The scope of human experience
Author: Joao Fabiano and Stephen ClareHow many people have ever lived?
How many life-years were ever experienced?
How many life-years have we spent in different activities?
References

How many people have ever lived?
In the Introduction, we discuss the scope of human experience - the cumulative
memories of every person who has ever lived. Our estimates of the amount of time
people have spent doing various activities rely on estimates of the size of the humanpopulation throughout history. The tradition of demographers trying to estimate thenumber of people ever born is centuries old.1 Recent estimates suggest that people
currently alive make up a surprisingly large share of all those that have ever existed.
Though humans have been around for 300,000 years, close to one in ten of those everborn are currently alive.
The basic approach to this question is fairly simple: we can use the current populationand historical birth rates to estimate the total number of people born in the past. The
earliest estimate made like this assumed two people were alive a million years ago, that
population had grown steadily to its current size, and that births per capita had been
constant, and derived the total ever born (Winkler, 1959). However, such estimates arevery sensitive to historical variations in fertility (Keyfitz, 1966).2 Rather than growing
steadily, the human population has grown in fits and starts, and even temporarily
shrunk, throughout history. A better estimate of the number of past people, therefore,
must rely on uncertain estimates of the population size and birth rates throughout all ofhistory.
For recent times, we can simply aggregate data from national censuses to get a global
total (UN, 2019). As we move further into the past, official censuses become sparse.
Luckily, populous societies tend to be complex and record census data for taxation.
1

The earliest attempt was perhaps by the 17th century English economist William Petty: "20,000,000,000,
as the full number of all that died from the beginning of the world to the year 1682, unto which, if
320,000,000, the number of those who are now alive, be added, the total of the quick and the dead willamount but unto one fifth part of the graves which the surface of Ireland will afford" (Petty, 1682, p. 54)
2
That is, there’s a big difference between using one smooth growth curve and a series of different growth
curves. To illustrate, compare a scenario of constant growth to that of growth, decline and explosive
growth.

Even for societies lacking records, demographers can still place upper and lower
bounds on their population by drawing inferences from anthropological findings. For
instance, the extent and type of farming can be paired with dietary needs to infermaximal population density; that is, population can be estimated by dividing the total
amount of farmland in an area by the amount of farmland required to feed one person.
Hunter-gatherers can be estimated similarly. Accounts of household size and residential
ruins can be used to infer how many people lived in a given urban settlement. Humanremains tell us about mortality and fertility. These indirect methods can at least suggest
rough upper and lower bounds for ancient populations.
Another challenge is deciding who counts as human and figuring out when they firstemerged. Fortunately for us, early humans lived in small populations that grew slowly,
so they don’t contribute very much to the total human population, or totality of humanexperience.3
Recent plausible estimates of total population are within the range of 50 to 150 billion(Cohen, 2014, p. 1563; Goldberg, 1983, p. 28).4 Demographers from the Population
Reference Bureau have updated their estimates since 1995 as new data have emerged(Haub & Kaneda, 2021). Their latest work suggests that 117 billion people total have
lived. However, this estimate implicitly assumes a prehistoric life expectancy that is
drastically lower than comparable recent estimates.5 That said, the Population
Reference Bureau works also presents the most detailed period breakdown available,with population and births per person estimated for 12 periods between 190,000 BCE
and AD 2020. The current world population of about 7.7 billion comprises about 6.7
percent of all humans, living or dead. The Population Reference Bureau’s detailed
period breakdown is reproduced in the table below.

3

For instance, adding four in-between periods reduced the ever-lived figure by a factor of 80 (Keyfitz,1966). Meanwhile, moving the start date from 50,000 to 190,000 BC only increased the figure by a factorof 1.1 (Haub & Kaneda 2018, 2021).
4
In a rapid review of primary sources, we found seven sources whose estimates ranged from 46 to 117
billion (Deevey, 1960; Desmond, 1964; Keyfitz, 1966; Westing, 1981; Goldberg, 1983; Sanchez-Romero
et al., 2017; Haub & Kaneda, 2018, 2021). As Homo Sapiens emergence has been pushed back recently(Schlebusch et al., 2017; Hublin et al., 2017; Galway-Witham & Stringer, 2018), recent estimates tend to
be on the higher end of this range.
5
They claim it “averaged only about 10 years” (Haub & Kaneda, 2021) but do not include specifics in theirdata table. Sanchez-Romero et al. (2017) calculated that the implied prehistoric life expectancy in their2011 estimate is 13 years. PRB’s figure comes from assuming a linear increase from prehistory to now.
However, there’s substantial archaeological evidence that life expectancy decreased with the inception ofagriculture (Diamond 1999; Gurven & Kaplan 2007). The average life expectancy among hunter-gatherer
societies is over 30 years (Kaplan et al. 2010; Gurven & Kaplan 2007), whereas the life expectancy inAncient Rome and Greece was 28 (Scheidel 2001; Morris 2004). See next section for life expectancy
data.

PeriodEnding Population

Births within period

Percentage of those
ever born alive at end

190,000-50,000 BC

2,030,625

7,856,100,000

0%50,000-8,000 BC

5,000,000

1,137,789,769

0.1%

8,000 BC-1 AD

300,000,000

46,025,332,354

0.5%

1-1200450,000,000

26,591,343,000

0.6%1200-1650

500,000,000

12,782,002,453

0.5%

1650-1750

795,000,000

3,171,931,513

0.8%

1750-1850

1,265,000,000

4,046,240,009

1.2%

1850-1900

1,656,000,000

2,900,237,856

1.6%

1900-1950

2,516,000,000

3,390,198,215

2.3%

1950-1995

5,760,000,000

5,427,305,000

5.1%

1995-2011

6,987,000,000

2,130,327,622

6.1%

2011-2020

7,772,850,162

1,302,594,620

6.7%

Table 1: People that ever lived, 12 periods (Source: PRB)

How many life-years were ever experienced?
A life of 75 years typically contains three times as many experiences as a life of 25
years. So, to estimate the number and distribution of total life-years, we have toconsider life expectancy in addition to population size.6 This has varied a lot throughout
history. Some estimates have life expectancy in Ancient Rome as low as 25 years(Scheidel, 2001), whereas today it is over 73 years globally (WHO, 2020). The 26 billion
people born between, say, AD 1 and AD 1200 had substantially shorter lives than thoseborn more recently. We can construct our own estimate of total life-years ever lived byadding life expectancy data to the Population Reference Bureau data. Multiplying aperiod’s births by the average length of a life within that period will tell us about how
many life-years were lived in that time.

6

The current life expectancy is a projection of how long a currently alive cohort is expected to live on
average. It uses current observed mortality rates at different ages adjusted by observed past trends inmortality to account for future changes. Historical life expectancy is something of a misnomer given that it
refers to past populations that are all dead; instead, it is a retroactive estimative of how long people lived
on average.

Unfortunately, historical life expectancy is even harder to estimate than historical
population. While rare, we do at least have some census data from the ancient world;
one of the earliest is a Chinese record from the year AD 2 (Nishijima, 1986). In contrast,
life expectancy has only been recorded in the last couple of centuries. The very idea of
estimating how long a cohort will live only came about in the 17th century, when it
couldn’t even be calculated due to a lack of mortality data (Greenwood, 1938; Tattersall
1996, pp. 333-334).7
Thus, to estimate historical life expectancy we must turn to indirect measures, such as
life expectancy in current hunter-gatherer societies, estimated age of death of humanremains, and tombstone inscriptions. Unsurprisingly, the available evidence suggeststhat life expectancy has generally increased. But it also shows that this increase has
been punctuated by the shocks of the agricultural and industrial revolutions.8
Period/Year

Life expectancy

Source300,000-15,000BC

35.5

Kaplan et al. (2000)9

15,000-7,000BC

30Galor & Moav (2005)

7,000-3,000BC

20Galor & Moav (2005)

3,000BC-650BC

35Galor & Moav (2005)

650BC-500AD

28

Scheidel (2001), Morris (2004)

500-150028**No sources, repeating above

1543 (UK)34

Zijdeman & Ribeira da Silva (2015)

1751 (Sweden)

35

Finch (2010)

1800

28.5

Riley (2005)

7

Given that censuses were conducted in order to levy taxes, it’s understandable there was little
motivation to record deaths.
8
More precisely, the data evince multiple temporary dips in life expectancy, and in fact suggest there may
have been two dips prior to industrialization. The first occurs between 7,000BC and 3,000 BC, and thesecond between 650 BC and AD 1500. The first dip is very plausible and coincides with the transition toagriculture during the Neolithic Revolution, when multiple sources attest an increase in mortality.
However, the second dip is uncertain and might be a spurious result from using different sources, which
use different estimation techniques for the pre-historical (Galor & Moav) and historical (Scheidel) periods.
It is still plausible that early urbanisation is responsible for the second dip.
In the end, historical populations are small enough, and the range of plausible values for life expectancy
narrow enough, that different plausible estimates don’t change our main conclusions.
9
Extrapolating from at 15 expectancy of 54.1 and 60% survival to 15 (Kaplan et al., 2000, p. 158).
Assuming who died before 15 lived 7.5 years (if 0, we get 32.5). Another source by the same main author
has 31 years life expectancy at birth (Gurven & Kaplan, 2007). A more recent source has an average life
expectancy of 27.5 and median of only 25.9 (Page & French, 2020, Tab. S1). However, it includes data on
several groups that are sedentary or have unusually low fertility that could not have been sustained
long-term (see comments here).

1900

32

Riley (2005)

1950

48

Riley (2005)

1973

60

Riley (2005)

2001

66.6

Riley (2005)

2016

72

WHO (2020)

2019

73.4

WHO (2020)

2020

73.4

WHO (2020)

Table 2: Life expectancy timeline

By adding these life expectancies to the population data, we can derive life-years lived
in each of the Population Research Bureau’s 12 periods, the total life-years ever lived,
and how much of the total was lived by the end of each period.10

PeriodBirthswithinperiod (bi)190,000-50,000BC

7.9

35.5

278.6

278.6

7.60%

50,000-8,000BC

1.1

34.6

39.3

317.9

8.67%

8,000 BC-1AD

46

26.3

1,200

1,529

41.69%

0.1%

1-1200

26.6

28.0

744.6

2,273

62.00%

0.5%

1200-1650

12.8

29.8

381.3

2,654

72.40%

0.6%

1650-1750

3.2

34.5

109.4

2,764

75.38%

0.5%

1750-1850

4

31.0

125.4

2,889

78.81%

0.8%

1850-1900

2.9

30.3

87.7

2,977

81.20%

1.2%

1900-1950

3.4

40.0

135.6

3,113

84.90%

1.6%

1950-1995

5.4

58.5

317.7

3,430

93.56%

2.3%

1995-2011

2.1

67.1

143

3,573

97.46%

5.1%

2011-2020

1.3

71.4

93

3,666

100.00%

6.1%

Life Life-years inExp. period (bi)

2020
Total
10

Life-years till
period's end(bi)Life-years
lived

Ever
born at
start

6.7%
116.8

3,666

The years in life expectancy data do not neatly correspond to the years in PRB’s periods. Therefore, we
calculated life expectancy within PRB’s periods using two simplifying assumptions. If the life expectancy
data was for a period, we used a simple weighted average, adding up periods’ life expectancy weighted
by how many years were within a given PRB’s period. If the life expectancy data was for point years, we
assumed that the life expectancy between point years is the average of both points' life expectancy. We
used the method for periods over that if necessary.

Life-years by the currently alive(1950-)

553.7

15%

Table 3: Life-years ever lived (see detailed spreadsheet here)

We get a total tally of 3.7 trillion life-years, with people currently alive expected tocontribute half a trillion. That means that about one of every seven life-years everexperienced has, or will be, experienced by someone currently alive. Over a third ofexisting life-years happened after 1200, and about one in five after 1650.
If we use the implausibly low life expectancy implied by the PRB model, life-years
become even more concentrated in the present, with 22% belonging to those currently
alive.11 The total life-years ever lived total 2.5 trillion. Using a lower-end estimate forhunter-gatherers’ life expectancy of 26 years decreases life-years from early periods bya couple percentage points, with everything else remaining roughly the same.12

How many life-years have we spent in different activities?
To get a sense of how humans have spent these life-years, we also made roughestimates of the amount of time people have spent doing various activities such as
giving birth, having sex, drinking coffee and being enslaved.
Giving birth
Currently, labour lasts about 6 hours for hospital births and 10.6 hours for at-homebirths (Laughon et al., 2012; Kilpatrick & Garrison, 2018; Hildingsson et al., 2015). Let
us assume, for simplicity, that all births prior to 1950 took place at home and all after
were at a hospital. By multiplying all post-1950 births in our table by 6 hours and all
pre-1950 births by 10.6 hours, we get a total of 1.2 trillion hours or 137 million years
spent in labour. However, 10.6 hours is the length of time spent in labour during at homebirths for modern, low-risk pregnancies in wealthy countries. It likely underestimates theduration of labour in other conditions. As an upper bound, we can assume active labourof home births lasted five times longer pre-1950, resulting in a total of 659 million
life-years spent in active labour from the dawn of humanity until now. That represents
between 0.004% and 0.018% of all human life-years.
Having sex

11
12

See footnote 5.
See footnote 8. As mentioned, this life expectancy figure seems likely wrong.

To estimate the total amount of time humans have spent having sex, we combine ourdata on the number of historical births with an estimate of how many times people havesex for each birth and data on how long intercourse usually lasts.13
From our earlier estimates, we know that about 117 billion humans have been born.
How many intimate acts occurred for each birth? Well, 89% of US couples using nocontraception become pregnant within one year (Trussell, 2011, p. 398). Let’s assume
that, of the remaining 11%, half take two years to become pregnant and half are
infertile.
One study found that married couples at the age of fertility have sex about 2.5 times per
week (130 per year) on average (Call et al., 1995, p. 646). This would imply that the
89% of couples who get pregnant within one year have sex about 130 times perpregnancy. The 5.5% who, we assumed, take two years to get pregnant have sex 260
times per pregnancy. And we have to account for the remaining 5.5% of couples, whohave sex about 130 times per year without getting pregnant. This averages out to about
142 intercourses for each pregnancy.
Unfortunately, around 35% of pregnancies result in miscarriage (McNair & Altman,
2012, p. 438), so this would correspond to about 220 times having sex for each live birth(this ignores stillbirths, which occur about 1.39% of the time (Danzhen et al., 2020, p.
11)). Finally, we can multiply this by the average duration of sexual intercourse: about
7.4 minutes, according to a 2004 study (Miller & Byers, 2004, p. 304). This adds up to
about 1600 minutes of sex for each live birth. Assuming two partners are involved, thisis 3200 minutes of experience. So, with 117 billion live births so far, we can estimatethat people have spent about 374 trillion minutes, or 700 million years, having sex.
That’s about 0.01% of all 3.7 trillion life-years so far.
Drinking coffee
Coffee consumption only became widespread around the middle of the 19th century(Wikipedia, 2022). In 2019, 168 million 60-kg bags of coffee were consumed by the 7.69
billion people alive (International Coffee Organization, 2020, p. 7). That works out to
1.31kg consumed per life-year. People probably drink more coffee today than they did in
the past. So let’s assume, quite arbitrarily, that the average yearly coffee consumption
since 1850 is half of what it was in 2019. A cup of coffee uses about 10g of beans(Giovanisci, 2021). And let’s say it takes about 30 minutes to drink. That gives us about
2000 minutes drinking coffee per life-year. According to our table above, about 780
13This is probably not what you expected to read about when you bought this book. All we can say is thatit’s not what we expected to be researching when we were asked to help research it!

billion life-years have been lived since 1850. So, a rough estimate for the amount ofyears spent drinking coffee is 3 billion life-years, which is about 0.8% of all life-years
ever lived.
I note that that’s much more time than humans have spent having sex. People really likedrinking coffee!
Time spent enslavedThis may be the trickiest estimate of all. We know that the prevalence of slavery hasvaried widely throughout history and across societies, but we don’t know much about
where it was high and where it was low. So any estimate of the average prevalence ofslavery over all of human history is going to be very uncertain.
Our broad assumptions, which seem relatively solid, are as follows. Slavery only started
with agriculture, likely peaked in the 18th century, and drastically declined in the 20th
century. Nearly two-thirds of the life-years ever lived were from before the 13th century,
so most of the life-years spent enslaved occurred before 1200 AD. However, over
one-fifth of life-years enslaved are from the 17-19th centuries even though less than
one-tenth of all life-years are from this period. This shows how widespread slavery wasin the early modern era.
The sources and calculations we used to get a rough estimate are described in note 6
to the Introduction of What We Owe The Future. Using that data, we calculate that 850
million life-years have been lived as a slave; nearly a quarter of all life-years.14 This
number is different than what appears in the text as we have continued to work on thisquestion long after the book was published.

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results of these estimations and sources used. This is a work in progress, email me for additional details
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