Early childhood and the developing brain

Author: Richard Batcheler, Isos Partnership
A child playing with building blocks

The period of rapid brain development that takes place during pregnancy and early childhood provides the foundations for every individual’s future development, health and wellbeing.

A growing body of research has proven beyond doubt the importance of both nature and nurture in shaping this. Our genes provide a blueprint for our development, but how we develop depends a great deal on our early environment and experiences. Central to the experiences during early childhood are our relationships with caregivers, placing parents and carers — and support for those parents and carers — at the heart of supporting young children’s brains development.   

Improving understanding of the evidence of the developing brain during early childhood is key for raising awareness of the extraordinary impact of the early years.1

Foundational connections

We are born with billions of neurons — specialised brain cells that transmit information to other nerve cells around the body. Neurons are connected by synapses, which allow neurons to communicate with one another. During the first years of life, as many as a million synapses are formed per second.2 Different areas of the brain form circuits of neurons and synapses that specialise in particular functions, such as vision, hearing, language, memory and emotion. By the age of two, our brains are approximately 80% of adult weight, reaching 90% of adult size by age five.3 After this rapid growth, the number of connections reduces through “pruning”, allowing circuits to become more efficient.  

Early brain development does not just support our thinking skills or “cognitive development”. Brain development is also important for physical development, with the way the brain grows influencing other systems within the body, including those supporting heart and lung function, digestion and fighting infection.4 How the brain grows is also inextricably linked to emotional development, with how we manage emotions built into the architecture of young children’s brains.5   

Brain development during early childhood is said to be particularly important as it is the period when the brain is most “plastic” or flexible.6 This flexibility allows young children to adapt to a wide range of environments. As we get older and the brain assumes more complex functions, it is less capable of adapting to new challenges (see Figure 1). For example, by age one, the parts of the brain that differentiate sound are becoming specialised to the languages it has been exposed to, with the brain already losing the ability to recognise the different sounds of other languages. So, while it is possible to learn a new language as we get older, our brain circuits become increasingly difficult to alter. 

Levitt image 16:9

The brain develops sequentially, meaning early childhood provides the foundation for future development. Sensory pathways such as for hearing develop first, followed by early language skills then higher cognitive functions (figure 2). So, while not all brain development occurs during early childhood, stronger early foundations acquired through the connections of those simpler circuits provide opportunities to support later development.  

In the proliferation and pruning process, simpler neural connections form first, followed by more complext circuits. The timing is genetic, but early experiences determine whether the circuits are strong or week.

— Image source: C.A. Nelson (2000). Credit: Center on the Developing Child

Nature plus nurture

While the importance of early brain development is clear, there has been a long-running debate over what matters more in shaping its development: nature — the approximate 23,000 genes a child inherits from their parents — or nurture — a child’s experiences and environment. This debate has often viewed nature and nurture to be independent from one another or to view the two as in conflict.7 However, an increasing body of evidence is revealing the interaction between the two. 

Research from the field of behavioural genetics has examined the extent to which both genetic and environmental factors and the interactions between them influence differences among children. Several mechanisms through which genes may affect children’s development have been identified, including:  

  • The direct impacts of a child’s genetic endowment on their development. For example, a genetic condition affecting a child’s sight may affect their ability to read. Genetic effects may be mediated by environment, such as the provision of ophthalmological care for children with eye conditions.   

  • A child’s genes may elicit different behavioural responses from parents. For example, a child with genes associated with greater interest in reading may prompt parents to read more with their child.  

  • A parents’ genetics may influence the environment their create for their children. For example, parent with genes associated with greater interest in reading may read more often with their child.8,9   

In recent years, the field of epigenetics has started to provide evidence that our early experiences and environment may in fact alter whether and how our genes express themselves. Rather than viewing genes as fixed, genes are “expressed,” so that they may be turned on or off. The epigenome consists of chemical compounds that mark genes and control their functioning. Some scientists describe genes as like the hardware of a computer, determining what it is possible for the computer to do, and the epigenome as the operating system, determining the functions the hardware performs.10 Others describe genes as a piece of music, with the epigenome the annotations on the manuscript that will dictate how the music sounds.11 So while the music does not change, each performance is unique. Similarly, while identical twins have the same genomes, their different experiences are said to result in different epigenomes, explaining in part why identical twins differ. Scientists at the Harvard Centre of the Developing Child state that the epigenome “provides an explanation, at the molecular level, for how and why early positive and negative experiences can have lifelong impacts.”12   

The dynamic interactions between genes and the environment mean that disentangling the genetic, epigenetic and environmental contributions to early brain development is difficult, with large differences in the estimated effect of genetic influences.13 However, there is now clear evidence that development is not biologically determined, but that it is nature plus nurture, with our early relationships, experiences and environment playing an important role in helping children to develop in a way that supports their longer-term health and wellbeing.

The importance of early experiences and relationships

Our early brain development can also be thought about as being shaped by the relative balance of two things: the positive and protective factors that nurture us and our exposure to adversity.14,15 Experiences can be positive and protective, creating stronger and faster connections in the brain. Neural circuits are said to have “sensitive periods,” during which experiences play an especially important role in shaping brain development. Examples of capacities that have been shown to be affected by sensitive periods include vision, hearing, language and responses to social cues.16  

Conversely, a lack of certain experiences during sensitive periods may inhibit development. For example, if the eyes do not receive visual input during the sensitive period for optical development, the respective neural circuit may not function properly. Lack of activation or use can leave connections weak, and connections that are not used or strengthened die off.17   

Similarly, adverse experiences may hamper development.  Magnetic resonance imaging (MRI) has provided evidence of how early adversity can affect brain structure, particularly during pregnancy and the early years. Adverse early experiences can change how the brain processes negative (‘threatening’) and positive (‘rewarding’) aspects of the environment and how emotions are regulated.18 These changes have the potential to create vulnerabilities that affect mental health, social development and learning over time.  

Central to the experiences during early childhood – and the brain development happening at that time – are the relationships with caregivers, particularly parents. The relationship between carer and child is dependent on early, reciprocal interaction. These interactions – sometimes referred to as ‘serve and return’ – shape the developing brain. When a baby or young child babbles, gestures or cries, and their caregiver responds appropriately with eye contact, words or a hug, neural connections are built and strengthened that support the development of communication and social skills.19 Evidence shows a secure relationship with an adult can also buffer against the effects of longer-lasting and more severe stress, protecting the developing brain from the potentially harmful impact of trauma.20 This evidence places parents and carers — and support for those parents and carers — at the heart of ensuring young children’s brains develop in a way that supports lifelong physical and emotional wellbeing.


[1] Raising awareness of the extraordinary impact of the early years is one of the six areas of opportunity in the early years identified in our Big Change Starts Small report.

[2] The Center on the Developing Child at Harvard University. Brain Architecture. https://developingchild.harvard.edu/science/key-concepts/brainarchitecture

[3] Lenroot, R.K. & Giedd, J.N. (2006). Brain development in children and adolescents: insights from anatomical magnetic resonance imaging. Neuroscience & Biobehavioral Reviews, 30(6), 718-729.

[4] The Center on the Developing Child at Harvard University. Connecting the Brain to the Rest of the Body: Early Childhood Development and Lifelong Health are Deeply Intertwined. https://46y5eh11fhgw3ve3ytpwxt9r-wpengine.netdna-ssl.com/wp-content/uploads/2020/06/wp15_health_FINALv2.pdf

[5] The Center on the Developing Child at Harvard University. (2020). Children’s Emotional Development is Built into the Architecture of Their Brains. https://46y5eh11fhgw3ve3ytpwxt9r-wpengine.netdna-ssl.com/wp-content/uploads/2004/04/Childrens-Emotional-Development-Is-Built-into-the-Architecture-of-Their-Brains.pdf

[6] The Center on the Developing Child at Harvard University. The Science of Early Childhood Development. https://developingchild.harvard.edu/resources/inbrief-science-of-ecd/

[7] Cattan, S., Fitzsimons, E., Goodman, A., Phimister, A., Ploubidis, G. B., and Wertz, J. (2022). Early childhood inequalities. IFS Deaton Review of Inequalities. IFS.

[8] Belsky, J., Caspi, A., Moffitt, T. E., and Poulton, R. (2020). The Origins of You: How Childhood Shapes Later Life. Harvard University Press.

[9] Cattan, S., Fitzsimons, E., Goodman, A., Phimister, A., Ploubidis, G. B., and Wertz, J. (2022). Early childhood inequalities. IFS Deaton Review of Inequalities. IFS.

[10] The Center on the Developing Child at Harvard University. (2010). Early experiences can alter gene expression and affect long-term development. https://46y5eh11fhgw3ve3ytpwxt9r-wpengine.netdna-ssl.com/wp-content/uploads/2010/05/Early-Experiences-Can-Alter-Gene-Expression-and-Affect-Long-Term-Development.pdf

[11] Rutherford, A. (2015). Beware the pseudo gene genies [online]. The Guardian. https://www.theguardian.com/science/2015/jul/19/epigenetics-dna--darwin-adam-rutherford

[12] The Center on the Developing Child at Harvard University. (2010). Early experiences can alter gene expression and affect long-term development. https://46y5eh11fhgw3ve3ytpwxt9r-wpengine.netdna-ssl.com/wp-content/uploads/2010/05/Early-Experiences-Can-Alter-Gene-Expression-and-Affect-Long-Term-Development.pdf

[13] For example, genetic influences have been said to account for as little as 4% or as much as 40% of the variation in cognitive skills in childhood. Cattan, S., Fitzsimons, E., Goodman, A., Phimister, A., Ploubidis, G. B., and Wertz, J. (2022). Early childhood inequalities. IFS Deaton Review of Inequalities. IFS.

[14] Royal Foundation Centre for Early Childhood. (2021). Big Change Starts Small. 

[15] Dana Suskind has framed this balance as nurturing care and protection from toxic stress. Oster, E. (2022, 18 April). How society fails parents. ParentData. https://www.parentdata.org/p/how-society-fails-parents

[16] The Center on the Developing Child at Harvard University. (2008). The timing and quality of early experiences combine to shape brain architecture. https://46y5eh11fhgw3ve3ytpwxt9r-wpengine.netdna-ssl.com/wp-content/uploads/2007/05/Timing_Quality_Early_Experiences-1.pdf

[17] Holtmat, A. & Svoboda, K. (2009). Experience-dependent structural synaptic plasticity in the mammalian brain. Nature Reviews Neuroscience 10(9): 647-58. https://pubmed.ncbi. nlm.nih.gov/19693029/

[18] McCrory, E.J. & Viding, E. (2017). Annual research review: childhood maltreatment, latent vulnerability and the shift to preventative psychiatry — the contribution of functional brain imaging. Journal of Child Psychology and Psychiatry and Allied Disciplines 58(4), 228-357

[19] The Center on the Developing Child, Harvard University. Serve and Return. https://developingchild.harvard.edu/science/key-concepts/serve-and-return/

[20] The Center on the Developing Child, Harvard University. InBrief: The Impact of Early Adversity on Childhood Development. https://developingchild.harvard.edu/resources/inbrief-the-impact-of-early-adversity-on-childrens-development/