"All social planning is based not only on the size of the population, but also on the age structure, and basically changes in a way that we don't yet understand," said George Leeson, University of Oxford Institute of Population Aging.
The Washington University Health Metric and Evaluation Institute (IHME) research in Seattle was published in the journal.
The Lancet compares the public health between 1950 and 2017 in the world.
Nearly half of the world Countries, particularly Europe, North and South America, do not have enough children to maintain their population size. When communities get more "grandparents than grandchildren", it's going to have great results.
The result came to the BBC as a "big surprise" for the authors.
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Researchers have caused almost half the world's birth rate since 1950: up to 4.7 children in women, up to 2.4 children per woman in 2017. But the variations are great, the researchers write. In Africa and Asia, birth continues to grow with average women in Niger, who feed seven children throughout their lives.
According to the IHME, Cyprus is the most fertile country in the world – an average Cypriot woman has a child in her life. On the other hand, women in Mali, Chad and Afghanistan have more than six children on average.
Ali Mokdad, a professor at IHME, says education is the most important factor for population growth.
Or If a woman trains herself, she will spend more years in school, postpone her pregnancies and therefore have fewer children, “he says.
Mokdad says the population in developing countries continues to grow, so their economies are usually increasing, which often leads to a drop in birth over time.
"Countries are expected to be economically better, and fertility is more likely to decline and rise to levels.
The critical point is that the average fertility level of a country reaches 2.1 children per woman. Then the birth begins to decline. When the study began in 1950, no country had reached that point.
Bir We reached a watershed where half of the countries have fertility levels below the level of compensation, so when nothing happens, the population in these countries decreases. This is Professor Christopher Murray at IHME, which is a remarkable transition.
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In many rich countries, the drop in birth rate does not mean that the population is so; because a country's population is a mixture of birth, death and immigration. There may be a generation before becoming aware of the change, but as more countries get better economy, phenomena will become more common than researchers.
We've been living longer than ever before. The expected global life expectancy for men increased from 48 to 1950 in the 1950s. Now women are expected to live in the 1950s.
Heart disease these days The most common cause of death worldwide is IHME. There were newborn problems in the late 1990s, followed by lung diseases and diarrhea.
Ors As countries become richer, they see less death than infectious diseases, but you also see more disability,. Says Ali Mokdad.
Since 1990, he has pointed out that, although deaths from infectious diseases such as malaria and tuberculosis have decreased significantly, new noncommunicable diseases have taken place.
– There are more cardiovascular diseases and certain behaviors that lead to cancer. Obesity is a number – it increases every year and our behavior contributes to it, he says.
If development does not break, we will have population growth and a lot of life with several children.
To counter the consequences of a falling population, there are three things a country can do, the researchers write: Increasing migration, women feeding more children through political reforms, and raising the age of retirement.
However, no measures have been successful, he says.
Generous migrants are struggling with social and political challenges, locks to increase birth rates did not have a major impact on productive women, and recommendations for high retirement age were usually met by protests.
Migration, young After all, the poor countries are not turning to rich countries, nor are they a global solution.
George Leeson still thinks that an optimistic and aging population doesn't have to be a problem if it's adapted to society.
Demography affects all parts of our lives; traffic, how we live, consumption. Everything is about demography, but we have to plan a changing age structure in a way that we didn't understand before, it told the BBC.
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