Seven countries with big (and small) population problems
A major new study published in the Lancet medical journal suggests falling fertility rates mean nearly every country could have shrinking populations by the end of the century, and warns of a "jaw-dropping" impact on societies.
We looked at seven countries facing some of the most dramatic population changes and the measures they are taking to combat them.
Japan's population will more than halve, from a peak of 128 million in 2017 to less than 53 million by the end of the century, the researchers behind the new Lancet study predict.
Japan already has the world's oldest population and the highest rate of people over the age of 100.
This has put strain on the country's workforce and the problem is only expected to worsen.
Official forecasts say elderly people will account for more than 35% of the population by 2040.
This, combined with a low fertility rate of just 1.4 births per woman, means that the number of people able to fill jobs in the country is in decline.
Countries need a fertility rate of about 2.1 births to maintain existing population sizes.
While Japan has traditionally been wary of immigration, it has eased rules in recent years in a bid to deal with the issue.
However, there have been widespread reports of exploitation of migrant workers.
The Italian population is also expected to more than halve, from 61 million in 2017 to 28 million by the end of the century, according to the Lancet study.
Like Japan, Italy is known for its ageing population. More than 23% of people there were over the age of 65 in 2019, according to World Bank data.
In 2015, the government launched a programme offering an €800 (£725) payment per couple per birth to try to boost fertility rates.
However, they still remain among the lowest rates in the European Union.
The country also sees high rates of emigration. About 157,000 people left the country in 2018, according to official data.
Several towns have introduced their own schemes to try to boost local populations and their economies. This includes selling homes for just €1 or even paying people to live in under-populated communities - if they set up a business.
Towns with dwindling populations in Spain - which is also projected to see its population more than halve - have launched similar schemes.
In 1979, China famously introduced its controversial one-child policy to try to slow population growth amid concerns over the effect it would have on its plans for economic growth. Today, the world's most populous country is facing the issue of major declines in birth rates.
The Lancet study predicts that China's population will peak at 1.4 billion in four years' time before nearly halving to 732 million by 2100.
Official data showed the country's birth rate had fallen to its lowest level in 70 years in 2019.
Some fear that the country is a "demographic time bomb", which will see a smaller working-age population having to support a bigger, retired population. As one of the world's biggest economies, this would have global ramifications.
Concerns over China's ageing population led the government to end the one-child policy in 2015, allowing couples to have two children. But while this sparked a brief increase in birth rates, it failed to reverse the trend long-term.
The one-child policy has been blamed for a severe gender imbalance in the country, with males still outnumbering females by more than 30 million in 2019. This has, in part, been blamed on some couples opting for sex-selective abortions.
Experts also said that the easing of population restrictions was not accompanied by additional support for families, meaning many people could not afford more than one child.
Iran is also expected to see its population significantly decline by the end of the century.
The country experienced a population boom after the Islamic Revolution in 1979, but went on to implement an effective population control policy.
Last month, the health ministry warned that annual population growth had dropped below 1%. Without action, it said it could become one of the world's oldest countries in the next 30 years.
State-run news agency Irna has reported that marriage and children within marriage are both in decline, largely because of economic hardship.
In an effort to boost its population, Iran ruled last month that vasectomies can no longer be carried out at state-run medical centres and contraceptives will only be offered to women whose health might be at risk.
Brazil has experienced a dramatic decline in fertility rates over the past 40 years, from about 6.3 births per woman in 1960 to 1.7 in the most recent estimates.
The Lancet study projects that Brazil's population will decline from about 211 million in 2017 to less than 164 million in 2100.
A 2012 study suggested that soap operas portraying small families had contributed to lower birth rates in the predominantly Catholic country.
While overall birth rates are in decline, Brazil is actively trying to stem high teenage pregnancy rates, launching a campaign called "Adolescence first, pregnancy after".
"We need to bring the numbers down. We had the courage to say we are going to talk about delaying the start of sexual relations," Damares Alves, the minister for women, family and human rights, told the BBC earlier this year.
India is expected to overtake China as the most populous country in the world by 2100, according to the new study.
This is despite the fact that the size of its population is expected to decline from today's numbers - dropping from 1.3 billion in 2017 to less than 1.1 billion at the end of the century, researchers say.
The birth rate in the country currently stands at about 2.24, down from 5.91 in 1960.
While other countries are trying to encourage growing fertility rates, Prime Minister Narendra Modi has called on people to have smaller families.
"Population explosion will cause many problems for our future generations. But there is a vigilant section of the public which stops to think, before bringing a child to the world, whether they can do justice to the child, give them all that she or he wants.
"They have a small family and express their patriotism to the country. Let's learn from them. There is a need for social awareness," he said in a speech last year.
Nigeria and other countries in Africa are bucking the trend of population decline.
According to the new Lancet study, the population of sub-Saharan Africa is expected to treble in size to more than three billion people by 2100.
Nigeria will become the world's second most populous country, with 791 million people, it says.
The new study predicts that Nigeria will have one of the largest working-age populations in the world by 2100 and see big increases in GDP.
But rapid population expansion puts a strain on infrastructure and social structures, and Nigerian officials have spoken out about trying to ease population growth.
In a 2018 interview with the BBC, Finance Minister Zainab Ahmed said there needed to be a discussion about the country's birth rate - which is among the highest in the world.
"We have a lot of families who cannot even feed the children they have, not to talk about good healthcare or even giving them good quality education, so we have to talk about these things," she said.