In 1800, it is estimated that approximately 9.4 million people lived in the region of modern-day South Korea (and 13.8 million on the entire peninsula). The population of this region would remain fairly constant through much of the 19th century, but would begin to grow gradually starting in the mid-1800s, as the fall of the Joseon dynasty and pressure from the U.S. and Japan would end centuries of Korean isolationism. Following the opening of the country to foreign trade, the Korean peninsula would begin to modernize, and by the start of the 20th century, it would have a population of just over ten million. The Korean peninsula was then annexed by Japan in 1910, whose regime implemented industrialization and modernization policies that saw the population of South Korea rising from just under ten million in 1900, to over fifteen million by the start of the Second World War in 1939.

The Korean War Like most regions, the end of the Second World War coincided with a baby boom, that helped see South Korea's population grow by almost two million between 1945 and 1950. However, this boom would stop suddenly in the early 1950s, due to disruption caused by the Korean War. After WWII, the peninsula was split along the 38th parallel, with governments on both sides claiming to be the legitimate rulers of all Korea. Five years of tensions then culminated in North Korea's invasion of the South in June 1950, in the first major conflict of the Cold War. In September, the UN-backed South then repelled the Soviet- and Chinese-backed Northern army, and the frontlines would then fluctuate on either side of the 38th parallel throughout the next three years. The war came to an end in July, 1953, and had an estimated death toll of three million fatalities. The majority of fatalities were civilians on both sides, although the North suffered a disproportionate amount due to extensive bombing campaigns of the U.S. Unlike North Korea, the South's total population did not fall during the war.

Post-war South Korea Between the war's end and the late 1980s, the South's total population more than doubled. In these decades, South Korea was generally viewed as a nominal democracy under authoritarian and military leadership; it was not until 1988 when South Korea transitioned into a stable democracy, and grew its international presence. Much of South Korea's rapid socio-economic growth in the late 20th century was based on the West German model, and was greatly assisted by Japanese and U.S. investment. Today, South Korea is considered one of the world's wealthiest and most developed nations, ranking highly in terms of GDP, human development and life expectancy; it is home to some of the most valuable brands in the world, such as Samsung and Hyundai; and has a growing international cultural presence in music and cinema. In the past decades, South Korea's population growth has somewhat slowed, however it remains one of the most densely populated countries in the world, with total population of more than 51 million people.

In 1800, it is estimated that approximately 4.35 million people lived in the region of present-day North Korea. This figure would remain stagnant through much of the 19th century, increasing by just one million people over the next 110 years. The dissolution of the Joseon dynasty in the Korean peninsula in the late-1800s marked the end of centuries of Korean isolationism, and the new Korean Empire then opened itself to foreign trade and influence. By the start of the 20th century, the region of North Korea had a population of just over five million. This growth would increase dramatically following the annexation of the Korean peninsula by Japan in 1910, as extensive industrialization and modernization efforts would result in the population of North Korea doubling from just over five million at the turn of the century, to over ten and a half million by the start of the Korean War in 1950. The Korean War Following Japan's defeat in the Second World War, the Korean Peninsula was split along the 38th parallel, with a Soviet-backed totalitarian government established in the north, and a U.S.-backed authoritarian government in the south. Neither government recognized the legitimacy of the other, and an escalation of tensions saw the North invade the South in June 1950. By 1953, the North's population dropped below ten million. It is estimated that there were approximately three million total fatalities in the war, with North Korea having the largest share of fatalities of all forces involved; however, the North's isolationist and secretive policies have made it difficult to calculate the full impact of the war on the North's population. Most modern estimates suggest that more than 1.5 million North Koreans died or went missing over these three years. Following the establishment of an armistice between North and South Korea in 1953, the population of the newly-formed Democratic People's Republic of North Korea would begin to rise sharply, from 10 million at the time of armistice to almost 15 million in 1970, just twenty years later. North Korea today North Korea's population has increased in each year since the war's end, however, the rate of growth decreased in the late 1970s, as a severe economic crisis and a resulting lack of foreign technology would lead to a scaling back of many of social welfare programs for the country’s citizens. Growth would slow even further following the dissolution of the Soviet Union in 1991, as the loss of Soviet foreign aid would send the North Korean economy into a sharp decline. In the 1990s, a series of flooding, droughts, famine and economic mismanagement led to the deaths of between 500,000 to 600,000* North Koreans. Recent years have seen an improvement in political relations between both the North and South, and even sporadic improvements with the U.S., which could lead to a less-isolated North Korea in the future; however, the continued testing of nuclear weapons and allegations of widespread human rights violations have drawn widespread criticism from the international community. In 2020, it is estimated that approximately 25.8 million people reside in North Korea.

In 1900, the crude birth rate in South Korea was just under 42 births for every thousand people, meaning that approximately 4.2 percent of the population was born in that year. The crude birth rate would rise briefly in the 1930s, as Japanese investment would lead to economic growth on the peninsula, but would fall sharply in the 1940s, as the Second World War and the Korean War would result in two decades of significant socio-economic turmoil. While the crude birth rate would recover quickly after the end of the Korean War in 1953, a sharp decline in fertility beginning in the 1960s would see a corresponding fall in the crude birth rate lasting until the late 1980s, as South Korea would go through a rapid demographic transition and modernization. While the crude birth rate would briefly rise in the early 1990s, partially due to governmental restrictions on sex-selective abortion; the rate of decline would slow going into the 21st century. As a result, in 2020, it is estimated that South Korea has a birth rate of seven births for every thousand people, which is one of the lowest birth rates in the world.

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.

In 1890, the average person born in the area of present-day North Korea could expect to live to 26 years old, a rate which would see only marginal change until the annexation of the Korean peninsula by the Empire of Japan in 1910. As Japanese large scale industrialization and modernization reforms would begin in the peninsula (building on the Gwangmu reforms of the previous administration), life expectancy would begin to increase rapidly as standards of living rose across the region. As a result, life expectancy would rise by twenty years between 1920 and 1940, however this growth would slow during the 1940s, as Korea lost approximately half a million people (or 2% of its population) during the Second World War. Following the war, the peninsula was split into two separate states, with the north and south each administered by the Soviet Union and the U.S. respectively (although both militaries had largely left the peninsula within a few years). In 1950, as tensions grew between the two governments regarding the legitimate rulers of the peninsula, North Korea invaded the South and set in motion the Korean War. The war would last for three years, and result in the deaths of approximately three million Koreans (with the North suffering heavier losses than the South); this caused the life expectancy in North Korea to plummet from over 49 years to less than 38 years in the first half of the 1950s.

Life expectancy in North Korea would quickly recover in the post-war years, as the Soviet Union would provide significant economic aid to the country. As a result, life expectancy would return to pre-war levels by the end of the decade and resume its steady rise until the collapse of the Soviet Union in 1991. After peaking at 70 years in the early 1990s, it is estimated that life expectancy fell by nearly seven years in the early 1990s as the country would face extreme medical and food shortages following the end of Soviet aid. U.S. estimates put the death toll of these famines at more than half a million people in the 1990s. In the past two decades, life expectancy has slowly increased once more in North Korea, returning to pre-famine levels once more in 2015, and it is now estimated that the life expectancy from birth in North Korea in 2020 is approximately 72 years old.

In 1900, the crude birth rate in the area of modern-day North Korea was estimated to be approximately 42 births per thousand people, meaning just over 4.2% of the population was born in that year. This rate would remain largely unchanged for the first few decades of the 20th century, but would fluctuate greatly in the between the 1940s and 1970s, due to the impact of the Second World War in the 1930s and 1940s, followed by the devastating Korean War in the early 1950s, and then the period of instability and adjustment that followed the war. After falling to just under 21 births per thousand people by 1980, the crude birth rate would remain largely stable until the turn of the century, before falling below 14 births per thousand people.