Global map reveals worrying divide
Developed countries, including Australia, aren't producing enough children to maintain population levels.
The human population is increasing by around 75 million per year. While that may sound like a lot, nearly half of all countries are now facing a "baby bust" meaning their fertility rates are below the replacement level of 2.1 children per woman. The 2.1 benchmark allows for replacement of both parents, accounting for some early-in-life deaths.
Ninety-one nations, mainly in Europe and North and South America, as well as Australia, aren't producing enough children to sustain their current populations, according to a global overview of birth trends from 1950 to 2017, published this month in the journal Lancet.
Those who are having large numbers of children are mainly in developing countries in Africa and Asia where fertility rates continue to grow. For example, the average woman in Niger is giving birth to seven children during her lifetime and women in Mali, Chad and Afghanistan have on average more than six babies.
Comparatively, Australia's fertility rate continues to hover just below the replacement rate. Except for a brief time when it rate reached 2.0 in 2009, it has remained around 1.8 to 1.9 since 2006. The birthrate in the US is also at 1.8 children per woman.
In China, since the government has relaxed its one child policy, from 2014 to 2017 the number of births increased by about 12 per cent. It too has a birthrate of about 1.8.
Professor Alan Lopez from the University of Melbourne is one of the authors of the report which produced a global overview of birth, death and disease rates by evaluating thousands of datasets on a country-by-country basis.
It found that while the world's population skyrocketed from 2.6 billion in 1950 to 7.6 billion last year, that growth was deeply uneven according to region and income.
Speaking to ABC Radio on Thursday, Prof Lopez described the shift that is happening across the global population since the middle of the last century.
"In 1950, 25 per cent of the world's population were in rich countries - high-income countries - today that's around 14 per cent," he said. "And that's declining, so that will go down lower and lower."
In the map below, published in Lancet, you can see that the US, Canada, Western Europe, Australia and New Zealand have seen the steepest decline in birthrates from 1975 to 2017, for women aged 30 to 54.
THE TREND BEHIND BABY BUST
So what is behind the decline in baby making in much of the Western world?
Researchers are quick to evoke one word when asked that question: Development.
"I think what we're seeing is the general impact of development," Prof Lopez said. "As countries develop, family sizes decrease for a number of reasons, primarily because with development comes modernisation and it's inconvenient to have larger family sizes in urban areas."
He also said an increase in educational and professional opportunities for women is a major determinant in sliding birthrates.
As a result, countries that can least afford to support a rising population are the ones seeing sustained high levels of fertility.
Meanwhile policymakers in developed countries like South Korea - where the birthrate is expected to fall to its lowest ever level this year, approaching just 1.0 - are worried about future economic and social problems such as supporting social programs like healthcare and the pension.
In Denmark, the government is so worried about the declining birthrate it has produced sexually charged ads encouraging its population to procreate in a campaign called Do It For Denmark.
EARTH'S CARRYING CAPACITY
The United Nations predicts there will be more than 10 billion humans on the planet by the middle of the century.
Not only are there now billions more of us than 70 years ago, but we are also living longer than ever before.
This raises the question of how many people our world can support, known as Earth's "carrying capacity".
Global male life expectancy had increased to 71 years - from 48 in 1950. Meanwhile women are now expected to live to 76, compared with 53 in 1950. Living longer brings its own health problems, as we age and deteriorate and place greater burdens on our healthcare systems.
The study found that heart disease was now the leading cause of death globally. As recently as 1990, neonatal disorders were the biggest killer, followed by lung disease and diarrhoea.
"You see less mortality from infectious diseases as countries get richer, but also more disability as people are living longer," said Professor Ali Mokdad, from the Institute for Health Metrics and Evaluation, who was involved in the study.
He pointed out that although deaths from infectious diseases like malaria and tuberculosis are down significantly since 1990, new, non-communicable killers have taken their place.
"There are certain behaviours that are leading to an increase in cardiovascular diseases and cancer. Obesity is number one - it is increasing every year and our behaviour is contributing to that."