"All social planning is based on population size and age structure and radically changes in a way that we have not yet understood," said George Leeson, Managing Director of the Oxford Institute of Oxidation Aging at the BBC.
The study by the Institute of Metric Health and Assessment (IHME) at Washington University, Seattle, has been published in The Lancet and compares public health in the world between 1950 and 2017.
In almost half the world countries, mainly in Europe and North and South America, are not enough children born to maintain the size of their population. Something that will have serious consequences when communities get more "Grandparents from grandchildren".
The result came as a "big surprise" to the researchers, the BBC writes.
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Since 1950, childbirth in the world has almost doubled: from an average of 4.7 children per woman to 2.4 children per woman in 2017. But the variations are great, the researchers write. In Africa and Asia, childbirth continues to grow with the average women in Niger, who feed seven children during their lives.
According to IHME, Cyprus is the least fertile country in the world – an average Cypriot woman is giving birth to a child in her life. On the other hand, women in Mali, Chad and Afghanistan have on average more than six children.
Ali Mokdad, a professor at IHME, says the most important factor for population growth is education.
"If a woman educates herself, she spends more time at school, postpones her pregnancy and therefore has fewer children," she says.
Mokdad says that while populations in developing countries continue to grow, and their economies are generally increasing, which usually has a diminishing effect on childbirth over time.
"Countries are expected to improve economically and fertility is more likely to decrease and run out.
The crucial point is when a country's average fertility rate reaches 2.1 children per woman. Then childbirth begins to decline. When the study began in 1950, no country had reached this point.
"We have reached a catchment area where half the countries have fertility levels below the level of compensation, so if nothing happens, the population will be reduced to these countries. This is a remarkable transition, says Professor Christopher Murray at IHME.
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The fact that the birth rate falls in many rich countries does not mean that the population does so because the population of a country is a mixture of childbirth, death and migration. It may also take a generation before change begins, but as more countries have better economies, the phenomenon will become more common, according to the researchers.
We also live more than ever. The expected global life expectancy for men has increased to 71 years from 48 in 1950. Women are now expected to live at 76 compared to 53 in 1950.
Heart disease is today the most common cause of death worldwide, says IHME. By 1990, there were newborn problems, followed by lung disease and diarrhea.
"You see less mortality from infectious diseases as countries become richer but more disabilities because people live longer," says Ali Mokdad.
It notes that, although deaths from infectious diseases such as malaria and tuberculosis have declined significantly since 1990, new non-communicable diseases have occurred.
There are some behaviors that lead to more cases of cardiovascular disease and cancer. Obesity is the first – it grows every year and our behavior contributes to it, he says.
If growth does not break, we will have population growth with few children but very many ages.
To counterbalance the effects of a declining population, there are three things a country can do, researchers write: They increase immigration, make women feed more children with political reforms and increase retirement age.
None of the measures were successful, however, the study says.
Countries with generous immigration are struggling with social and political challenges, blocking the rise in birth rates has not had a significant impact on fertile women, and proposals for a higher retirement age have often been tackled with protests.
Migration to young people from poor countries are moving to rich countries, nor is it a global solution, according to the study.
George Leeson is still optimistic and believes that the aging of the population should not be a problem, provided it is adapted to society.
Demography hits all parts of our lives. traffic, how we live, consumption. It's a matter of demography, but we have to design a change of age structure in a way we have not yet understood, he told the BBC.
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