IB Psychology SLTopic 2 — Human DevelopmentPaper 1 & 2Biological approach~11 min read
Brain Maturation and Development
A newborn baby has almost all the brain cells it will ever have — but the brain is nowhere near finished. What happens over the next twenty years is a slow rewiring job, and it happens in a fixed order. That order explains a lot about why children think the way they do.
📘 What you need to know
Maturation means brain changes that follow a biological timetable. They happen with age, in roughly the same order, in almost everyone.
In the first years of life the brain forms new connections (synapses) at an enormous rate — over a million a second at the peak.
Connections that get used a lot are strengthened. This is neural branching.
Connections that are hardly used get deleted. This is synaptic pruning, and about half of them go.
The brain matures back to front: senses and movement first, planning and self-control last.
A window of opportunity is a stretch of time when the brain is especially ready to pick up a particular skill.
Key study: Chugani (1998) used PET scans of glucose use to track this from birth to late adolescence.
What “maturation” actually means
Maturation is the part of development that is driven by biology rather than by teaching. Nobody has to train a baby’s visual cortex to work. Give it light and time, and it comes online by itself.
That does not mean experience is irrelevant. Think of it like a house being built. Maturation lays the wiring in a set order, room by room. Experience decides which switches get flicked and which rooms actually get used. You need both.
Definition
Maturation = changes in the brain and body that unfold with age on a biological timetable, in the same broad sequence for everyone.
Examiners like it when you say maturation is sequential. The order is fixed; the exact timing is not. Two children can hit the same milestone a year apart and both be completely normal.
Building connections, then cutting them
Here is the part that surprises most students. A young brain does not slowly add connections until it has enough. It massively overproduces them first, and then deletes the ones it does not need.
Connections that fire together often get thicker and better insulated — that is neural branching. Connections that sit unused get removed — that is synaptic pruning. By late adolescence roughly half the peak number are gone.
Pruning is not damage. It is the brain getting faster by deleting clutter, in the same way a shortcut path across a field becomes clearer the more people walk it.
🧠 A way to remember it
Think of a sculptor with a block of marble. The block starts far bigger than the statue. Nothing is added — everything is chipped away until the shape appears. Early brain development works the same way round: too much first, then carve.
The brain matures back to front
The areas that mature earliest are the ones doing the simplest jobs: seeing, hearing, moving. The area that matures last is the prefrontal cortex, right behind your forehead, which handles planning, weighing consequences and holding back an impulse. It is still finishing in the mid-twenties.
This single fact explains a huge amount. A four-year-old is not being difficult when they cannot wait for a reward — the brain part that does waiting is barely built. A teenager who takes a risk they later call stupid has a fully mature reward system sitting on top of a not-yet-finished braking system.
The three regions here are simplified on purpose. What matters for the exam is the direction of travel, not the exact anatomy.
Windows of opportunity
Some skills are much easier to pick up during a particular stretch of childhood, because that part of the brain is in its build-and-prune phase. Miss the window and the skill can still be learned, but it takes far more effort and often never becomes automatic.
The clearest example is language. Vocabulary can be added at any age — adults learn new words all the time. Grammar is the fussy one. Children who get very little language input in the first decade tend to keep making grammatical errors for life, even after years of teaching.
Treat these ages as rough guides, not rules. Individual children vary, and that variation is itself a limitation of research like this.
Careful with the word “critical”. A critical period is strict — miss it and the skill may never develop properly. A sensitive period is softer — it is just the easiest time. Most modern researchers prefer “sensitive”, because so few human windows turn out to be truly all-or-nothing.
The key study
KEY STUDY
Chugani (1998) — glucose use in the developing brain
Aim
To track how the brain uses glucose from birth to late adolescence, and to link that to when different mental abilities appear.
Method
A review of PET scan studies. PET measures how much glucose an area is burning; a busy, developing area burns more. So the scan acts as a map of which regions are currently under construction.
Findings
In newborns, activity is low in the cortex but present in deeper emotional areas — feeling comes before thinking.
Between roughly 6 and 9 months, frontal lobe activity rises, matching the point where babies start solving simple problems.
Glucose use climbs through childhood, peaks at around age 10, then settles to adult levels.
The peak lines up with the period when children learn fastest — the “window” idea.
Conclusion
Cognitive development tracks physical brain development. The abilities appear roughly when the hardware for them is ready.
If you only remember one line from this study, make it this one: the timing of the brain changes matches the timing of the mental changes. That correlation is the whole point — and, as you will see below, it is also the weakness.
Evaluating the research
Strengths
Limitations
PET is objective. It measures something physical rather than relying on what someone reports about themselves.
It is correlational. Brain change and skill change happen together, but the scan cannot tell you which one caused which.
Replicable where the equipment and trained staff exist, and later work agreed the prefrontal cortex finishes last.
Reductionist. It explains development purely biologically and ignores parenting, poverty, schooling and culture.
Useful. Knowing the typical timetable helps professionals spot developmental delay early, when support works best.
Individual differences are flattened out. Plenty of healthy children sit well outside the average window.
Gives a biological explanation for behaviour that used to be blamed on character, such as poor impulse control in teenagers.
Ethics and access. PET involves a radioactive tracer, so scanning healthy children just for research is hard to justify.
Link to concepts
Measurement: scanning lets researchers turn something invisible into numbers. But a scan measures blood flow or glucose, not “thinking” — that link is an interpretation, not a direct reading.
Causality: the honest claim is that brain maturation and cognitive change are linked. Saying the brain change caused the new ability goes further than the evidence.
Exam practice
EXAM QUESTION
Explain one study of brain development in childhood. [9]
The trap here is spending seven lines on the method and none on the explaining. “Explain” wants the study and what it tells us about development.
1. Set up the idea (2–3 lines)
Brain maturation follows a fixed order; researchers wanted physical evidence for it.
2. Name and describe the study
Chugani (1998), review of PET studies, glucose use from birth to late adolescence.
3. Give the findings that matterlow cortical activity in newborns → frontal activity rises at 6–9 months → peak around age 104. Explain the link
Each rise in activity lines up with new abilities appearing — so cognitive development tracks brain development.
5. Add one critical point
But this is correlational, and PET is reductionist: it leaves out environment.
Describe + explain + one limitation = full marksone study done properly beats three studies mentioned in passing
💡 Exam tips
Use the technical words precisely: neural branching for strengthening, synaptic pruning for removal. Swapping them costs marks.
Whenever you mention a brain scan, have one evaluation line ready: correlational, not causal.
Say sensitive period unless the question itself says critical period. It shows you know the difference.
Link maturation to real behaviour — teenage risk-taking, toddler tantrums. Examiners reward application.
One well-explained study is worth more than a list of names with no detail.
Do not forget the other side: maturation is biological, but the environment decides what the brain gets to practise.
⚠ Common mix-ups
Thinking the brain only adds connections. It overbuilds first, then prunes. Pruning is normal and useful, not a sign of damage.
Thinking new brain cells are being made at that rate. The million-per-second figure is connections, not neurons.
Saying the brain is “fully developed at 18”. The prefrontal cortex carries on into the mid-twenties.
Mixing up maturation and learning. Maturation is the timetable; learning is what you put into it.
Treating windows as hard deadlines. Most are sensitive periods — harder afterwards, not impossible.
Writing that a PET scan shows a cause. It shows a pattern over time. That is a correlation.
Up next: Neuroplasticity in the Developing Brain — maturation is the timetable the brain is born with; plasticity is how it rewrites that timetable when life pushes back.
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