The brain has to be grown, ultimately starting from a single fertilised egg. A lot of neurons have to be made during prenatal development, as well as an equivalent number of other brain cells (so-called glia) to support their operation. The average rate of generating neurons prenatally is 250,000 a minute. Postnatally, the brain grows in size by a factor of 4 between birth and the teenage years.
It’s all very miraculous. But for us, not that exciting – unless the process of brain development changes the way the brain works. If someone told you that your mobile phone was growing, and that if you kept it plugged in and waited a bit, it would have a better operating system, a brighter screen, and more memory, that’s kind of important to know.
The fact that the brain has to be grown places certain limits on the way it works. Depending on the evolutionary history of the brain structure, the construction plans may be more or less detailed. The more detailed construction plans are for the parts with a longer evolutionary history – having a spine with a brain stem at one end, having a limbic system with emotion structures. The cortex is old too, but the large size of cortex in humans is a new evolutionary innovation. There’s a lot of cortex, but few detailed instructions for the functions that will develop in it.
The best way to design a machine is often to build specialised, pre-fabricated components, and to slot them together. A computer has a motherboard; you attach a power unit, a CPU, memory, hard disk, video and audio cards and so forth. Pre-fabricated, slotted together. Because the cortex has to be grown, because there isn’t a detailed plan, specialised components cannot be pre-fabricated. It’s more like a sheet of general thinking power. The broad wiring diagram can be drawn up, which bits of cortex are connected to which other bits.1 But the ‘special function’ of different parts of the cortex comes about during development, and depends on how each area of the cortex is being used. That specialisation takes a bit of time to happen.
In contrast to pre-fabrication and assembly of separate parts, you might image a cortical sheet machine, which produces a sheet of thinking power on which experience will draw patterns.
While development is taking place, the infant-toddler-child-adolescent-adult is learning all the time. How are learning and development different? They are related ideas and there’s no clear cut-off between them. But broadly, learning is what happens to the individual and is reversible (we forget!). I learned Russian, my brother never did. Then I forgot Russian. Development is what happens to all healthy humans, and is mostly irreversible. Both my brother’s brain and my brain grew in similar ways. Notably, development happens fast early on, and then has a diminishing rate. If we compared brain scans of a 3-month-old infant, a 3-year-old toddler, and a 30-year-old adult, the picture of the 3-year-old toddler brain would look more similar to the 30-year-old adult brain than to the 3-month-old infant brain.
Here’s something surprising. Development produces too much brain early on – mostly in terms of connections. Way too many connections. This gives the brain great flexibility to adapt to the world it finds itself in. Flexibility even to adapt to early damage. 2 But after a while, the brain starts trimming away the connections it’s not using (by a process called pruning) and optimising the connections that it is using (by a process called myelination, increasing the transmission rate of electrical signalling). There’s something paradoxical here. As we get older, more developed, cleverer, the brain is reducing its connections (its ‘grey matter’), shrinking, getting slimmer and trimmer.
So what, if anything, changes in the way the brain works? Recall the main principles: the brain is a set of specialised systems, whose activation is controlled by a goal-oriented modulatory system, which in turn is in conversation with emotion systems.
Here are half a dozen aspects that change with development. First, the detail and resolution of the specialised systems increases. They get better at their job. Second, the speed of communication between neurons increases, which is particularly evident in the speed and finesse of motor movements. Better quality of neural processing improves accuracy, and how much information can be ‘kept in mind’ from moment to moment. Third, the modulatory system is slow to develop. It gradually improves in its ability to control the specialised systems and to strategically activate the appropriate knowledge and memories in the right contexts, to stay on task. Fourth, the brain becomes better able to build plans that stretch deeper into the future. Fifth, the brain learns basic lessons about ‘what works to get what I want’, whether the world is generally a mean or a friendly place, and who are the crucial other people to rely on. Sixth, the dynamic coordination of all the parts of the brain improves, including the interface of plans, emotions, and actions.
The teenage years deserve a special shout-out. The brain is mostly built by then (there’s still some myelination to be done, bits of the temporal and frontal lobe are just coming into their own). But behaviour seems to change a lot. Teenagers can behave in impulsive, irrational, or dangerous ways, seeming not to think things through, not to fully consider the consequences of their actions. Has something gone wrong? Adolescence is an important stage of development, and probably similar across other social primates. Primate offspring must leave the protection of the family unit, and compete for a place in the adult social hierarchy, seeking status and a mate. In tandem with the development of secondary sexual characteristics, there are hormonal changes that alter motivation, elevating the importance of peer relationships. The hormonal changes perhaps prompt new changes in the modulatory system and its interface with emotions, heightening its facility to learn about social relationships and decision making in the adult world.
Less formally, the teenage brain says: ‘You gotta get out there, make something of yourself, compete with your peers, impress your friends, find a mate, take some chances, make some mistakes, get your heart broken, learn lessons about what risks are sensible to take – all while your hormones are raging and things are, like, really intense!’
As it trims away its spare connections, in some ways the brain becomes less able to change as it gets older. To understand this, let’s dust off a metaphor we used earlier: that the sensory and motor systems are like towers, with the lowest levels connected to the senses and motor outputs, and higher levels seeing patterns in information, patterns-within-patterns, patterns-within-patterns-within-patterns, and so on up the floors of the tower. The lowest levels develop first. Higher levels can only pick out further patterns when there’s meaningful information on levels below. Crucially, the gradual reduction in the ability to change (improve the best connections, trim away the spare) starts at the lowest level, and then works its way up the towers. But it doesn’t get to the top. So-called ‘sensitive’ periods in development, then, mainly refer to the low levels of perceptual processing or motor output. When we’re older, we find it hard to learn subtle sounds in a foreign language, or to speak a foreign language without an accent. Sensitive periods don’t really affect concepts, ideas, plans, or goals, which are flexible throughout life. 3 S
Development continues across the lifespan, eventually blending into ageing, as reaction times slow, wisdom and experience come to the fore, and whisky becomes palatable. But some plasticity always remains in the healthy brain. Never too late to learn
 For a taste of proper neuroscience, here’s an example of research on early wiring. Ghislaine Dehaene-Lambertz investigates how initial brain wiring helps children learn language. Here’s some more on plasticity  Find out here about educating the adult brain