In the early 1970's one, now forgotten, writer estimated that 75% of the people who have ever been born were alive at that time. This guess has taken hold and persisted, even though a little reflection would show how unlikely this figure actually is.
For this estimate to be true it would mean either that births in the 20th century far, far outnumbered those in the past or that there were an extreme number of old people in the early 1970's.
This was faulty reasoning that wrongly allowed social scientists to explain the rapid pace of population growth in the 20th century. The study was accepted despite the facts which showed that these conditions were not true at the time of the estimate.
Modern demographers like Nathan Keyfitz, Judah Matras, Colin McEvedy and Richard Jones among others, have been working on a more realistic assessment of population history since the inaccurate statement was released.
But, by necessity, even these estimates are semi-scientific, as much of the information relating to population history is sketchy at best, and the modern demographers had to use many educated guesses to come up with a reasonably accurate figure.
The accuracy of the new study is dependent on two factors: (1) the length of time humans are thought to be on earth and (2) the average size of the population at different periods.
The first factor: did the population rise at a constant rate or rise to a point and fluctuate wildly in response to famine or climate change? This new projection assumed that the rise was constant for the purpose of producing a linear prediction with as much accuracy as possible.
The birth rates were set at 80 per 1,000 per year through 1 AD and at 60 per 1,000 from 2 AD through 1750. Higher birth rates such as these were required just to maintain a replaceable population before the arrival of modern medicine, better diet, and improved sanitation. (The graph demonstrates this by the very slow growth rate up to that point.)
From 1750 through 1998, the birth rates drop down to a low of 24 in 1998 This reflects the spread of modern medicine as well as birth control programs that are widespread throughout the world in the 1990's.
To deal with the second factor, the demographers placed the very beginning - with just two people at 50,000 years ago. This Adam and Eve starting point is referred to as a "minimalist approach" by researchers. A second benchmark of 5 million people at 8,000 BC, and a third at 300,000,000 at the time of Christ were also used to complete the study.
This projection yields a total of about 110 billion persons that have ever lived on the planet Earth.
The period 8000 BC to 1 AD is the key to the magnitude of the number, but little is known about that era. Some scientists may disagree with the results of the study because the starting point 50,000 BC is in serious question. Humans may have appeared much earlier.
Because of the many assumptions, the study should be taken in the spirit of an interesting intellectual exercise rather than a scientific fact. The results will surely be revised when and if new information becomes available.
The new estimate is certainly more fact based, and considerably more accurate than the one done in the 1970's. The study states that about 5.5 percent of all people that were ever born are alive today, about 5.8 billion. That is a fairly large percentage considering the history of human population dynamics.
Source: Modified from Carl Haub in Population Today, February 1995
Link to the Population Reference Bureau for More Information