How many have lived? A short history of everyone

Looking for an appropriate topic to start this blog (which will take over the population side from my other pages), the best candidate seems to be that popular topic, "How many of us have ever lived?" The question appears to interest us beyond the old urban myth of more living today than have ever died. And the answer can tell us a great deal about the long-run evolution of human numbers.

The now classic response to the query is that of the Population Reference Bureau's Carl Haub, who in 1995 ventured a total of 105,472 million anatomically modern human beings born over the past 50,000 years, since raised by an additional couple of billion to cover intervening growth. His sensible conclusion was that only 5½% of those who had ever lived were alive in the present, a proportion that would now have risen to 7%, still far short of the reported claims of up to 75% which he so rigorously debunked.

Needless to say Haub’s estimate has been criticised, sometimes as being on the low side, at other times as too high. The author himself stressed the "semi-scientific" character of his speculation, hazarding a guess that the true number might be higher, though his approach – estimating the population in given periods and assigning a likely approximate birth rate in the light of probable mortality and growth – was a sound one in the absence of reliable data until comparatively recently. His allowance of only fifty millennia of Homo sapiens would today generally be doubled or even quadrupled, though as he observed the number of additional lives involved in such an extension would constitute only a tiny fraction of the total as global human population remained sparse – at most 5-10 million – until the dawn of agriculture little over ten thousand years ago.

A more fundamental problem with Haub’s figures is his assumption of an 8% annual birth rate until the start of the Christian era. This is certainly too high for the later part of the epoch as settled societies developed toward the 4-5% mortality of the Classical age, but its plausibility as a norm in earlier millennia is also questionable. As annual population growth over this vast period averaged well under 0.1%, the higher figure implies a crude death rate of barely under 8% with an average expectation of life at birth of just over 12½ years, infant mortality of around a half, and 70% of girls dying before childbearing age, placing an enormous reproductive burden on the survivors. The total fertility rate – live births per woman making it through reproductive age (conventionally 15-45, now increasingly 15-50, but possibly shorter with the poorer nutrition and health of earlier times though modern least-developed country patterns suggest otherwise) – becomes difficult to calculate at such extreme levels, but is likely to be in in the region of 9½-10, at the high end of the range for the strongly pro-natal Hutterite community whose 9-10 births are commonly thought to approach the biological maximum even for such a relatively healthy and well-nourished population. While such numbers may have characterised pre-agricultural populations in particular periods or areas, births at this level seem unsustainable worldwide over hundreds or thousands of generations.

If 8% is improbably high for annual births (and deaths), what might be appropriate in its place? High-fertility populations until recent times have regularly exceeded a birth rate of 5% and sometimes attained completed fertility of 8, with Niger (a society with a strong cultural preference for large families) maintaining a CBR of 5.7% into the 1960s and fertility of around 7½ to the present. The TFR indicated by a mesolithic 6% birth and death rate – still consistent with life expectancy of only 16½ years but now giving around 7¼-7½ births per woman completing childbearing age – is thus not impossible. An average birth rate of around 7% for the long prehistoric period incorporates the notion that childbearing was at times higher, at others lower.

The cumulative impact of a difference between a birth rate of 8% over the millennia before the Roman and Han empires and one descending gradually to 5-6% dwarfs the addition to overall human numbers from an additional 150,000 years of hunting and gathering. But another serious question mark hangs over Haub’s adoption of the common 300 million as his population for the year 1 CE. There is really no foundation for this figure, which still surfaces regularly in UN projections but seems to rest on outdated notions of African and Indian stagnation over centuries. We now know that like Europe and China, sub-Saharan Africa was in the throes of internal agricultural colonisation for at least two millennia before the arrival of Europeans, and there is no reason to assume that India alone of the world’s major regions was becalmed in centuries of demographic inertia. While McEvedy's 170 million is likely to be an underestimate, 210 million seems a reasonable allowance given 60 million in China and still probably under 40 million in Europe, with India at perhaps something around 45-50 million.

This point is important because Haub's 1 CE benchmark underpins the biggest single element of his total, the 46 billion assumed to have been born over the preceding eight millennia. Adjusting both the birth rate and the final population as outlined above reduces that portion by three-fifths, even after raising the number of people – and hence also of births – at the start of the period to take account of progress in the mesolithic. Add to that figures for 1200 and 1750-1900 that seem high by around 50 million (although the intervening estimate for 1650 may conversely be low by 70 million) coupled with excessive birth rates down to at least 1650 (though those for 1750-1900 should be raised, offsetting the overstatement of population), and the case for a lower total seems overwhelming.

The table below illustrates the results of such a reworking, with the latest UN estimates appended to Haub's original estimate to bring the numbers down to 2015. The numbers of births differ little between the two series after 1650, but my revisions for the preceding ten millennia bring the total down by 30 billion to under 80 billion, still a formidable sum to contemplate: on this showing those living today would be 9.3% of all those ever born, up from 1% in 1650, 2% in 1860, 3% in 1930, 5% in 1970 and 8% in 2000: looking ahead, the ratio should peak at 12% toward the end of this century and be back down below 11% a couple of centuries later, still well short of anything approaching a majority as Haub correctly surmised.

The exercise is of use not just as a piece of sterile enquiry, but in revealing population as a complex of dynamic flows rather than mere stocks of people. In particular, it invites us to consider the conditions under which societies of the past survived and eventually grew. And it may explain something of present-day reproductive behaviour: if birth rates of 8% were as unsustainable as calculations of completed fertility seem to indicate, a similar prolonged death rate might have imperilled the very survival even at greatly reduced numbers of communities at the margins of subsistence. Lingering large-family preferences in some regions may go deeper than cultural traditions or the need for children to provide support in older age, perhaps offering an indication of why some fertility levels have been harder to bring down than others.