"All social planning is based on the size of the population, but also on the age structure, and it is fundamentally changing in a way that we have yet to understand," said George Leeson, director of the Oxford Institute for Population Aging.
The research from the Institute of Health and Values (IHME) at the University of Washington, Seattle, was published in Lancet and compares public health in the world between 1950 and 2017.
Almost half the world Countries, especially in Europe and North and South America, are not born enough children to maintain their population size. Something that has significant implications when communities get more "grandparents than grandchildren".
The result was a "big surprise" for the researchers, the BBC writes.
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Since 1950, birth in the world has been almost half: from an average of 4.7 children per woman to 2.4 children per woman in 2017. But the differences are great, the researchers write. In Africa and Asia, childbirth continues to grow with average women in Niize eating seven children throughout their lives.
According to IHME, Cyprus is the least fertile country in the world – an average Cypriot woman is a child girl in her life. On the other hand, women in Mali, Chad and Afghanistan have an average of more than six children.
Ali Muqdad, a professor at IHME, says that the most important factor in population growth is education.
"If a woman practices, she spends more years in school, rejects her pregnancies and therefore has fewer children," he says.
Muqdad says that in time Populations in developing countries continue to rise, so do their economies generally, which usually have an impact on birth over time.
"Countries are expected to improve economically, and it is more likely that fertility will decrease and increase.
The critical point is when the average fertility level of the country reaches 2.1 children per woman. Then the birth begins to decline. When the study began in 1950, no country reached that point.
"We have reached a watershed where half of the countries have fertility levels below the level of compensation, so if nothing happens, the population will be reduced in these countries."
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The fact that fertility rates are declining in many rich countries does not mean that the population does so because the country's population is a mixture of birth, death and immigration. It may also take a generation before the change begins to notice, but as more countries get better economies, the phenomenon will become more widespread, according to researchers.
We live more than ever. The expected global life expectancy for men increased to 71 years from 48 in 1950. Women are expected to live up to 76 compared with 53 in 1950.
Heart disease today The most common cause of death in the world, IHME states. Until 1990 there were problems in newborns, followed by lung diseases and diarrhea.
"You see less mortality from infectious diseases like countries get richer, but also more handicapped, because people live longer," says Ali Mukdad.
He noted that although deaths from infectious diseases such as malaria and tuberculosis have declined significantly since 1990, new diseases that are transmissible have taken place.
– There are certain behaviors that lead to additional cases of cardiovascular disease and cancer. Obesity is number one – it increases every year and our behavior contributes to it, he says.
If development does not break, we will have a population development with little children but very many ages.
To prevent the consequences of declining population, there are three things that the state can do, researchers write: increasing immigration, making women feed more children with political reforms and increase retirement age.
None of the steps were successful, however, says the study.
Countries with generous migration are struggling with social and political challenges, the lock to increase birth rates has had no major impact on fertile women and suggestions for higher retirement ages have often been met by protests.
Immigration, for young people from Poor countries are moving to rich countries, and no solution at the global level, according to the study.
George & amp; Leeson is still optimistic and believes that an aging population should not be a problem, provided it is adapted to the company.
Demography affects all parts of our lives; The movement, how we live, consumption. It's all a matter of demography, but we need to design a different age structure in a way we have not yet understood, he told the BBC.
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