Part 2 made the strongest case I could for nursery, and I meant all of it.

But there is research pointing the other way. Some of it is very solid. Some of it is weaker than people claim. And one popular argument against nursery has actually been disproved, which I will come to.

Here are four things.

First: a day at nursery is physically harder on a toddler than a day at home.

Our bodies produce a hormone called cortisol. It rises when we are under pressure. Normally it is high in the morning and falls slowly through the day.

Researchers measured it in the same children, at home and at nursery. At home it fell as expected. At nursery it often rose instead.

Be careful with this finding. Cortisol also rises when a child runs, gets excited or does something new. No study has shown that this daily rise causes long-term harm.

Why this is a point for home: whatever a nursery day asks of a small child, home asks less. If both options are decent, one of them is easier on his body.

Second: the first weeks are hard for him.

One study followed children at fifteen months, close to my son’s age. It measured them at home, then while their mothers stayed with them at nursery, then after the mothers began leaving. In the first hour after the mother left, their cortisol nearly doubled.

There is a second half to this. Another study followed children for three months. The early weeks were hard. By the third month the levels were normal again, and both parents and staff said the children were doing well.

Why this is a point for home: starting nursery has a real cost, and the child pays all of it. Staying home has no such cost. Most children recover, but you should know you are asking him to pay something.

Third: at home, nobody else is using the adult.

This is the biggest point, and several others come out of it.

To be fair, a nursery room is usually not one adult alone. Many have a teacher plus two or three helpers for around fifteen children. That is a reasonable amount of adult attention.

But it is shared attention, and shared attention is not the same as attention that belongs to one child. Four things follow from that.

He waits. One child falls, one cries, one needs changing, one is hungry. The adults have to choose an order. At home there is no order, because there is nobody else in the queue. That immediate response is one of the main ways very young children learn.

He may hear less language, not more. This surprised me, because language was the strongest argument in Part 2. One study recorded children’s whole days and counted the words adults said, the sounds the child made, and the exchanges between them. The children got more of all three at home. The reason is simple. A toddler points at a bird. The adult says “bird.” He makes a sound. She says “yes, it is flying.” He points again. That conversation can continue because nobody else needs her.

Nobody knows him as well. A person who looks after the same child every day learns a great deal. That sound means tired. That cry means hurt. That gesture means water. This song calms him. Children can form real relationships with nursery staff, as I said in Part 2, but knowing one child that well is harder when you are responsible for fifteen.

He loses control of his own day. A toddler gets tired at unpredictable times. He eats badly one morning and is hungry by ten. At home, someone can simply react. Fifteen children need a schedule. Meals at a set time, naps in a system.

Why this is a point for home: at home the day is built around one child. At nursery the child fits into a day built for fifteen. The staffing may be perfectly good, and it is still not the same as having someone to himself.

Fourth: he will get sick more. This one is not in dispute.

A Danish study followed more than one million children. Starting nursery caused a clear rise in infections, and the rise was larger the younger the child was when they started.

And the extra illness was not repaid later. The common idea that they get sick now so they get sick less at school did not appear in the data.

Why this is a point for home: this is the only cost on either side that is certain. Nursery means more illness, for him and for the rest of the house, and nothing later cancels it out.

And one argument that is weaker than people think

This is the one you hear most often, so it needs care.

The large American study found that children who spent more total hours in childcare over their first four and a half years showed more difficult behaviour later. More aggression, more conflict with adults.

If I wanted to make a dishonest case against nursery, I would stop there.

But newer research combined data on more than ten thousand children across seven studies in five countries. Instead of comparing different children with each other, they checked whether a change in the same child’s hours changed that same child’s behaviour. The result was zero.

Why it is still worth knowing: because you will hear this argument, and you should know that the older studies found a link and the better ones found no evidence that the hours cause it. It is a reason not to assume more hours is better. It is not proof of harm.

And one thing that is not on this list at all

Staying at home does not put a child behind.

The same large study checked children’s language and thinking at one, two and three. Children cared for only by their mothers did not do worse than children in childcare.

Why this matters most: it removes the fear behind a lot of these decisions. There is no evidence that a child who does not go to nursery is missing something he needs.

Where this leaves us

All of this is real, but the points are not equal.

The illness finding is certain. The physical cost of a nursery day is real, though we do not know whether it matters in the long term. The undivided attention at home is a genuine advantage. The behaviour argument is much weaker than the internet suggests.

But notice something. Every point above assumes one thing: that home means a warm, attentive adult who talks to him, takes him outside and answers him when he needs something.

If home means an adult busy with housework, a television on in the background, few books and no other children, then every advantage on this list disappears.

Which is the same problem as before, from the other direction. The question was never nursery or home.

It is which nursery, and which home.

So I need to know what a good nursery actually looks like.

That is Part 4.