Understanding Your Child’s Brain - Part 1

This is the first article in a four-part series exploring brain development and children’s behaviour. Together, we’ll look at some of the brain systems involved in survival, emotions, thinking and self-regulation, and how understanding them can help us respond to children with greater empathy and more realistic expectations.

REPTILE → MAMMAL → HUMAN → INTEGRATION

PART 1 OF 4 — THE REPTILE BRAIN: THE BRAIN THAT KEEPS US SAFE

🦎 REPTILE — Survival & Automatic Regulation

→ Brainstem

→ Autonomic nervous system

→ Arousal and bodily regulation

→ Protective responses such as fight, flight and freeze

→ Other interconnected brain networks involved in detecting and responding to potential threat

A NOTE BEFORE WE BEGIN

The terms “reptile brain,” “mammal brain” and “human brain” are sometimes used as a simple way of explaining different brain systems and their functions. Modern neuroscience tells us that the brain is much more interconnected than this, and these systems do not work as three separate brains. They communicate and influence one another constantly.

In this series, I will use these terms as a simple framework for understanding some of the major systems involved in children’s behaviour, while drawing on contemporary neuroscience to explain what is actually happening.

So, let’s start with the part of the brain that has been protecting us for a very long time.

THE REPTILE BRAIN: THE BRAIN THAT KEEPS US SAFE

Imagine that you are walking through the bush and suddenly notice something moving in the grass.

Before you have time to stop and think:

“What was that?”

your body may already have reacted.

Your heart beats faster.

Your muscles become ready to move.

Your attention narrows.

You become more alert.

You might freeze for a moment.

You are not consciously deciding to do these things.

Our bodies are constantly receiving and processing information about what is happening around us. Some responses are automatic, particularly when the nervous system detects something that may require rapid action, while other responses involve conscious processing and interpretation.

This ability to respond quickly is one of the important ways our nervous system helps keep us safe.

And this is where our “reptile brain” metaphor can be helpful.

WHY CALL IT THE “REPTILE BRAIN”?

The idea of a reptile brain comes from the older “triune brain” model, which described the brain as developing through evolutionary layers. Modern neuroscience does not support the idea that humans have three separate brains stacked on top of each other.

But there is still something useful in the image.

Think about a reptile. 🐊

It doesn’t need to sit down and reason through:

“I wonder whether that movement in the environment represents a potential threat, and what might be the most appropriate response?”

Its nervous system is designed to respond rapidly to important changes in the environment.

Humans have these ancient survival systems too, alongside much more complex emotional, social and thinking systems.

The brainstem is one important part of this foundational system. It contains networks involved in essential functions such as breathing, cardiovascular regulation, sleep and arousal. It also communicates continuously with other brain regions and with the body’s autonomic nervous system (ANS).

The autonomic nervous system helps regulate functions that largely occur outside conscious awareness, including heart rate, blood pressure, digestion and changes in breathing. Together with other systems, these networks help the body adjust its state in response to what is happening internally and externally.

So when we talk about the “reptile brain” in this series, we are really talking about a group of ancient and automatic survival and regulation systems, rather than one little part of the brain.

And these systems are incredibly important.

They keep us alive.

YOUR BODY IS CONSTANTLY REGULATING ITSELF

Think about your heartbeat.

You don’t have to remind your heart:

“Okay, it’s time for another beat.”

You don’t have to consciously tell your body to keep breathing while you are asleep.

You don’t have to think about digesting your lunch.

Much of this regulation happens automatically through interconnected brain and body systems, including the brainstem and autonomic nervous system.

When we are resting, different physiological systems support restoration and digestion.

When we are active or facing a challenge, other systems increase alertness and mobilise energy.

The sympathetic nervous system, for example, helps prepare the body for action during challenge, while the parasympathetic system contributes to recovery and restoration.

These systems aren’t simply an on/off switch. They are continuously interacting and adjusting to what is happening inside and around us.

And that becomes particularly important when we talk about children’s behaviour.

BEFORE YOUR CHILD CAN THINK ABOUT WHAT TO DO, THEIR BODY MAY ALREADY BE RESPONDING

Imagine that your child is playing happily and suddenly hears a very loud noise.

They jump.

Their heart may beat faster.

They might cover their ears.

They may run towards you.

They might cry.

You could explain:

“It was only a balloon popping. You’re safe.”

And you would be right.

But your child’s body doesn’t necessarily wait for your explanation before responding.

The nervous system can react to a potentially important or threatening stimulus before conscious reasoning has had time to fully process what is happening.

Research into children’s stress development shows that the systems involved in detecting and responding to threat, including autonomic and neuroendocrine systems, continue to develop and are shaped by experience throughout childhood and adolescence. Filetti, Kane-Grade & Gunnar (2024) describe how both the stress-response systems and the brain systems involved in regulating them change throughout development.

So when your child suddenly reacts strongly, it is worth remembering:

The reaction may have started in the body before your child had a chance to think about it.

THE BRAIN IS CONSTANTLY ASKING: “IS THIS SAFE?”

Of course, the brain doesn’t literally sit there asking this question in words.

But our nervous system is continuously processing information about our internal and external environment and adjusting our level of arousal accordingly.

Is something unexpected happening?

Is there something potentially threatening?

Is the environment unfamiliar?

Is there a sudden change?

Is someone approaching quickly?

Is the situation becoming overwhelming?

These systems help us respond rapidly when something might require our attention.

This is why a new environment can sometimes feel so intense for a young child.

A new childcare room.

A new educator.

A crowded birthday party.

A swimming lesson.

A new group of children.

A loud shopping centre.

A parent leaving.

A change to the usual routine.

To an adult, these things may seem manageable.

But your child’s nervous system doesn’t have the same history of experience that you have.

They don’t always know:

“I’ve been here before.”

“This person is safe.”

“Mum always comes back.”

“The noise will stop soon.”

“This feeling will pass.”

They are still learning these things.

And their nervous system is learning from experience.

FIGHT, FLIGHT, FREEZE AND FAWN

When the nervous system detects a significant threat or challenge, one of the responses we commonly talk about is fight or flight.

Your child may:

• hit

• push

• yell

• run away

• refuse

• become physically restless

• try desperately to escape the situation.

But not every child fights or runs.

Some children freeze.

They may become very still.

They may stop talking.

They may hide.

They may look as though they are ignoring you.

They may seem unable to move or respond.

Freezing is also recognised as part of defensive responding to threat. Contemporary research describes defensive responses as involving coordinated neural and physiological systems, with freezing and active fight-or-flight responses occurring in different circumstances. Developmental Neuroendocrinology of Early-Life Stress (2024) describes the involvement of autonomic and neuroendocrine systems in these defensive responses.

So when a child freezes, it isn’t necessarily:

“They’re refusing to cooperate.”

It may be:

“Their nervous system is overwhelmed, and their body is responding protectively.”

There is also another protective response you may have heard about called fawn. This can happen when a child responds to perceived threat by becoming very compliant, agreeing quickly, trying to please others or putting their own needs aside in an attempt to maintain safety or avoid conflict. For some children, staying close to or pleasing the person they perceive as threatening may feel safer than fighting, running away or freezing.

That doesn’t mean every refusal, tantrum or moment of silence is a fight, flight, freeze or fawn response. Children also say NO because they don’t want to do something!

Context matters.

But understanding the nervous system gives us another question to ask:

“What is happening for this child right now?”

WHEN “THREAT” LOOKS LIKE AGGRESSION

Imagine your child is playing with another child.

The other child suddenly grabs the toy your child has been playing with.

Your child pushes them.

Perhaps they hit, yell:

“Give it back!”

or even throw the toy across the room.

From the outside, we might see:

“They know they’re not allowed to hit.”

“Why did they do that?”

“They need to learn to share.”

And yes, your child still needs to learn that hitting isn’t okay.

But before we get there, something else may have happened.

The sudden loss of the toy may have been experienced as a social threat or significant stressor.

Their nervous system reacted rapidly. Their body moved towards action before they had time to consider:

“I could use my words.”

“Maybe I can ask for it back.”

“The other child might give it back.”

“I could ask an adult for help.”

Those are more complex responses that require emotional regulation, impulse control, flexible thinking and problem-solving.

The push happened first. The thinking came later.

That doesn’t mean we excuse the behaviour.

We can calmly stop the hitting:

“You really wanted that toy back but I’m not going to let you hit.”

And once your child is calmer, we can teach the alternative:

“Next time, you can say, ‘I was playing with that. Can I have it back?’ Or you can come and ask me for help.”

This is one possible pathway to aggressive behaviour, but it is important to remember that not every aggressive behaviour is a fight response. Children may hit or push for many different reasons, including frustration, impulsivity, communication difficulties, boundary testing or learned behaviour.

Understanding the nervous system gives us another possible piece of the puzzle.

AND THIS IS WHERE BEHAVIOUR STARTS TO MAKE MORE SENSE

Imagine asking your child to leave the playground.

You have given them plenty of warning.

You have explained what is happening.

They know the routine.

And yet, when you say:

“Okay, it’s time to go.”

they scream:

“NO!”

They throw themselves on the ground.

Perhaps they kick.

Maybe they run away.

It’s very easy to think:

“They know the rules.”

“I’ve told them five times.”

“They’re just being difficult.”

And perhaps sometimes your child really is testing a boundary.

But there may also be something happening beneath the behaviour.

The transition has suddenly become emotionally significant.

Their body is activated.

They are disappointed.

Their nervous system is responding to a change they don’t want.

And now we are asking them to stop what they are doing, control their body, manage their disappointment, listen to us, remember what we said earlier, accept the change and move to the next activity.

We will talk much more about those emotional and thinking skills in Parts 2 and 3.

For now, the important thing to understand is this:

Your child’s behaviour is happening within a nervous system.

Before we ask:

“Why are they behaving like this?”

we can sometimes ask:

“What state is their nervous system in?”

That question can completely change the way we respond.

SAFETY COMES BEFORE REASONING

When your child is highly activated, explaining more isn’t always the answer.

You might give a long explanation:

“I’ve already told you that we need to leave because we have to go home, and we’ve had plenty of time, and you knew this was going to happen…”

But your child may not be in a state where they can take in all that information.

This doesn’t mean children can’t learn rules.

They absolutely can.

It doesn’t mean we shouldn’t set boundaries.

Children need boundaries.

It means that the ability to reason, process language and regulate behaviour is influenced by the child’s current state of arousal.

The stress-response system involves both rapid autonomic responses and slower hormonal processes, and children’s capacity to regulate these systems develops over time. Filetti, Kane-Grade & Gunnar (2024) highlight the developmental changes in both stress-reactivity and the systems that regulate it.

Sometimes the first thing your child needs is for us to help create enough safety and regulation for their thinking to become more available.

That might sound like:

“You’re really upset. I’m here.”

“I’m not going to let you hit.”

“It’s hard to stop when you’re having so much fun.”

Then, when your child’s body is calmer, we can talk about what happened.

Connection and regulation don’t replace boundaries.

They help make boundaries possible.

CHILDREN DON’T CHOOSE THEIR NERVOUS SYSTEM RESPONSE

This is a really important distinction.

Your child can learn to recognise their feelings.

They can learn strategies.

They can gradually develop self-regulation.

They can learn what behaviour is expected.

But they don’t consciously choose the first physiological response that happens in their body.

A child doesn’t usually think:

“I’d like to become completely overwhelmed now so I can make this situation more difficult for everyone.”

Their nervous system is responding to information.

And over time, through development, relationships and repeated experiences of safety, children become increasingly able to notice what is happening inside them and influence how they respond.

This is one reason why co-regulation is so important.

A calm, responsive adult can become part of the child’s regulatory environment.

Research on children’s stress development highlights the role of social regulation and social buffering, the ways supportive relationships can influence children’s physiological stress responses.

Filetti, Kane-Grade & Gunnar (2024) describe the important role of attachment figures in buffering children’s stress responses.

In other words, your presence matters.

Your voice matters.

Your facial expression matters.

Your ability to stay regulated matters.

Not because you can magically switch off your child’s nervous system, but because children develop their own regulatory capacities within relationships and through repeated experiences of being supported.

WHAT DOES THIS MEAN FOR PARENTS AND EDUCATORS?

It means that when we see behaviour we don’t understand, we can pause before jumping straight to:

“They’re being naughty.”

“They’re manipulating me.”

“They know better.”

“They should be able to control themselves.”

And instead ask:

What happened?

What changed?

What might their body be responding to?

Are they overwhelmed, frightened, tired, hungry, overstimulated or struggling with a transition?

What does their nervous system need right now?

This does NOT mean excusing behaviour.

A child can be overwhelmed and still need a boundary.

A child can be dysregulated and still need to learn that hitting is not okay.

Understanding the brain is not about removing responsibility.

It is about making our expectations more developmentally realistic and choosing responses that actually help children develop the skills we want them to have.

These survival and automatic regulation systems are already functioning from birth because babies need them to breathe, regulate their bodies and respond to their environment before they have developed the more complex emotional and thinking skills that develop later.

AND THIS IS ONLY THE BEGINNING

The “reptile brain” helps us understand something fundamental:

Your child has a body and nervous system designed to keep them safe.

Before your child can reason, negotiate, reflect or choose a different response, some of the most basic systems of the brain and body are already working to detect change, regulate vital functions and prepare for action.

But safety isn’t the whole story.

Your child doesn’t only have a survival system.

They also have a developing emotional brain, the systems involved in emotions, memory, connection and the meaning we give to experiences.

You may have heard of another brain structure called the amygdala, often described as part of our brain’s alarm system. We’ll come back to the amygdala in Part 2, because understanding its role in emotions, learning and memory will help us understand why children can sometimes have such big reactions to experiences that seem small to us.

And that’s where we are going next.

WHAT TO TAKE AWAY FROM PART 1

When your child’s nervous system detects something important or threatening, their body may respond before they have had time to think about what is happening.

Understanding this doesn’t mean excusing behaviour or removing boundaries. It means remembering that behaviour happens within a developing nervous system.

Sometimes, before we ask “Why are they behaving like this?”, it helps to ask:

“What might their nervous system be responding to?”

NEXT IN THE SERIES:

PART 2 — THE MAMMAL BRAIN: THE BRAIN THAT FEELS

We’ll explore emotions, the limbic system, the amygdala, memory, connection and why your child’s feelings can sometimes become much bigger than their ability to explain or regulate them.

Because once we understand why the body reacts, the next question becomes:

What happens when the brain starts to feel?

REFERENCES — PART 1

Steffen, P. R., Hedges, D., & Matheson, R. (2022). The Brain Is Adaptive Not Triune: How the Brain Responds to Threat, Challenge, and Change. Frontiers in Psychiatry, 13, 802606.

Center on the Developing Child at Harvard University. Brain Architecture. Harvard University.

Center on the Developing Child at Harvard University. A Guide to Toxic Stress and Early Childhood Development. Harvard University.

Center on the Developing Child at Harvard University. Three Principles to Improve Outcomes for Children and Families. Harvard University.

Center on the Developing Child at Harvard University. Executive Function and Self-Regulation. Harvard University.