It is only in the past two centuries where demographics and the development of human populations has emerged as a subject in its own right, as industrialization and improvements in medicine gave way to exponential growth of the world's population. There are very few known demographic studies conducted before the 1800s, which means that modern scholars have had to use a variety of documents from centuries gone by, along with archeological and anthropological studies, to try and gain a better understanding of the world's demographic development. Genealogical records One such method is the study of genealogical records from the past; luckily, there are many genealogies relating to European families that date back as far as medieval times. Unfortunately, however, all of these studies relate to families in the upper and elite classes; this is not entirely representative of the overall population as these families had a much higher standard of living and were less susceptible to famine or malnutrition than the average person (although elites were more likely to die during times of war). Nonetheless, there is much to be learned from this data. Impact of the Black Death In the centuries between 1200 and 1745, English male aristocrats who made it to their 21st birthday were generally expected to live to an age between 62 and 72 years old. The only century where life expectancy among this group was much lower was in the 1300s, where the Black Death caused life expectancy among adult English noblemen to drop to just 45 years. Experts assume that the pre-plague population of England was somewhere between four and seven million people in the thirteenth century, and just two million in the fourteenth century, meaning that Britain lost at least half of its population due to the plague. Although the plague only peaked in England for approximately eighteen months, between 1348 and 1350, it devastated the entire population, and further outbreaks in the following decades caused life expectancy in the decade to drop further. The bubonic plague did return to England sporadically until the mid-seventeenth century, although life expectancy among English male aristocrats rose again in the centuries following the worst outbreak, and even peaked at more than 71 years in the first half of the sixteenth century.

For most of the world, throughout most of human history, the average life expectancy from birth was around 24. This figure fluctuated greatly depending on the time or region, and was higher than 24 in most individual years, but factors such as pandemics, famines, and conflicts caused regular spikes in mortality and reduced life expectancy. Child mortality The most significant difference between historical mortality rates and modern figures is that child and infant mortality was so high in pre-industrial times; before the introduction of vaccination, water treatment, and other medical knowledge or technologies, women would have around seven children throughout their lifetime, but around half of these would not make it to adulthood. Accurate, historical figures for infant mortality are difficult to ascertain, as it was so prevalent, it took place in the home, and was rarely recorded in censuses; however, figures from this source suggest that the rate was around 300 deaths per 1,000 live births in some years, meaning that almost one in three infants did not make it to their first birthday in certain periods. For those who survived to adolescence, they could expect to live into their forties or fifties on average. Modern figures It was not until the eradication of plague and improvements in housing and infrastructure in recent centuries where life expectancy began to rise in some parts of Europe, before industrialization and medical advances led to the onset of the demographic transition across the world. Today, global life expectancy from birth is roughly three times higher than in pre-industrial times, at almost 73 years. It is higher still in more demographically and economically developed countries; life expectancy is over 82 years in the three European countries shown, and over 84 in Japan. For the least developed countries, mostly found in Sub-Saharan Africa, life expectancy from birth can be as low as 53 years.

A global phenomenon, known as the demographic transition, has seen life expectancy from birth increase rapidly over the past two centuries. In pre-industrial societies, the average life expectancy was around 24 years, and it is believed that this was the case throughout most of history, and in all regions. The demographic transition then began in the industrial societies of Europe, North America, and the West Pacific around the turn of the 19th century, and life expectancy rose accordingly. Latin America was the next region to follow, before Africa and most Asian populations saw their life expectancy rise throughout the 20th century.

Global life expactancy at birth has risen significantly since the mid-1900s, from roughly 46 years in 1950 to 73.5 years in 2025. Post-COVID-19 projections There was a drop of 1.7 years during the COVID-19 pandemic, between 2019 and 2021, however figures resumed upon their previous trajectory the following year due to the implementation of vaccination campaigns and the lower severity of later strains of the virus. By the end of the century it is believed that global life expectancy from birth will reach 82 years, although growth will slow in the coming decades as many of the more-populous Asian countries reach demographic maturity. However, there is still expected to be a wide gap between various regions at the end of the 2100s, with the Europe and North America expected to have life expectancies around 90 years, whereas Sub-Saharan Africa is predicted to be in the low-70s. The Great Leap Forward While a decrease of one year during the COVID-19 pandemic may appear insignificant, this is the largest decline in life expectancy since the "Great Leap Forward" in China in 1958, which caused global life expectancy to fall by almost four years between by 1960. The "Great Leap Forward" was a series of modernizing reforms, which sought to rapidly transition China's agrarian economy into an industrial economy, but mismanagement led to tens of millions of deaths through famine and disease.

Chart and table of India life expectancy from 1950 to 2025. United Nations projections are also included through the year 2100.

Life expectancy in the United Kingdom was below 39 years in the year 1765, and over the course of the next two and a half centuries, it is expected to have increased by more than double, to 81.1 by the year 2020. Although life expectancy has generally increased throughout the UK's history, there were several times where the rate deviated from its previous trajectory. These changes were the result of smallpox epidemics in the late eighteenth and early nineteenth centuries, new sanitary and medical advancements throughout time (such as compulsory vaccination), and the First world War and Spanish Flu epidemic in the 1910s.

Life expectancy in India was 25.4 in the year 1800, and over the course of the next 220 years, it has increased to almost 70. Between 1800 and 1920, life expectancy in India remained in the mid to low twenties, with the largest declines coming in the 1870s and 1910s; this was because of the Great Famine of 1876-1878, and the Spanish Flu Pandemic of 1918-1919, both of which were responsible for the deaths of up to six and seventeen million Indians respectively; as well as the presence of other endemic diseases in the region, such as smallpox. From 1920 onwards, India's life expectancy has consistently increased, but it is still below the global average.

Life expectancy in Japan was 36.4 in the year 1860, and over the course of the next 160 years, it is expected to have increased to 84.4, which is the second highest in the world (after Monaco). Although life expectancy has generally increased throughout Japan's history, there were several times where the rate deviated from its previous trajectory. These changes were a result of the Spanish Flu in the 1910s, the Second World War in the 1940s, and the sharp increase was due to the high rate of industrialization and economic prosperity in Japan, in the mid-twentieth century.

Life expectancy in China was just 32 in the year 1850, and over the course of the next 170 years, it is expected to more than double to 76.6 years in 2020. Between 1850 and 1950, finding reliable data proved difficult for anthropologists, however some events, such as the Taiping Rebellion and Dungan Revolt in the nineteenth century did reduce life expectancy by a few years, and also the Chinese Civil War and Second World War in the first half of the twentieth century. In the second half of the 1900s, Chinese life expectancy increased greatly, as the country became more industrialized and the standard of living increased.

Life expectancy in Russia was 29.6 in the year 1845, and over the course of the next 175 years, it is expected to have increased to 72.3 years by 2020. Generally speaking, Russian life expectancy has increased over this 175 year period, however events such as the World Wars, Russian Revolution and a series of famines caused fluctuations before the mid-twentieth century, where the rate fluctuated sporadically. Between 1945 and 1950, Russian life expectancy more than doubled in this five year period, and it then proceeded to increase until the 1970s, when it then began to fall again. Between 1970 and 2005, the number fell from 68.5 to 65, before it then grew again in more recent years.

In 1870, the average person born in the Philippines could expect to live to just under the age of 31 years old. This figure would remain unchanged until the early 1900s, when life expectancy would fall to just over 25 years in the Philippine-American War of 1899-1902, as disruptions in food supply and healthcare would result in the loss of several hundred thousand Filipinos to famine and disease. This drop would be accompanied by another drop in the 1920s as the Spanish Flu would ravage the country. However, life expectancy would quickly recover and begin to rise under the United States military administration of the island, as investment by the American government would result in significant expansion in access to nutrition and healthcare. As a result, life expectancy would rise to over 41 years by 1940.

Life expectancy in the Philippines would decline once more in the 1940s, however, in the 1941 invasion and subsequent occupation of the island nation by the Empire of Japan in the Second World War, in which famine and causalities of war would result in the death of an estimated 500,000 Filipinos. Despite significant destruction in the Second World War, and an ending to the bulk of American investment in the country following its independence from the U.S. in 1946, life expectancy in the Philippines would quickly rise in the post-war years as the country would modernize; almost doubling in the two decades between 1945 and 1965 alone. It then plateaued throughout the 1970s and 1980s, during the authoritarian regime of Ferdinand Marcos, before the People Power Revolution in 1986 returned democracy to the country, and living standards began to improve once more. Life expectancy has also increased since this time, and in 2020, it is estimated that the average person born in the Philippines can expect to live to just over the age of 71 years old.

In 1870, the average life expectancy in South Africa was 33.5 years from birth. This life expectancy would remain largely unchanged until the late-1910s, where life expectancy would drop to as low as thirty years as a result of the 1918 Spanish Flu epidemic. In the 1930s, life expectancy in South Africa would begin to steadily rise, peaking at over 63 years in 1995, as industrialization and greater access to healthcare and vaccinations led to significantly reduced child mortality rates across the region. However, life expectancy experienced a sudden drop beginning after 1995, as the HIV/AIDS epidemic spread throughout the country, beginning in the early 1990s. As the epidemic spread through the country, life expectancy would fall by almost 10 years, bottoming out below 54 years in 2005. Life expectancy would begin to rise again beginning in the early 2010s however, as access to HIV counselling and treatments, such as antiretroviral therapy, became more widely available throughout the region. Life expectancy in the country is estimated to be almost 64 years from birth in 2020; a return to the pre-HIV figures of the early 1990s.

In the past four centuries, the population of the United States has grown from a recorded 350 people around the Jamestown colony of Virginia in 1610, to an estimated 331 million people in 2020. The pre-colonization populations of the indigenous peoples of the Americas have proven difficult for historians to estimate, as their numbers decreased rapidly following the introduction of European diseases (namely smallpox, plague and influenza). Native Americans were also omitted from most censuses conducted before the twentieth century, therefore the actual population of what we now know as the United States would have been much higher than the official census data from before 1800, but it is unclear by how much. Population growth in the colonies throughout the eighteenth century has primarily been attributed to migration from the British Isles and the Transatlantic slave trade; however it is also difficult to assert the ethnic-makeup of the population in these years as accurate migration records were not kept until after the 1820s, at which point the importation of slaves had also been illegalized. Nineteenth century In the year 1800, it is estimated that the population across the present-day United States was around six million people, with the population in the 16 admitted states numbering at 5.3 million. Migration to the United States began to happen on a large scale in the mid-nineteenth century, with the first major waves coming from Ireland, Britain and Germany. In some aspects, this wave of mass migration balanced out the demographic impacts of the American Civil War, which was the deadliest war in U.S. history with approximately 620 thousand fatalities between 1861 and 1865. The civil war also resulted in the emancipation of around four million slaves across the south; many of whose ancestors would take part in the Great Northern Migration in the early 1900s, which saw around six million black Americans migrate away from the south in one of the largest demographic shifts in U.S. history. By the end of the nineteenth century, improvements in transport technology and increasing economic opportunities saw migration to the United States increase further, particularly from southern and Eastern Europe, and in the first decade of the 1900s the number of migrants to the U.S. exceeded one million people in some years. Twentieth and twenty-first century The U.S. population has grown steadily throughout the past 120 years, reaching one hundred million in the 1910s, two hundred million in the 1960s, and three hundred million in 2007. In the past century, the U.S. established itself as a global superpower, with the world's largest economy (by nominal GDP) and most powerful military. Involvement in foreign wars has resulted in over 620,000 further U.S. fatalities since the Civil War, and migration fell drastically during the World Wars and Great Depression; however the population continuously grew in these years as the total fertility rate remained above two births per woman, and life expectancy increased (except during the Spanish Flu pandemic of 1918).

Since the Second World War, Latin America has replaced Europe as the most common point of origin for migrants, with Hispanic populations growing rapidly across the south and border states. Because of this, the proportion of non-Hispanic whites, which has been the most dominant ethnicity in the U.S. since records began, has dropped more rapidly in recent decades. Ethnic minorities also have a much higher birth rate than non-Hispanic whites, further contributing to this decline, and the share of non-Hispanic whites is expected to fall below fifty percent of the U.S. population by the mid-2000s. In 2020, the United States has the third-largest population in the world (after China and India), and the population is expected to reach four hundred million in the 2050s.

Brazil and the United States are the two most populous countries in the Americas today. In 1500, the year that Pedro Álvares Cabral made landfall in present-day Brazil and claimed it for the Portuguese crown, it is estimated that there were roughly one million people living in the region. Some estimates for the present-day United States give a population of two million in the year 1500, although estimates vary greatly. By 1820, the population of the U.S. was still roughly double that of Brazil, but rapid growth in the 19th century would see it grow 4.5 times larger by 1890, before the difference shrunk during the 20th century. In 2024, the U.S. has a population over 340 million people, making it the third most populous country in the world, while Brazil has a population of almost 218 million and is the sixth most populous. Looking to the future, population growth is expected to be lower in Brazil than in the U.S. in the coming decades, as Brazil's fertility rates are already lower, and migration rates into the United States will be much higher. Historical development The indigenous peoples of present-day Brazil and the U.S. were highly susceptible to diseases brought from the Old World; combined with mass displacement and violence, their population growth rates were generally low, therefore migration from Europe and the import of enslaved Africans drove population growth in both regions. In absolute numbers, more Europeans migrated to North America than Brazil, whereas more slaves were transported to Brazil than the U.S., but European migration to Brazil increased significantly in the early 1900s. The U.S. also underwent its demographic transition much earlier than in Brazil, therefore its peak period of population growth was almost a century earlier than Brazil. Impact of ethnicity The demographics of these countries are often compared, not only because of their size, location, and historical development, but also due to the role played by ethnicity. In the mid-1800s, these countries had the largest slave societies in the world, but a major difference between the two was the attitude towards interracial procreation. In Brazil, relationships between people of different ethnic groups were more common and less stigmatized than in the U.S., where anti-miscegenation laws prohibited interracial relationships in many states until the 1960s. Racial classification was also more rigid in the U.S., and those of mixed ethnicity were usually classified by their non-white background. In contrast, as Brazil has a higher degree of mixing between those of ethnic African, American, and European heritage, classification is less obvious, and factors such as physical appearance or societal background were often used to determine racial standing. For most of the 20th century, Brazil's government promoted the idea that race was a non-issue and that Brazil was racially harmonious, but most now acknowledge that this actually ignored inequality and hindered progress. Racial inequality has been a prevalent problem in both countries since their founding, and today, whites generally fare better in terms of education, income, political representation, and even life expectancy. Despite this adversity, significant progress has been made in recent decades, as public awareness of inequality has increased, and authorities in both countries have made steps to tackle disparities in areas such as education, housing, and employment.

Throughout the Common Era, Japan's population saw relatively steady growth between each century. Failed invasions and distance from Asia's mainland meant that Japan was unaffected by many pandemics, primarily bubonic plague, therefore its development was not drastically impeded in the same way as areas such as China or Europe. Additionally, religious practices meant that hygiene was prioritized much more in Japan than in other regions, and dietary customs saw lower rates of meat consumption and regular boiling of water in meals or tea; both of these factors contributed to lower rates of infection for many parasitic or water-borne diseases. Fewer international conflicts and domestic stability also saw lower mortality in this regard, and Japan was an considered an outlier by Asian standards, as some shifting trends associated with the demographic transition (such as lower child mortality and fertility) began taking place in the 17th century; much earlier time than anywhere else in the world. Yet the most significant changes came in the 20th century, as Japan's advanced healthcare and sanitation systems saw drastic reductions in mortality. Challenges Japan's isolation meant that, when pandemics did arrive, the population had less protection and viruses could have higher mortality rates; smallpox has been cited as the deadliest of these pandemics, although increased international contact in the late 19th century brought new viruses, and population growth slowed. Earlier isolation also meant that crop failure or food shortages could leave large sections of the population vulnerable, and, as mentioned, the Japanese diet contained relatively little meat, therefore there was a higher reliance on crops and vegetables. It is believed that the shortage of arable land and the acidity of the soil due to volcanic activity meant that agriculture was more challenging in Japan than on the Asian mainland. For most of history, paddy fields were the most efficient source of food production in Japan, but the challenging nature of this form of agriculture and changes in employment trends gradually led to an increased reliance in imported crops. Post-Sakoku Japan Distance from the Asian mainland was not the only reason for Japan's isolation; from 1603 to 1853, under the Tokugawa shogunate, international trade was restricted, migration abroad was forbidden, and most foreign interaction was centered around Nagasaki. American neo-imperialism then forced Japan to open trade with the west, and Japan became an imperial power by the early-1900s. Japanese expansion began with a series of military victories against China and Russia at the turn of the century, and the annexation of Taiwan, Korea, and Manchuria by the 1930s, before things escalated further during its invasion of China and the Second World War. Despite its involvement in so many wars, the majority of conflicts involving Japan were overseas, therefore civilian casualties were much lower than those suffered by other Asian countries during this time. After Japan's defeat in 1945, its imperial ambitions were abandoned, it developed strong economic ties with the West, and had the fastest economic growth of any industrial country in the post-WWII period. Today, Japan is one of the most demographically advanced countries in the world, with the highest life expectancy in most years. However, its population has been in a steady decline for over a decade, and low fertility and an over-aged society are considered some of the biggest challenges to Japanese society today.