Owing to several factors, most notably the decline in fertility, a newly published study has suggested that the world's population will begin to decline over the next few decades, especially in developed countries, where ageing is high.

The world population now stands at 7.8 billion, with experts predicting a peak of 2064, when we reach 9.7 billion.

After reaching this peak, the world's population will continue to decline, falling to 8.79 billion by 2100, 79 years from now.

The study, published in the scientific journal Lancet, reported that the population of some 23 countries in the world would be halved by low fertility and high ageing.

The list of countries threatened by severe population decline includes Japan, Thailand, Italy, Portugal and South Korea, which means that developed countries will be most affected by population shortages.

Even China, which is the world's most populous population at the moment, will not be spared the decline, because this Asian country's population will also fall from 1.4 billion to 732 million in 2100.

Sixty Emil Volsett, a researcher from the Institute of Health Metrics and Evaluation of the University of Washington, gives an explanation of the reasons for this anticipated decline.

The last decline in the world's population occurred in the mid-14th century, and was due to the black plague or so-called "bubonic" plague.

"If these expectations are correct, the world population will have declined for the first time in history due to fertility shortages, whereas the decline was previously caused by diseases and famine, that is, by compelling factors."

In contrast, there are countries that will see their population rise, with the Middle East and sub-Saharan Africa expected to increase by 3 times, from 1.03 billion in 2017 to 3.07 billion in 2100.

Volsett stated: "Africa and the Arab world will shape the future, while the influence of Europe and Asia will decline," he added: "By the end of the present century, the world will be multipolar, while India, Nigeria, China and the United States will become the dominant powers."

The American researcher attributes the decline in fertility to two prominent factors, namely, easy access to contraceptives thanks to modern medicine, as well as the education of girls and women.