Rapid population growth is a very new phenomenon. After 300,000 years on the planet, we only hit one billion people in 1804. That doubled 123 years later. Since 1974 we have added another billion every 12-14 years.¹

Now that trend is coming to an end just as quickly. By 2085 we will hit our peak: 10 billion people. After that, no-one knows. According to one estimate, we might be back down to 2 billion by 2400. And it would not stop there.

This is because most countries today have fertility ratesThe number of children born, on average, to each woman within a studied group. A total fertility rate of 2.1, is the minimum needed for a population to remain steady and is known as the "replacement rate". that are below the replacement rateThe rate of births needed for babies to replace their parents. . In order for the population to remain steady, each couple needs to have 2.1 children - two to replace themselves and an extra 0.1 to make up for people who die or do not have children of their own.

But in Europe, the current figure is just 1.5. In East Asia it is 1.2. Africa has the highest rate at 4.5, but that is a fall from 6.6 in 1980, and it keeps dropping.

The whole of human progress has taken place against a backdrop of population growth. So we simply have no idea what a world with falling birth rates looks like.

But we do know it means an ageing population. That means more and more elderly people who cannot work and who need expensive medical treatments that have to be paid for by a smaller working population. It could become a huge crisis.

Experts also worry that having fewer people means fewer ideas, inventions, discoveries. Progress could grind to a halt.

Some say it is difficult to halt the fall in birth rates because it is driven by people's own preferences. 

Ready access to contraception and more freedoms for women mean most people can choose the size of their family. And it turns out they tend to want fewer children than they had in the past. That means all we can do, they say, is try to adapt ourselves to it.

But others think we are misreading the data. Fertility rates, they say, do not drop just because people want fewer children. It is because we make it very difficult for them.

Children are extremely expensive. It is estimated that it costs a couple £157,562 to raise a child in the UK. For a lone parent, it is £208,735.² That is just the cost of food, housing and childcare - leaving out luxuries like holidays, birthday gifts and funding hobbies.

For people working unstable jobs, renting a home rather than owning one, life is just too uncertain to make this kind of investment. But that is how most young people, those who are of an age to have children, now live.

Even for people with more stable lives, having a child means one or both parents have to take time out of work to look after them - a burden that still falls mostly on women. But it can be very difficult to get back into a good career after taking time out.

So it is no wonder many people decide having lots of children is just not worth it. But we could change all of these conditions. If people had better work and shorter hours, if childcare was more readily available, the fertility rate might start pushing upwards again.

Yes: The way we live our lives today is based on the assumption of endless population growth. It is going to be a mighty shock to the system when it goes into reverse.

No: Humans are very adaptable. We took rapid population growth in our stride, and we will easily be able to manage the opposite. We should not worry about making dramatic changes now.

Or... The problem is not with the way we live. Right now we are discouraging people from having children with high costs and poor work. Change that, and the population will stabilise.