Stages of Development, the Brain and Intelligence
The Four Stages of Development
Developmental psychologists study how people change across the whole of their lives, not just in childhood. Psychologists divide this process into four broad stages: pre-natal development, childhood, adolescence and adulthood. Each stage is defined by a different pattern of physical, cognitive and brain change, which is why a toddler, a teenager and a forty-year-old think and behave so differently.
The pre-natal stage begins the whole process. Within weeks of conception, a simple tube of cells forms and swells at one end, laying down the earliest version of the brain. By birth, the basic brain regions are already in place — but they are far from finished. Childhood is the stage where those regions grow fastest: a baby’s brain increases in size dramatically in the first years of life, and basic skills such as walking, talking and recognising faces are all being learned during this period.
The four stages of development are pre-natal (conception to birth), childhood (birth to puberty), adolescence (the teenage transition to adulthood) and adulthood (from the early twenties). Brain and body change fastest in the earliest stages.
Adolescence: A Stage of Its Own
Adolescence is treated as a distinct stage rather than simply the end of childhood, because the brain is still changing in important ways. The cortex — the part of the brain responsible for reasoning, planning and self-control — is still maturing throughout the teenage years, while the brain regions linked to emotion and reward develop earlier. This mismatch helps explain why a teenager can be capable of complex reasoning one moment and impulsive risk-taking the next: the emotional side of the brain is fully formed before the reasoning side has caught up.
Adulthood begins once physical growth and the major stages of brain maturation are largely complete, typically from the early twenties onwards. This does not mean development stops. Adults continue to learn new skills and form new memories throughout life, but the rapid, large-scale restructuring of the brain seen in the pre-natal, childhood and adolescent stages has slowed considerably by this point.
The Nervous System, Neurons and the Developing Brain
Every thought, movement and memory depends on the nervous system. Its basic building block is the neuron. Billions of neurons form before birth, and they keep connecting to one another throughout childhood. Development is not just about growing more neurons — it is about how well those neurons connect to each other.
Brain development follows a bottom-up sequence: the oldest, simplest structures mature first, and the newest, most complex structures mature last. The brain stem controls automatic survival functions such as breathing and heart rate, and is working from birth. Above it, the thalamus matures next, relaying sensory information from the eyes, ears and skin towards the areas of the brain that interpret it.
The cerebellum develops next and coordinates movement and balance. It is why a newborn cannot walk immediately — the cerebellum needs time and repeated practice to build a smooth programme of movement, which is one reason motor milestones such as crawling and walking happen gradually across infancy and early childhood rather than all at once.
The cortex is the last structure to mature, and it keeps developing throughout childhood and adolescence. It handles language, reasoning, planning and problem-solving — skills that keep improving well into the teenage years, which is one reason adolescence is treated as its own stage of development rather than an extension of childhood.
Neurons do not work alone. They communicate across tiny gaps called synapses. When a neuron fires, chemicals cross the synapse and trigger the next neuron — this is how a signal travels through the nervous system, from a sense organ all the way to the part of the brain that responds to it.
During childhood, the brain produces far more synapses than it will keep. Connections used repeatedly — through experience, practice and learning — are strengthened, while connections rarely used weaken and are pruned away. This interaction between neurons and synapses, not just the raw number of neurons a person has, is what makes the developing brain more efficient across childhood and adolescence, and it is one reason early experience matters so much for later development.
Neurons connect to each other across synapses. Synapses used often are strengthened; those rarely used are pruned away. This interaction — not just the number of neurons — drives brain development.
IQ Tests as a Measure of Intelligence
Intelligence is hard to define, but psychologists have tried to measure it using IQ tests (Intelligence Quotient tests). These use standardised questions covering verbal reasoning, memory, spatial awareness and problem-solving, then compare a person’s score to the average score for their age group.
Because the questions and scoring procedure are fixed and applied in the same way to everyone, IQ tests are described as standardised: every test-taker faces the same tasks under the same conditions, which makes it possible to compare one person’s score fairly against another’s. Most people score close to the average, with fewer people scoring very high or very low — a pattern psychologists describe as a normal distribution.
IQ tests give a single, comparable number that can be tracked over time or used to identify children who might need extra support or extension at school. Schools and educational psychologists have used IQ scores to help decide what kind of support a pupil might benefit from, precisely because the score is easy to compare across a large population.
However, IQ tests mainly measure logical and verbal skills, and critics argue they say little about creativity, practical skill or emotional understanding. A score is also affected by a person’s education, language and cultural background, which raises questions about how fair the comparison really is between people from very different backgrounds.