Across the Lifespan
Minds develop in a partly ordered sequence — from infant attachment through Piaget's stages of reasoning — and keep changing, gaining and losing different capacities, into old age. · 12 min
You were once a person who believed a toy stopped existing the moment it left your sight. You are not that person now. Between those two states lies development: the ordered set of changes that build a mind from an infant's reflexes to an adult's reasoning — and the slower changes that keep reshaping it for the rest of your life.
Guess before you learn
You pour water from a short, wide glass into a tall, thin glass. The amount of water does not change. You ask a typical four-year-old which glass holds more. What does she say?
A typical four-year-old points to the tall glass, and says it with certainty. Before about age seven, children judge amount by a single feature — here, height — and miss that pouring changes shape but not quantity. Keep your pencil mark: whether you predicted the child's error or the adult's logic, this lesson is about the ordered stages that separate them.
9–12
3–5
Little kids think in simple ways. A tall glass looks like more water, even when it is not. As you grow, your thinking gets more careful, and you start to notice when your eyes are fooling you.
Growing up is not one big jump. It is many small steps, and they usually come in the same order for everyone. And you never stop learning — grown-ups pick up new things too.
6–8
Psychologists describe development as a sequence of stages. Jean Piaget argued that children pass through four in a fixed order: sensing and moving, imagining but not yet reasoning logically, reasoning about concrete things, and finally reasoning about abstract ideas. The ages vary from child to child; the order does not.
Before the stages begin, infants form an attachment — a strong bond with a caregiver that shapes how safe they feel exploring the world. And development does not end at adulthood. Some abilities keep growing for decades while others slowly fade.
9–12
Piaget's four stages remain the backbone of the account: the sensorimotor stage (birth to about two), when infants learn that objects still exist when hidden — object permanence; the preoperational stage (two to seven), marked by vivid imagination but no conservation; the concrete operational stage (seven to eleven), when logical operations work on concrete cases; and formal operational thought (eleven and up), which handles abstraction and hypotheticals.
Two cautions temper the model. First, the boundaries are softer and earlier than Piaget thought — infants show signs of object permanence before they can reach for hidden toys. Second, development is lifelong: Mary Ainsworth's work on infant attachment and later research on aging both describe a mind that keeps gaining and losing capacities across every decade.
K–2
Play peekaboo with a baby. Cover your face. A very young baby thinks you are gone. An older baby laughs, because now she knows you are still there. That is her mind growing.
You learn things in an order. First you crawl, then you walk, then you run. Minds grow in an order too, a little at a time, and they keep growing your whole life.
Undergrad
The stage model's great rival is the continuous view: development as gradual, domain-specific change rather than global leaps. Modern evidence favors a hybrid. Object permanence, for instance, appears far earlier in looking-time studies (Baillargeon) than in Piaget's reaching tasks, suggesting his method underestimated infants by demanding motor responses they lacked.
Cognition across the lifespan splits in two. Fluid abilities — working memory, processing speed, novel reasoning — peak in early adulthood and decline. Crystallized abilities — vocabulary, accumulated knowledge — hold or rise into old age. This is why a study comparing 25- and 70-year-olds at one moment confounds aging with cohort, a problem only longitudinal designs resolve.
Postgrad
Piaget's constructivism — knowledge built through assimilation and accommodation — competes with information-processing accounts that model development as gains in memory capacity and strategy rather than qualitative restructuring. Neo-Piagetian theories (Case, Fischer) attempt a synthesis, tying stage-like shifts to measurable working-memory limits.
Baltes's lifespan framework reframes aging as selective optimization with compensation: as fluid resources fall, adults sustain performance by narrowing goals and leaning on crystallized expertise. Disentangling maturation is a methodological knot — cross-sectional designs confound age with cohort, longitudinal designs confound it with practice and attrition; only sequential designs partly separate the three.
object permanence
The understanding that objects continue to exist even when you cannot see them. It emerges across the first two years, in the sensorimotor stage.
Why is this true?
Why can the order of the stages be fixed even though the ages are not?
Because each stage supplies equipment the next one needs — you cannot reason about hidden possibilities until you can reason about concrete ones. The prerequisites force the sequence; nutrition, culture, and schooling only adjust the pace.
Development does not stop when the stages end. From your twenties onward, different abilities move in different directions at the same time — which is why 'getting older' is not a single slope of decline, but two trends crossing.
Practice — new ink and old, interleaved
1.Match each school to how it gathered its evidence.
2.In an experiment, the variable the researcher deliberately changes is the —
3.Name the three stages of memory and say which one fails in a tip-of-the-tongue moment.
Encoding, storage, and retrieval; a tip-of-the-tongue moment is a retrieval failure — the memory is stored but momentarily inaccessible.
How close were you? Grade yourself honestly — it sets your review date.
4.Without looking back, name Piaget's four stages in order.
Sensorimotor, preoperational, concrete operational, formal operational.
How close were you? Grade yourself honestly — it sets your review date.
5.For a bell to become a conditioned stimulus, what matters most?
6.Ebbinghaus's forgetting curve shows that newly learned material is —
7.Around what age do most children first pass conservation tasks?
8.What is the best description of inattentional blindness, as shown by the gorilla study?
9.In Pavlov's experiment, the bell that came to make dogs salivate on its own is the —