The world's total fertility rate has been cut in half since 1950, but the population is still rising, according to a study published Thursday in The Lancet.
The total fertility rate -- or the average number of children a woman would have if she lived through all her reproductive years -- declined from 4.7 live births in 1950 to 2.4 in 2017.
Females (demographic group)
Fertility and infertility
Health and medical
Population and demographics
Sexual and reproductive health
Meanwhile, the global population has nearly tripled since 1950, from 2.6 billion people to 7.6 billion, the report says. An average of nearly 84 million people have been added to the Earth's population every year since 1985.
"As women have gotten more educated and participate more in the workforce and they get access to health services, no surprise, fertility has come down tremendously," said study author Dr. Christopher Murray, director of the Institute for Health Metrics and Evaluation at the University of Washington. "And it comes down faster in younger women."
Other factors have been shown to predict falling fertility rates, including better infant survival rates and later marriage.
"The age at which women are getting married is increasing," said Dr. James Kiarie, coordinator for the World Health Organization's Human Reproduction Team in the Department of Reproductive Health and Research.
"Marriage is one of the biggest drivers of having children all over the world," said Kiarie, who is not an author on the new report.
'The world is really divided'
While total fertility rates fell across all 195 countries and territories in the data, they were split roughly down the middle between those below replacement level and those above, Murray said. "Replacement" describes the total fertility rate "at which a population replaces itself from generation to generation, assuming no migration," which comes out to about 2.05 live births, the authors say.
For example, a woman in Cyprus had one child on average in 2017, while a woman in Niger had 7.1. This range is lower than 1950's, in which total fertility rates ranged from 1.7 live births in Andorra to 8.9 in Jordan.
"The world is really divided into two groups," Murray said. "In a generation, the issue's not going to be about population growth. It's going to be about population decline or relaxing immigration policies."
In countries that want to boost fertility rates, the creation of financial incentives for families, including parental leave, has been shown to have a small effect, Murray said. Only 33 countries, largely in Europe, were falling in population between 2010 and 2017, according to the report.
"The country that's probably the most concerned about this already is China, where the number of workers is now starting to decline, and that has an immediate effect on economic growth potential," Murray said. "In a place like India -- that is still above replacement but very soon going to be below replacement fertility -- that's just such a dramatic change."
That doesn't mean the global population will soon reverse course. A United Nations report last year predicted that the world population would swell to 9.8 billion by 2050 and 11.2 billion in 2100. That report forecast that over half of the expected growth between 2017 and 2050 is likely to occur in Africa.
In just the past several years, Kiarie said, parts of Africa and Asia have substantially lowered fertility rates. The countries that have seen the sharpest declines are those that previously had lower rates of contraception, where the introduction of family planning made a more significant impact, he added.
"There's been rapid progress, but I think in terms of ... the areas which have the biggest unmet need for family planning, it's still largely in Africa," he said.
Lifestyle continues to kill millions
Thursday's report comes alongside six others, funded by the Bill & Melinda Gates Foundation, on the global burden of disease. The findings include a rising prevalence of obesity in nearly every nation, idling or worsening mortality rates in some countries, and a high burden of non-communicable diseases, contributing to nearly three-quarters of deaths worldwide in 2017. The top risk factors were high blood sugar, high blood pressure, high body-mass index and smoking.
Lifespans have also gotten longer on average since 1950, climbing from 48.1 to 70.5 years for men and from 52.9 to 75.6 years for women, according to the study. However, the study authors say that just because women live longer doesn't mean they are living in better health.
An editorial published by The Lancet points out that countries around the world are falling behind the United Nations' global health goals in some way, and the study "should be an electric shock, galvanising national governments and international agencies not only to redouble their efforts to avoid the imminent loss of hard-won gains but also to adopt a fresh approach to growing threats."
When it comes to fertility rates, Kiarie said that the UN goals "are about the ability for people to have the number of children they want," as well as when and with whomever they choose. "What is key is for that ability to be there, in the women's hands."
What often gets lost in discussing fertility statistics and population numbers, Kiarie said, is the focus on individual people, their desires and how countries can empower them to achieve those goals.
"How can we ensure that people do what they think is right for themselves?" he asked.