Lecture: Development Across the Lifespan
Lecture script · about 15–20 minutes spoken · Week 11
Opening
A preschool teacher watches a 4-year-old insist that the tall glass holds more juice. A nurse on a children's ward watches a toddler cry when a parent steps out, and watches again when the parent comes back. A manager notices that the most experienced person on the team solves problems nobody else can. Each of them is reading the same thing: a mind at one point in a very long story.
That story is this lecture's subject. In Weeks 1 and 2, you learned to ask "how do we know?" In Week 3, you met a brain that rewires itself with experience. In Week 6, you saw how practice and reward shape behavior, and in Week 9, you met intelligence and the order in which language arrives. Week 10 asked what moves you and what a feeling is made of. This week, we follow a person from the first year of life to the last. Next week, in Week 12, the course asks what makes each person's personality their own.
Here's the plan. First, the three big questions every developmental psychologist asks. Then Piaget's four stages, one at a time, with four children. Then the helper who stretches what a learner can do. Then what a baby needs besides food, the challenges Erikson placed across a whole life, and what changes, and what doesn't, as we age. If you keep one sentence, keep this one: a stage is a kind of thinking, not a birthday.
1. Ask the three big questions
Picture two cousins. One grows tall like her father. The other speaks two languages because his grandmother lives with the family. You might say height is biology and the second language is experience. But height depends on food, and learning any language depends on a brain built to learn one.
That tangle is where developmental psychology begins: the branch of psychology that studies how people change, and how they stay the same, in body, thinking, and social life from conception to old age. It keeps returning to three questions.
The first is nature and nurture: the question of how inherited biology and experience shape development. The two always interact, so neither side wins a contest. A baby is born ready to learn a language. That's nature. Which language it learns at the dinner table is nurture.
The second is continuity vs. stages. Picture a ramp, where each step is a little higher than the last. Now picture a staircase, where you are on one step and then on the next. Is development a ramp or a staircase?
The third is stability vs. change: whether a person's traits stay much the same across the lifespan or shift as the person grows older. Were you a shy toddler? Are you a shy adult?
Pause and try this. Someone says, "Musical talent is all genes." Which big question is this, and what's wrong with the claim?
Hold that thought for a moment.
Here's the answer. It's the nature and nurture question, and the claim treats it as a contest. Even a child with a strong inherited ear needs an instrument, lessons, and hours of practice, so nature and nurture work together.
2. Climb Piaget's staircase
Jean Piaget spent years listening closely to children's wrong answers. Children of similar ages made the same wrong answers, and those errors changed as the children grew. To Piaget, that meant a child isn't a small adult who knows less. A child thinks in a different way.
So Piaget voted for the staircase. He described four stages of thinking that always arrive in the same order, with no stage skipped: sensorimotor, then preoperational, then concrete operational, then formal operational. This course uses the approximate boundary ages 2, 7, and 11. Other courses sometimes shift a boundary by about a year, so hear every age as "about."
Here's how the steps line up. The sensorimotor stage runs from birth to about 2, so it lasts about 2 years. The preoperational stage runs from about 2 to about 7, so 7 minus 2 gives about 5 years. The concrete operational stage runs from about 7 to about 11, so 11 minus 7 gives about 4 years. Formal operational thinking begins at about 11 and has no upper boundary. It continues through adulthood.
To place a child, compare the child's age with 2, 7, and 11. That's the whole rule.
Pause and try this. Of the three stages that have an upper boundary, which one lasts longest?
Run the three subtractions before you go on.
Here's the answer. The preoperational stage lasts longest. Its width is 7 minus 2, which is about 5 years, compared with about 2 years for sensorimotor and about 4 years for concrete operational.
3. Start with the senses
Watch a baby in a high chair drop a spoon, look down, and laugh. Then she does it again. She's running an experiment with her hands and her eyes.
That's the sensorimotor stage: Piaget's first stage, from birth to about age 2, in which a baby knows the world through the senses and through actions such as looking, grasping, and mouthing. Its great achievement is object permanence: the understanding that an object or person still exists when it cannot be seen, heard, or touched.
Now play peekaboo with a baby who is 1 year old. You hide your face behind your hands, then pull them away, and she squeals. Place her first. She's 1, and 1 is below 2, so she's in the sensorimotor stage. For a baby who isn't yet sure your face still exists behind your hands, its return is a real surprise. Object permanence is still being built.
Pause and try this. Someone says object permanence is a hallmark of the preoperational stage. Agree or disagree?
Think about the timeline before you answer.
Here's the answer. Disagree. Object permanence belongs to the first stage and is built before about age 2, so it is already in place by the time a child reaches the preoperational stage.
4. Meet the preschool mind
Call a 3-year-old on a video chat and ask what he's holding. He may hold a drawing up to his own eyes and say, "This!" He can see it, so he assumes you can see it too.
That's the preoperational stage: Piaget's second stage, from about age 2 to about age 7, in which a child uses words, images, and pretend play but cannot yet reason logically about changes such as pouring or spreading out. It has two hallmarks. The first is egocentrism: a young child's difficulty seeing a situation from another person's point of view. In Piaget's sense, that isn't selfishness; taking a second viewpoint is the hard part.
The second hallmark is about amounts. Conservation is the understanding that an amount stays the same when only its shape or arrangement changes. Here's the test. A child who is 4 watches you pour all the juice from a wide glass into a tall, thin one. Nothing is added or spilled. She says the tall glass holds more. She's 4, between 2 and 7, so she's preoperational, and her answer shows no conservation yet. She judges by one thing she can see, the height of the juice.
Pause and try this. Why isn't the 4-year-old's answer a careless mistake?
Consider what she does every time you repeat the test.
Here's the answer. Her answer is consistent: she reliably picks the taller glass, because she reasons from one visible feature at a time. A steady pattern like that is exactly what Piaget meant by a stage.
5. Watch logic arrive
Now line up two rows of coins at a lemonade stand, the same number in each row. Ask a child who is 8 which row has more. "The same." Spread one row out so it's longer, and ask again. "Still the same. You only moved them."
That shrug is the concrete operational stage: Piaget's third stage, from about age 7 to about age 11, in which a child conserves and reasons logically about real, concrete objects and events. He's 8, between 7 and 11, and he conserves. He holds two facts at once, a longer row and wider gaps, and sees that they cancel out.
The fourth step adds something new. Ask a teen who is 14 how sports would change if everyone in the world were the same height. She reasons about a world that doesn't exist. That's the formal operational stage: Piaget's fourth stage, from about age 11 onward, in which a person can reason about abstract ideas and hypothetical situations. She's 14, above 11, so she's formal operational.
Put the four children together: 4 children, 4 stages, 1 child per stage, in order from sensorimotor to formal operational.
Pause and try this. An 8-year-old is asked, "What is freedom?" and answers, "Staying up late." Which stage does the answer fit, and why?
Picture what kind of logic each stage can use.
Here's the answer. It fits the concrete operational stage. The child reasons well about real, concrete things, so an abstract idea comes back as a concrete example; reasoning about the abstract idea itself is the formal operational hallmark.
6. Add the helper
A 6-year-old is learning to ride a bike in a park. On her own, she wobbles and falls. With training wheels and a parent's hand on the seat, she rides the whole path. Weeks later, the hand lets go, then the wheels come off, and she rides alone.
Lev Vygotsky stressed something Piaget gave less weight to: much of what children learn, they learn with other people. His term for the space where that happens is the zone of proximal development, the gap between what a learner can do alone and what the learner can do with help from a more skilled person. The support that bridges the gap is scaffolding: temporary support a more skilled person gives a learner inside that zone, removed step by step as the learner's skill grows.
Run the bike story through those terms. Alone, she falls. With help, she rides. That gap is her zone. The wheels and the hand are the scaffolding. And here's the part people forget: scaffolding is meant to come down.
You meet scaffolding in this course every week: a worked example before the practice set carries you through the first problem.
Pause and try this. A task the learner can't do even with help: is it inside the zone of proximal development?
Look at both edges of the zone.
Here's the answer. No. The zone is the middle band where help makes the difference, so a task the learner can't do even with help sits above it, and support aimed there won't work yet.
7. Ask what a baby needs
At a family gathering, a toddler's father steps out of the room. She cries and crawls toward the door. When he comes back, she reaches up, is held for a moment, and then wriggles down to play. That reach toward one particular person is attachment: the strong emotional bond between an infant and a caregiver.
For a long time, many psychologists assumed babies attach to whoever feeds them. In 1958, Harry Harlow tested that idea with infant monkeys. Each one had two stand-in mothers: one of bare wire that held the bottle, and one covered in soft cloth that didn't. The monkeys spent most of their time clinging to the cloth mother. They visited the wire one to feed and came back to the cloth one for comfort. That's contact comfort, the soothing that comes from touching something soft and warm. Food alone didn't build the bond. One honesty note: Harlow raised these monkeys apart from their real mothers, which is why his work is also taught as an ethics case.
Mary Ainsworth brought attachment into a playroom. In her Strange Situation, a toddler goes through brief separations from a caregiver and reunions while observers watch. A toddler who is comforted at the reunion and goes back to play shows secure attachment. One who clings without settling, or avoids the caregiver, shows insecure attachment.
Pause and try this. A toddler cries hard when her caregiver leaves. Does that make her insecurely attached?
Think about which moment the Strange Situation watches most closely.
Here's the answer. Not by itself. The reunion is the key moment, and a securely attached toddler may cry at the separation and still be comforted quickly when the caregiver returns.
8. Stretch development across a life
Piaget's staircase ends at about 11. But a person starting a new career, or looking back over old photographs, is still developing. Erik Erikson, writing in 1950, wanted a map of the whole life. He proposed eight psychosocial stages, each centered on one social and emotional challenge, written as a healthy outcome versus an unhealthy one.
Three of them anchor the lifespan. In infancy comes trust vs. mistrust: a baby whose needs are met reliably learns that people can be counted on. In adolescence comes identity vs. role confusion: a teenager tries out roles and values in order to form a stable sense of who they are. In late adulthood comes integrity vs. despair: a person looks back and feels either that life was meaningful or that it was wasted.
Picture a retired bus driver telling her grandchildren which choices she's proud of. She's meeting the last challenge, leaning toward integrity. A high school junior who joins the robotics team, then quits for theater, is meeting the adolescent challenge.
Erikson described a balance in each stage, not a pass-or-fail test. Later relationships can shift it.
Pause and try this. Put these three in lifespan order: integrity vs. despair, trust vs. mistrust, identity vs. role confusion.
Start from the very beginning of life.
Here's the answer. Trust vs. mistrust comes first, in infancy; identity vs. role confusion comes in adolescence; and integrity vs. despair comes last, in late adulthood.
9. Finish with adolescence and aging
A teenager changes her style three times in one school year. That's adolescence at work: the period from puberty to independent adulthood, marked by rapid physical change and by the search for a stable identity, a person's stable sense of self. Trying on styles and opinions is how a teen finds out which ones fit.
Now picture an older crossword champion who fills in rare words in seconds but takes longer than his grandson to learn a new phone game. Psychologists split one part of intelligence into two kinds that age differently, following Horn and Cattell in 1967. Fluid intelligence is the ability to reason quickly and solve new problems that do not depend on what a person already knows; it tends to decline in later adulthood. Crystallized intelligence is the knowledge, vocabulary, and skills a person has built up over a lifetime; it tends to hold steady.
So the slow start on the phone game is fluid intelligence, and the instant crossword words are crystallized intelligence. That's why an experienced nurse, mechanic, or teacher can outperform a quicker newcomer. Development doesn't stop at 11. It's lifelong, with gains and losses side by side.
Pause and try this. An older adult knows far more words than her grandchild but solves a brand-new puzzle more slowly. Which kind of intelligence explains each result?
Sort the two results one at a time.
Here's the answer. The vocabulary is crystallized intelligence, which holds steady with age, and the slower new puzzle is fluid intelligence, which tends to decline in later adulthood.
Closing
Let me pull it together in the order we built it. Development asks three big questions, and nature and nurture always work together. Piaget's four stages run sensorimotor, preoperational, concrete operational, formal operational, at about 2, 7, and 11. The sensorimotor baby builds object permanence. The preoperational child shows egocentrism and doesn't yet conserve. The concrete operational child conserves and reasons about real things, and the formal operational teen reasons about what-ifs. Vygotsky's zone of proximal development is the gap that help can bridge, and scaffolding comes down as skill grows. Harlow's monkeys chose contact comfort over food, and the Strange Situation reads attachment at the reunion. Erikson's challenges run from trust in infancy to identity in adolescence to integrity in late adulthood. And as we age, fluid intelligence tends to slip while crystallized intelligence holds.
Next in this module come the slide deck, the Readings & Resources page, the lecture tutorial, the practice set, and the quiz, then the discussion, the assignment, and the Analysis Workshop. Next week, in Week 12, Personality, the course asks what makes each person's pattern of thinking, feeling, and behaving their own.
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