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IB DP Psychology — Biological Approach | SL & HL | Paper 1
🎯 Learning Intentions By the end of this note, you will be able to:
- Define brain development, critical periods, and sensitive periods using IB-aligned terminology.
- Explain the processes of synaptic pruning and neural plasticity and how they shape cognitive and behavioural development.
- Describe the role of experience and environment in shaping brain structure during early development.
- Evaluate research evidence for critical periods using supporting and opposing studies in ACAS format.
- Apply concepts of brain development and critical periods to a real-world case study from the biological perspective.
- Discuss the extent to which critical periods provide a complete explanation of developmental outcomes in humans.
- Analyse research on brain development using relevant concept areas: Causality, Bias, Perspective, and Responsibility.
Introduction & Context
Imagine a child who has never been spoken to in their first few years of life. When they finally receive language input, will they learn to speak as fluently as a child who was surrounded by language from birth? Or consider a kitten raised in a room where one eye is kept covered during the first weeks of life — when the cover is removed, will the eye function normally? These are not hypothetical curiosities; they are questions that have driven decades of research into how the brain develops and whether there are windows of time during which experience is uniquely powerful.
The human brain is not a finished product at birth. It undergoes extraordinary change during childhood and adolescence — forming connections, eliminating unnecessary ones, and reorganising itself in response to the environment. Understanding this process helps psychologists explain why early experiences can have such lasting effects on cognition, language, emotion, and behaviour. It also raises important questions about what happens when those early experiences are absent, disrupted, or harmful.
🔧 Analogy — The Sculptor and the Clay Think of the developing brain as a block of clay that is most malleable when it is fresh. A sculptor can shape it easily at first — adding detail, refining structure, removing excess. Over time, the clay hardens. It can still be worked, but it takes far more effort to make significant changes, and some fine details become impossible to add once the clay has set. Early experience is the sculptor; the brain is the clay. The earlier the shaping occurs, the more profound and lasting its effects.
Core Concepts
To understand why early experience matters so profoundly, we first need to understand what is actually happening inside the brain during development. The brain does not simply grow larger — it undergoes a complex process of building connections, testing them, and then selectively eliminating those that are not being used. This process is both biological and experience-dependent, meaning that nature and nurture are inseparable in shaping the developing brain.
📖 Key Definition — Brain Development Brain development refers to the biological process by which the structure and function of the brain change across the lifespan, including the formation of neurons, the growth of synaptic connections (synaptogenesis), and the selective elimination of unused connections (synaptic pruning). This process is influenced by both genetic programming and environmental experience.
At birth, the human brain contains approximately 100 billion neurons — roughly the same number as in adulthood. What changes dramatically after birth is not the number of neurons but the number of connections between them. In the first two years of life, the