"All social planning is based on population size and age structure, and fundamentally changes in a way we have not yet understood," said George Leeson, Oxford Institute's CEO for Population Aging
The study of the Institute of Metrology and Health Assessment (IHME) at Washington University in Seattle was published in the journal
Lancet and compares public health in the world between 1950 and 2017.
In almost half the world countries, especially in Europe and North and South America, are not enough babies to maintain the size of their population. Something that will have major consequences when communities become more "grandparents than grandchildren".
The result came as a "big surprise" for researchers, the BBC writes.
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Since 1950, birth in the world has fallen by nearly half: from an average of 4.7 children per woman to 2.4 children per woman in 2017. But variations are high, say researchers. In Africa and Asia, birth continues to grow with the average women in Niger feeding seven children in their lives.
According to IHME, Cyprus is the least fertile 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 average more than six children.
Ali Mokdad, a professor at IHME, says education is the most important factor for population growth.
"If a woman trains, she spends several years at school, postpones pregnancies, and therefore will have fewer children," he says.
Mokdad says that while populations in developing countries continue to grow, and their economies generally increase, which usually has a decreasing effect on birth over time.
"Countries are expected to become economically better and fertility is more likely to decline and decline.
The critical point is when the average fertility rate of a country reaches 2.1 per woman per woman. Then the birth begins to decrease. When the study began in 1950, no country has reached this point.
"We have reached a point where half of the countries have fertility levels below the level of compensation, so if nothing happens, the population will fall in these countries. This is a remarkable transition," says Professor Christopher Murray from IHME.
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The fact that the birth rate decreases in many rich countries does not mean that the population does it, too, because the population of a country is a mixture of birth, death and immigration. It can also take a generation before the change begins to observe, but, given that many countries get better economies, the phenomenon will become more common, according to researchers.
We also live more than ever. The overall expected life expectancy for men rose to 71 years from 48 in 1950. Currently, women expect to live in 76 people, compared with 53 in 1950.
Heart disease is today the most common cause of death worldwide, says IHME. Since 1990, neonatal problems have occurred, followed by lung disease and diarrhea.
"You see less mortality due to infectious diseases, because countries become richer, but more disabilities, because people live longer," says Ali Mokdad.
He stressed that although deaths from infectious diseases such as malaria and tuberculosis have declined significantly since 1990, new non-communicable diseases have emerged.
– There are certain behaviors that lead to more cases of cardiovascular disease and cancer. Obesity is number one – it grows every year and our behavior contributes to it, he says.
If development does not break, we will have a population growth with few children, but with very many ages.
In order to counteract the consequences of a declining population, there are three things a country can do, researchers write: Increasing immigration, increasing the number of children feeding children with political reforms and increasing the retirement age.
None of the measures was successful, but the study says.
Countries with generous immigration face social and political challenges, the rise in birth rates has not had a major impact on fertile women, and proposals for higher retirement age have often been met with protests.
Migration from young people 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 an aging population should not be a problem, provided it is adapted to society.
Demography affects all parts of our lives; traffic, how we live, consumption. It's about demographics, but we have to plan a modified age structure in a way we have not yet understood, he told the BBC.
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