Estimates of historical world population

This article lists current estimates of the world population in history. In summary, estimates for the progression of world population since the Late Middle Ages are in the following ranges:

Estimates for pre-modern times are necessarily fraught with great uncertainties, and few of the published estimates have confidence intervals; in the absence of a straightforward means to assess the error of such estimates, a rough idea of expert consensus can be gained by comparing the values given in independent publications. Population estimates cannot be considered accurate to more than two decimal digits; for example, the world population for the year 2012 was estimated at 7.02, 7.06, and 7.08 billion by the United States Census Bureau, the Population Reference Bureau, and the United Nations Department of Economic and Social Affairs, respectively, corresponding to a spread of estimates of the order of 0.8%.

Deep prehistory
As a general rule, the confidence of estimates on historical world population decreases for the more distant past. Robust population data exist only for the last two or three centuries. Until the late 18th century, few governments had ever performed an accurate census. In many early attempts, such as in Ancient Egypt and the Persian Empire, the focus was on counting merely a subset of the population for purposes of taxation or military service. Published estimates for the 1st century ("AD 1") suggest uncertainty of the order of 50% (estimates range between 150 and 330 million). Some estimates extend their timeline into deep prehistory, to "10,000 BC", i.e., the early Holocene, when world population estimates range roughly between 1 and 10 million (with an uncertainty of up to an order of magnitude).

Estimates for yet deeper prehistory, into the Paleolithic, are of a different nature. At this time, human populations consisted entirely of non-sedentary hunter-gatherer populations, with anatomically modern humans existing alongside archaic human varieties, some of which are still ancestral to the modern human population due to interbreeding with modern humans during the Upper Paleolithic. Estimates of the size of these populations are a topic of paleoanthropology. A late human population bottleneck is postulated by some scholars at approximately 70,000 years ago, during the Toba catastrophe, when Homo sapiens population may have dropped to as low as between 1,000 and 10,000 individuals. For the time of speciation of Homo sapiens, some 200,000 years ago, an effective population size of the order of 10,000 to 30,000 individuals has been estimated, with an actual "census population" of early Homo sapiens of roughly 100,000 to 300,000 individuals. Per Sjödin, Agnès E Sjöstrand, Mattias Jakobsson and Michael G B Blum, "Resequencing data provide no evidence for a human bottleneck in Africa during the penultimate glacial period" Mol Biol Evol (2012) DOI: 10.1093/molbev/mss061. "A small human effective population size, on the order of 10,000 individuals, which is smaller than the effective population size of most great apes, has been interpreted as a result of a very long history, starting ~ 2 mya, of small population size, coined as the long-necked bottle model (Harpending et al. 1998; Hawks et al. 2000). Our findings are consistent with this hypothesis, but, depending on the mutation rate, we find either an effective population size of NA = 12,000 (95% C.I. = 9,000–15,500 when averaging over all three demographic models) using the mutation rate calibrated with the human-chimp divergence or an effective population size of NA = 32,500 individuals (95% C.I. = 27,500–34,500) using the mutation rate given by whole-genome trio analysis (The 1000 Genomes Project Consortium 2010) (supplementary figure 4 and table 6, Supplementary Material online). Not surprisingly, the estimated effective mutation rates θ = 4NAμ are comparable for the two mutation rates we considered, and are equal to 1.4 × 10−3/bp/generation (95% C.I. = (1.1–1.7) × 10−3). Relating the estimated effective population size to the census population size during the Pleistocene is a difficult task because there are many factors affecting the effective population size (Charlesworth 2009). Nevertheless, based on published estimates of the ratio between effective and census population size, a comprehensive value on the order of 10% has been found by Frankham (1995). This 10% rule roughly predicts that 120,000−325,[0]00 individuals (depending on the assumed mutation rate)"

Estimates regarding the questions of "how many people have ever lived?" or "what percentage of people who have ever lived are alive today?" can be traced to the 1970s. The more dramatic phrasing of "the living outnumber the dead" also dates to the 1970s, a time of population explosion and growing fears of human overpopulation in the wake of decolonization and before the adoption of China's one-child policy. The claim that "the living outnumber the dead" was never accurate. Arthur C. Clarke in 2001: A Space Odyssey (1968) has the claim that "Behind every man, now alive stand 30 ghosts, for that is the ratio by which the dead outnumber the living", which was roughly accurate at the time of writing.

Recent estimates of the "total number of people who have ever lived" are in the order of 100 billion. The answer depends on the definition of "people", i.e., whether only Homo sapiens are to be counted, or all of the genus Homo; due to the small population sizes in the Lower Paleolithic, however, the order of magnitude of the estimate is not affected by the choice of cut-off date substantially more than by the uncertainty of estimates throughout the Neolithic to Iron Age. Importantly, the estimate is also affected by the estimate of infant mortalities vs. stillborn infants, due to the very high rate of infant mortality throughout the pre-modern period. An estimate on the "total number of people who have ever lived" as of 1995 was calculated by Haub (1995) at "about 105 billion births since the dawn of the human race" with a cut-off date at 50,000 BC (beginning of the Upper Paleolithic), and inclusion of a high infant mortality rate throughout pre-modern history.

Before 1950
The following table uses astronomical year numbering for dates, negative numbers corresponding roughly to the corresponding year BC (for example, −10,000 = 8,000 BC, etc.). The table starts counting approximately 10,000 years before present, or around 8,000 BC, during the middle Greenlandian, about 1,700 years after the end of the Younger Dryas and 1,800 years before the 8.2-kiloyear event.

From the beginning of the early modern period until the 20th century, world population has been characterized by a faster-than-exponential growth. For the period of Classical antiquity to the Middle Ages, roughly 500 BC to AD 1500, there was also a general tendency of growth (estimated at a factor 4 to 5 over the 2,000-year period), but not strictly monotonic: A noticeable dip in world population is assumed due to the Black Death in the mid-14th century.

1950 to 2016
After World War II, demographic data of some accuracy becomes available for a significant number of countries, and population estimates are often given as grand totals of numbers (typically given by country) of widely diverging accuracies. Some sources give these numbers rounded to the nearest million or the nearest thousand, while others give them without any rounding.

Taking these numbers at face value would be false precision; in spite of being stated to four, seven, or even ten digits, they should not be interpreted as accurate to more than three digits at best (estimates by the United States Census Bureau and by the United Nations differ by about 0.5–1.5%).

By world region
[[File:Pop continents 2000 2050.png|thumb|250px|UN estimates (as of 2017) for world population by continent in 2000 and in 2050 (pie chart size to scale)

]] Population estimates for world regions based on Maddison (2007), in millions. The row showing total world population includes the average growth rate per year over the period separating each column from the preceding one.