Elon Musk has taken to X, formerly known as Twitter, to share his agreement in a post that population decline now stands as the world’s biggest problem.

A graph showing the expected change in population size between 2018 and 2100 for the predicted most populous nations on Earth, which also include Nigeria, the US, Indonesia and Pakistan, was posted to X Tesla Owners Silicon Valley account.

The account wrote: “Population collapse is our greatest threat to humanity… Elon Musk”.

Musk - who has 12 children - retweeted the post, simply responding: “Yes”.

Experts have for many years agreed that the world is set on a path for future population decline, though there is disagreement about how fast this process will be, and which countries will be affected the most.

Among the reasons for population decline are reduced fertility rates, emigration and ageing populations. In many countries, the average number of children per woman is below 2.1 - the level needed to maintain a constant population.

In 2023, the average number of children per woman in England and Wales was 1.44, the lowest on record, according to the Office for National Statistics. Fertility rates across the world have more than halved since 1963, when the average woman in the world had 5.3 children.

The graph - first published in 2020 - shows that while both the populations of India and China stand at around 1.5 billion as of 2018, India is expected to fall by just under 1.1 billion by 2100 - a fall of around four million.

China, meanwhile, is anticipated to decrease to just 731.9 million - a staggering fall of 731 million. This fall would see the country overtaken by Nigeria, who is set to reach a size of 790.1 million by 2100.

In the 2020 report, the researchers at the University of Washington suggested that population decline could be more rapid than previously thought, especially in China and India.

They assumed the world population will already peak around the middle of the century - earlier than projected by the UN Population Division.

The researchers did not see the same factors at play in most African nations, where population growth would continue to 2100 and beyond.

The US would remain the fourth-largest nation by 2100, despite below-replacement levels of fertility. The US, along with Canada and Australia, is set to keep population levels semi-stable through positive net migration instead.

Also by 2100, current growth nations Indonesia and Pakistan would have entered a slight population decline, with the Democratic Republic of the Congo and Ethiopia overtaking them in size around the turn of the new century.

Musk has also previously shared concerns about world population decline, claiming that "population collapse is coming" and sharing a screengrab of an article by Australia's ABC News, reposting the Bureau of Statistics figures, showing that 286,998 births were registered in 2023.