In 1800, the population of Japan was just over 30 million, a figure which would grow by just two million in the first half of the 19th century. However, with the fall of the Tokugawa shogunate and the restoration of the emperor in the Meiji Restoration of 1868, Japan would begin transforming from an isolated feudal island, to a modernized empire built on Western models. The Meiji period would see a rapid rise in the population of Japan, as industrialization and advancements in healthcare lead to a significant reduction in child mortality rates, while the creation overseas colonies would lead to a strong economic boom. However, this growth would slow beginning in 1937, as Japan entered a prolonged war with the Republic of China, which later grew into a major theater of the Second World War. The war was eventually brought to Japan's home front, with the escalation of Allied air raids on Japanese urban centers from 1944 onwards (Tokyo was the most-bombed city of the Second World War). By the war's end in 1945 and the subsequent occupation of the island by the Allied military, Japan had suffered over two and a half million military fatalities, and over one million civilian deaths.

The population figures of Japan were quick to recover, as the post-war “economic miracle” would see an unprecedented expansion of the Japanese economy, and would lead to the country becoming one of the first fully industrialized nations in East Asia. As living standards rose, the population of Japan would increase from 77 million in 1945, to over 127 million by the end of the century. However, growth would begin to slow in the late 1980s, as birth rates and migration rates fell, and Japan eventually grew to have one of the oldest populations in the world. The population would peak in 2008 at just over 128 million, but has consistently fallen each year since then, as the fertility rate of the country remains below replacement level (despite government initiatives to counter this) and the country's immigrant population remains relatively stable. The population of Japan is expected to continue its decline in the coming years, and in 2020, it is estimated that approximately 126 million people inhabit the island country.

In 1900, the fertility rate in the region of present-day South Korea was six children per woman, meaning that the average woman born in South Korea in that year could expect to have six children over the course of their reproductive years. This number began to fluctuate in the 1930s, when the Japanese administration (the Korean peninsula had been annexed by Japan in 1910) promoted fertility as part of the war effort, before fertility dropped below 5.2 births per woman in the aftermath of the war. It then increased above 6.3 in the 1950s due to the devastation and mass-displacement caused by the Korean War. As stability returned to the region, South Korea's fertility rate would fall sharply throughout the remainder of the century, as modernization, urbanization, and the implementation of family planning programs would see fertility fall to just over 1.5 children per woman by 1990.

Sex-selective abortion and gender ratios Abortion was illegal in South Korea between 1953 and 2020, although it was permitted in some cases from 1973 onward. Despite this, these laws were rarely enforced, and sex-selective abortion became widespread following advancements in ultrasound technology. In many Asian societies, it was often preferred to have male children as they were viewed as being better long-term providers for their parents and they would carry on the family name. In South Korea in the early 1990s, the practice of sex-selective abortion became so widespread that the gender ratio at birth was 114 males for every 100 females (reportedly as high as 125 in some cities), compared to the historical and natural average of approximately 105 males per 100 females. The government then prohibited doctors from revealing the gender of unborn babies to the parents in 1987, and introduced more severe penalties in 1994, in an attempt to revert this trend. The gender imbalance then reduced in the following decades, and has been at 106 males per 100 females since the 2010s (roughly the natural average). Abortion rights in South Korea were expanded in 2021.

Lowest in the world? Despite government initiatives aimed at increasing fertility, including financial incentives, South Korea's fertility rate has continued to fall in recent years, and today is at around half of replacement level. In 2020, it is estimated that the average woman born in South Korea will have just over one child over the course of their reproductive years. Some critics cite economic factors, such as high education and housing costs, for the reason that young couples are postponing marriage and having families; today, South Korea has the lowest adolescent fertility rate, and the lowest overall fertility rate in the Asia Pacific region. Due to the current trajectory of South Korea's fertility rate, in January 2021, it was announced that the South Korean population experienced a natural decline for the first time in it's history.