fable)

Summary

Puberty's timing varies widely among healthy teens; no single age is a deadline.
The secular trend is the earlier average start of puberty over the past century.
Early or late timing relates to a teen's experience through context, as an association, not a destiny.
The adolescent brain model: reward sensitivity rises early; the prefrontal cortex keeps maturing into the twenties.
The model describes groups and never excuses a choice.
The AAP's 2014 position: middle and high schools aim to start no earlier than 8:30 a.m.
Piaget's formal operational stage, at about 11 onward, is reasoning about ideas and possibilities.
On the pendulum problem, the formal response tests one variable at a time.
Many adolescents and adults do not use formal reasoning consistently.
Imaginary audience: being watched. Personal fable: being special or safe.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

An illustrative middle school starts at 7:45 a.m. How many minutes earlier is that than the AAP's 2014 position of no earlier than 8:30 a.m.? Show the steps.
At an illustrative community garden, a 13-year-old wants to know whether fertilizer makes tomato plants grow taller. The plan gives one row fertilizer, more water, and a sunnier spot, and gives the other row none of these. Name the problem with the plan, say which typical response on the pendulum problem it matches, and fix it.
A teen at an illustrative skate park says, "I don't need a helmet. Falls happen to other people." A second teen says, "I can't go back; everyone saw me fall last time." Name the form of adolescent egocentrism each sentence shows.
A relative says, "Teen brains are broken, so you can't expect them to plan anything." Name the misconception and rewrite the sentence so it matches the adolescent brain model.

Answers (worked reasoning — read after attempting)

Subtract the times: 8:30 − 7:45 = 45 minutes, since 7:45 to 8:00 is 15 minutes and 8:00 to 8:30 is 30 minutes, and 15 + 30 = 45. The school starts 45 minutes earlier than the AAP's position. That compares a schedule with a recommendation; it says nothing about any one student's sleep.
The plan changes three things at once, fertilizer, water, and sunlight, so a taller row could come from any of them. That matches the typical concrete operational response, changing several things at once. A fix: give both rows the same water and the same sunny spot, and change only the fertilizer. That is the formal operational response, testing one variable at a time.
"Falls happen to other people" is about being safe, so it shows the personal fable, the feeling of being unique and invulnerable. "Everyone saw me fall" is about being watched, so it shows the imaginary audience. The trap is calling both the imaginary audience because both mention falling.
The sentence treats the teenage brain as broken and the model as an excuse. A fixed sentence: "In the adolescent brain model, reward sensitivity rises early and the planning regions keep maturing into the twenties, so planning is still growing; many teens plan well, and the model is about groups, not one person."

Next up in this module: the Readings & Resources page, the lecture tutorial, the practice set, the quiz, and the discussion — and next week, Erikson's identity vs. role confusion model and Marcia's identity statuses give the robotics-and-art sentence in record R8 its name.

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