Soaring birth rates in developing nations are fuelling a global baby boom while women in dozens of richer countries aren't producing enough children to maintain population levels there, according to figures released Friday.
A global overview of birth, death and disease rates evaluating thousands of datasets on a country-by-country basis also found that heart disease was now the single leading cause of death worldwide.
The Institute for Health Metrics and Evaluation (IHME), set up at the University of Washington by the Bill and Melinda Gates Foundation, used more than 8,000 data sources -- more than 600 of them new -- to compile one of the most detailed looks at global public health.
Their sources included in-country investigations, social media and open-source material.
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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.
Ninety-one nations, mainly in Europe and North and South America, weren't producing enough children to sustain their current populations, according to the IHME study.
But in Africa and Asia fertility rates continued to grow, with the average woman in Niger giving birth to seven children during her lifetime.
Ali Mokdad, professor of Health Metrics Sciences at IHME, told AFP that the single most important factor in determining population growth was education.
"It is down to socioeconomic factors but it's a function of a woman's education," he said. "The more a woman is educated, she is spending more years in school, she is delaying her pregnancies and so will have fewer babies."
The IHME found that Cyprus was the least fertile nation on Earth, with the average woman giving birth just once in her life.
By contrast, women in Mali, Chad and Afghanistan have on average more than six babies.
- 'Less mortality, more disability' -
The United Nations predicts there will be more than 10 billion humans on the planet by the middle of the century, broadly in line with IHME's projection.
This raises the question of how many people our world can support, known as Earth's "carrying capacity".
Mokdad said that while populations in developing nations continue to rise, so in general are their economies growing.
This typically has a knock-on effect on fertility rates over time.
"In Asia and Africa the population is still increasing and people are moving from poverty to better income -- unless there are wars or unrest," he said.
"Countries are expected to fare better economically and it's more likely that fertility there will decline and level out."
Not only are there now billions more of us than 70 years ago, but we are also living longer than ever before.
The study, published in The Lancet medical journal, showed male life expectancy had increased to 71 years from 48 in 1950. 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 IHME said 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.
Uzbekistan, Ukraine and Azerbaijan had the highest death rates from heart disease, whereas South Korea, Japan and France had among the lowest.
"You see less mortality from infectious diseases as countries get richer, but also more disability as people are living longer," said Mokdad.
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."