"All social planning is based on population size, but also age structure, and it is fundamentally changing in ways that we don't understand," said George Leeson, CEO of the Oxford Institute of Population Aging to the BBC.
Studies from the Institute of Health Metrics and Evaluation (IHME) at the University of Washington, Seattle, have been published in The Lancet magazine and compare public health in the world between 1950 and 2017.
In almost half of the world countries, especially in Europe and North and South America, not enough children are born to maintain their population size. Something that will have big consequences when the community gets more "grandparents than grandchildren".
The results came as a "big surprise" for researchers, the BBC wrote.
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Since 1950, births in the world have almost halved from an average of 4.7 children per woman to 2.4 children per woman in 2017. But the variation is very good, the researchers wrote. In Africa and Asia, labor continues to increase with the average woman in Niger who feeds seven children during their lifetime.
According to IHME, Cyprus is the most infertile country in the world – an average Cypriot woman gives birth to a child in her life. On the other hand, women in Mali, Chad and Afghanistan have an average of more than six children.
Ali Mokdad, professor at IHME, said that one of the most important factors for population growth was education.
"If a woman trains herself, she spends more years in school, postponing her pregnancy and therefore will have fewer children," he said.
Mokdad said it was temporary populations in developing countries continue to increase, so their economies generally increase, which usually has the effect of decreasing labor over time.
"Countries are expected to be better economically and it is likely that fertility will decrease and narrow.
The critical point is when a country's average fertility rate reaches 2.1 children per woman. Then labor begins to decline. When the study began in 1950, no country reached that point.
"We have reached a waterline where half of the countries have fertility rates below the compensation level, so if nothing happens, the population will decline in these countries. This is an extraordinary transition, said IHME Professor Christopher Murray .
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The fact that birth rates fall in many rich countries does not mean that the population does so, because the population of a country is a mixture of birth, death and immigration. It might also take a generation before change begins to pay attention, but as more countries get better economies, this phenomenon will become more common, according to the researchers.
We also live longer than before. The expected global life expectancy for men has increased to 71 years from 48 in 1950. Women are now expected to live up to 76 compared with 53 in 1950.
Current heart disease the most common cause of death globally, IHME said. Until the end of 1990 there was a neonatal problem, followed by lung disease and diarrhea.
"You see fewer deaths from infectious diseases because the country is richer, but also more disabled, because people live longer," Ali Mokdad said.
He pointed out that although deaths from infectious diseases such as malaria and tuberculosis have declined significantly since 1990, new non-communicable diseases have occurred.
– There are certain behaviors that cause more cases of cardiovascular disease and cancer. Obesity is number one – it increases every year and our behavior contributes to it, he said.
If the construction does not break, we will have a population development with several children but very many ages.
To overcome the consequences of a declining population, there are three things a country can do, the researchers write: Increasing immigration, making women feed more children with political reform and increasing retirement age.
None of the steps have been successful, however, said the study.
Countries with generous immigration struggle with social and political challenges, lockwork to increase birth rates does not have a large impact on fertile women and proposals for higher retirement ages are often filled with protests.
Migration, to young than poor countries move to rich countries, not solutions at the global level, according to the study.
George Leeson is still optimistic and believes that an aging population does not have to be a problem, as long as it is adapted to society.
Demography affects all parts of our lives; traffic, how we live, consumption. "It's all about demographics, but we have to plan for age structures that change in ways that we don't understand," he told the BBC.
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