The first five years matter enormously. They are also not a deadline, and they do not run to a timetable. Here is a plain-language look at what is actually developing between birth and five, and what the research does and does not say about it.

There is a sentence that appears on a lot of posters and parenting websites: "90 percent of the brain develops before age five." I understand why people like it. It makes the early years feel important, and they are. There is even some truth in it, in the sense that a young child's brain gets close to adult size surprisingly early. But it tends to land on parents as pressure, as if there is a window slamming shut on a child's fifth birthday and everything not done by then is lost.

That part is not what the science says, and I think it is worth slowing down and looking at what it does say.

Why the first five years matter

The Center on the Developing Child at Harvard University describes the brain as being built from the bottom up, through an ongoing process that starts before birth. Simple connections form first, and more complex ones are built on top of them. In their words, more than a million new neural connections form every second in the first few years of life.

Two things follow from that. The first is that early experiences really do shape the foundations. The second, which gets quoted far less often, is in the same place on the same page: new connections can form throughout life. The early years are the most active period. They are not the only period.

What builds those foundations is not, mostly, special activities or flashcards. It is relationships. Harvard calls the key ingredient "serve and return": a child does something, a look, a sound, a point, a cry, and an adult responds to it. Then the child responds to that. Back and forth, thousands of times a day, in completely ordinary moments. Nappy changes. The walk to the car. Arguing about socks.

The World Health Organization, UNICEF and the World Bank put this into what they call the Nurturing Care Framework, which names five things young children need: good health, adequate nutrition, safety and security, responsive caregiving, and opportunities for early learning. I find that list useful precisely because it is so unglamorous. Most of it is about the ordinary conditions of a child's life, not about enrichment.

What is actually developing

It is common to divide development into domains: physical, communication, cognitive, social and emotional. That is a useful way to talk about it, as long as we remember that a child does not experience it that way. A toddler climbing onto a chair to reach a cup is doing all five at once.

Physical. From lifting a head to rolling, sitting, crawling or bottom-shuffling, standing, walking, running, climbing, and then all the fine control of hands: picking up a blueberry, holding a crayon, doing up a button. Physical development is the most visible and the one people compare most, which is a shame, because the range of normal is wide.

Communication. Long before words there is a whole language of looking, pointing, crying and babbling. Words usually arrive somewhere in the second year and then accelerate, but the timing varies a great deal. One finding I find genuinely useful is that the amount of back-and-forth conversation a child experiences seems to matter more than the sheer number of words they hear. A study by Rachel Romeo and colleagues found that conversational turns were linked to language-related brain function in a way that raw word counts were not. The older, famous idea of a "30 million word gap" has itself been seriously challenged since. In practice, this means talking with a child rather than at them, and leaving gaps for them to fill.

Cognitive. Young children are remarkably good scientists. They test what happens when things fall, roll, splash, stack and hide. They notice when something surprising happens and pay close attention to it. Repetition, which can drive adults to distraction, is usually a child collecting data.

Social. The ability to understand that another person has thoughts and wants different from your own develops gradually across the preschool years, with an important shift often around four or five. That has big consequences for things like sharing, which I write about in a separate piece. Before that, a lot of social life happens alongside other children rather than with them.

Emotional. Feelings arrive fully formed. The ability to manage them does not. The skills that let us wait, shift attention, hold back an impulse and keep a goal in mind, often grouped together as executive function, develop slowly through childhood and keep developing well into adolescence and early adulthood. A two-year-old who cannot wait is not being difficult. They are two. I go into this more in the piece on toddler behaviour.

Do all children develop on the same timetable?

No. And this is the part I most want parents to hear.

Developmental milestones describe what most children do by a certain age. They do not describe what your child should be doing on a particular Tuesday. In 2022 the milestone checklists used in the United States were revised by a group led by paediatrician Jennifer Zubler. One of the main changes was to list skills that around 75 percent of children would be expected to show by a given age, rather than about half. The reason was practical: if a milestone is something only half of children have reached, then half of perfectly ordinary children will appear to be "behind" at every check.

Te Whāriki, our early childhood curriculum in Aotearoa, puts the same idea in different words. It asks teachers to "avoid making unnecessary developmental comparisons between children, recognising that developmental progress varies."

I learnt this the hard way. My son was slow to talk. I am an early childhood teacher and I still spent a lot of nights doing arithmetic about everything he was not yet doing. A chart describes a thousand children. You only have the one.

That said, variation is not a reason to ignore a worry. If you have a concern about your child's hearing, speech, movement, eating, sleep, behaviour or anything else, take it to your Well Child Tamariki Ora provider, your GP or your child's teachers. In New Zealand the B4 School Check at around four is a good opportunity, but you do not have to wait for it. Early support helps, and asking costs nothing. Parents are often right about something before they can explain why.

What about children who had a hard start?

My son's second and third years were spent largely at home during New Zealand's lockdowns, so this is a question I have asked myself many times.

The honest answer is that research on children born or raised through difficult periods is mixed. Some studies of the pandemic years found small average differences in areas like early communication. Others found little or none. Even where differences were found, they were small at the level of a whole population and cannot tell you anything about a particular child.

What is much better established is more hopeful. Children keep developing. The relationships a child is in now are doing their work now. Warmth, responsiveness, conversation, play and an adult who keeps coming back go on shaping a child long after a difficult stretch has ended. The first five years are a foundation. A foundation is not the whole house.

So what can you actually do?

Much less than the internet suggests, and much more than it feels like at 3am.

  • Respond to what your child is doing. Notice the look, the point, the sound, and answer it. That is serve and return, and you are probably already doing it more than you think.
  • Talk with them, and leave gaps. Count to three after you say something. Children need room to reply.
  • Give them time to play without being steered. I write more about that in the piece on learning through play.
  • Look after the ordinary things: sleep, food, safety, health, and your own wellbeing. A child's development happens inside a family, and the family's wellbeing is part of it.
  • When you find yourself wondering why your child is doing something, try asking what they might be practising.

The first five years are not a test that you pass or fail. They are a long, ordinary stretch of a child getting to know the world, mostly through the people closest to them.

Sources

  • Center on the Developing Child, Harvard University. Brain architecture. developingchild.harvard.edu
  • Center on the Developing Child, Harvard University. Serve and return. developingchild.harvard.edu
  • World Health Organization, UNICEF and World Bank Group. (2018). Nurturing care for early childhood development. nurturing-care.org
  • Romeo, R. R., Leonard, J. A., Robinson, S. T., West, M. R., Mackey, A. P., Rowe, M. L., & Gabrieli, J. D. E. (2018). Beyond the 30-million-word gap: Children's conversational exposure is associated with language-related brain function. Psychological Science, 29(5), 700-710. doi.org/10.1177/0956797617742725
  • Sperry, D. E., Sperry, L. L., & Miller, P. J. (2019). Reexamining the verbal environments of children from different socioeconomic backgrounds. Child Development, 90(4), 1303-1318. doi.org/10.1111/cdev.13072
  • Diamond, A. (2013). Executive functions. Annual Review of Psychology, 64, 135-168. doi.org/10.1146/annurev-psych-113011-143750
  • Zubler, J. M., Wiggins, L. D., Macias, M. M., et al. (2022). Evidence-informed milestones for developmental surveillance tools. Pediatrics, 149(3), e2021052138. doi.org/10.1542/peds.2021-052138
  • Ministry of Education. (2017). Te Whāriki. Mana tangata | Contribution strand. Te Whāriki online
  • Johnson, S. B., et al. (2024). Developmental milestone attainment in US children before and during the COVID-19 pandemic. JAMA Pediatrics, 178(6), 586-594. doi.org/10.1001/jamapediatrics.2024.0683