A new study published in a leading British medical journal predicts that the total fertility rate worldwide will drop well below replacement level by 2100, and that the population of many countries will drop by half in the coming century. The article, published in The Lancet on Tuesday, is titled “Fertility, mortality, migration, and population scenarios for 195 countries and territories from 2017 to 2100: a forecasting analysis for the Global Burden of Disease Study.” The research was conducted by the University of Washington’s Institute for Health Metrics and Evaluation, and was funded by the Bill & Melinda Gates Foundation. Researchers found that total fertility rate (TFR), the average number of children born to a woman throughout her lifetime, will drop to 1.66 worldwide by 2100. The global population is expected to peak at 9.73 billion in 2064, before dropping due to falling birthrates. A TFR of 2.1 is considered to be “replacement-level.” This means that adults will be producing enough children to eventually replace them in society. The figure is 2.1 rather than 2 to account for the number of children who do not survive to adulthood. With a TFR of 2.1, the population of an area is expected to remain stable. If it is higher, it will grow, and if it is smaller, the population will contract. “Our findings suggest that continued trends in female educational attainment and access to contraception will hasten declines in fertility and slow population growth,” says the article, noting the results could have dramatic effects worldwide. “A sustained TFR lower than the replacement level in many countries, including China and India, would have economic, social, environmental, and geopolitical consequences. Policy options to adapt to continued low fertility, while sustaining and enhancing female reproductive health, will be crucial in the years to come,” they said. The research found that 23 countries, including Japan, Thailand, and Spain, would lose more than 50% of their populations between 2017 and 2100 due to low birthrates if trends do not improve. The approximate current populations of Japan, Thailand, and Spain are 125 million, 69 million, and 47 million, respectively. China, which has a current population of about 1.4 billion people, will experience a population decline of 48% by 2100. The researchers forecast that the Chinese economy will eclipse that of the United States by 2035, but due to falling birthrates the United States will again be the largest economy by 2098. The countries projected to continue to have growing populations are mostly in sub-Saharan Africa. Presently, the top five biggest countries in the world by population are, in order, China, India, United States, Indonesia, and Pakistan. The researchers predict that by 2100, the five biggest countries by population will be India, Nigeria, United States, China, and Pakistan.