Developmental psychology

Core idea

Developmental psychology asks the deepest question in the field: how does a person get made? A newborn cannot speak, walk, reason about hidden objects, or distinguish self from world. Two decades later they can do all of that and run a country. The transformation runs along three tracks that develop in parallel — emotional (attachment), cognitive (Piaget, Vygotsky), and moral (Kohlberg) — and each track has stages with characteristic features.

The recurring lesson across all three tracks is that development is staged but not separated: capacities emerge in a roughly fixed order, but they build on each other, and an early gap (insecure attachment, missed object permanence, premature moral conformity) can constrain later stages. Harlow's monkeys cling to a cloth surrogate even when the wire one feeds them — comfort beats nutrition. Ainsworth's "strange situation" reveals that the security of an attachment, not its mere existence, is what predicts later relationships.

Why it matters

Most of what you can do later in life is built on developmental capacities you acquired without remembering acquiring them — object permanence, language, theory of mind, secure-base behaviour. When those substrates are missing or wobbly, adult outcomes are predictably distorted, and the distortions are extraordinarily expensive to repair compared to building them right the first time. This is why early-childhood investment is the highest-leverage public policy and the most fraught domain of parenting decisions.

Mental model

Three parallel tracks

A child is not building one thing — they are building three at once, and each track has its own stages running on a partly-overlapping clock.

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Ainsworth's four attachment patterns

The "strange situation" — caregiver leaves, stranger arrives, caregiver returns — sorts babies into four predictable patterns based on how they respond to the reunion.

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Piaget — the child as scientist

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Vygotsky's zone of proximal development

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Practical application

Example

A primary-school teacher has a class of seven-year-olds and wants to teach them why we have rules. Piaget puts them squarely in concrete operational — they can reason logically but only about things they can see. Kohlberg puts them roughly in stage 2 (individualism) edging toward stage 3 (interpersonal approval). Vygotsky says the leap to stage 3 happens with adult scaffolding.

A poor lesson would ask the class to discuss the social contract in the abstract — formal-operational content delivered to concrete-operational minds, doomed to blank stares.

A better lesson sets up a concrete scenario. "Imagine the playground had no rules. What would happen?" The children answer in concrete terms — Tom would take the ball, Sara would push, no one would have fun. The teacher then scaffolds the leap. "So we make a rule that we share the ball. Is that rule for Tom, or for everyone?" This pushes the children from stage 2 ("what do I get?") into stage 3 ("what do we all get?") by routing the abstract concept through concrete instances they can actually represent. By the end of the lesson, several children have internalised the move from "rules stop me from doing what I want" to "rules let us all play together" — a stage transition produced by appropriate scaffolding in their zone of proximal development.

The same lesson tried with five-year-olds would fail because the cognitive substrate is not yet there. Tried with eleven-year-olds it would be patronisingly concrete because they can now reason abstractly. Match the method to the stage.

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