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And no, you are not like Charlemagne.
February 15, 2010 11:45 AM   Subscribe

For the Genealogists, armchair or otherwise: You are a Man or Woman living in the 1500s. All things being average, how many descendants would you have today?

Thanks for the help...
posted by ryecatcher to Human Relations (15 answers total) 7 users marked this as a favorite
I've seen estimates of the world populate in 1500 being 450 million people. There are almost 7 billion now. So, all things being average: 15.5 descendants. Maybe?
posted by roue at 11:54 AM on February 15, 2010

Roue: Shouldn't there be a lot of inbreeding though?
posted by Jazzwick at 12:01 PM on February 15, 2010

Wouldn't the odds of having living descendants be really small? I would bet that if I'm a man or a woman of the 1500's, I've survived childhood (which is a good start), but that I've got to look out for disease, war, famine, etc., and that any children I have must do the same. I bet there are a few men, who having fathered a lot of children, have good odds of living descendants. Women less so, since they must invest so much time and energy in each child, and also have the associated risks of childbirth.

I would guess (and this is a total guess) that this is one of those situations where the average number of descendants, and the mean number of descendants, would be very different. I would also guess that men, since they can sustain a reproductive strategy where they father a lot of children, would have a higher average than females (who, since they have the associated risks of childbirth and must devote large amounts of resources to raising their young, are, evolutionarily, a much more conservative reproductive investment.)

I'm pulling some of my reasoning from Dawkin's "The Selfish Gene", which has a chapter or two on reproductive strategies, if you're interested.
posted by HabeasCorpus at 12:07 PM on February 15, 2010 [1 favorite]

Straight division doesn't hack it, either. Each person has many, many ancestors, so the number of descendants per ancestor could be huge.

It would depend on average number of children per family, carried forth in burgeoning combinatoric fashion through countable generations. Also, the length of a generation would probably be shorter, and the number of viable offspring potentially higher (or not), for earlier historical periods.
posted by amtho at 12:08 PM on February 15, 2010 [1 favorite]

Roue's estimate seems as good a place as any to start. There will be a lot of variance, though. This is from a few hundred years earlier, but it seems Genghis Khan has 16 million descendants.
posted by echo target at 12:09 PM on February 15, 2010

This article goes over some of the math and suggests you need the following information to calculate the average number of descendants from someone who lived n generations ago:

C, number of individuals in Current generation
A, number of individuals in Ancestral generation
n, number of generations in between
c, a correction factor

and would put it into this formula:

D = C * 2n / A * c
posted by jedicus at 12:13 PM on February 15, 2010 [4 favorites]

I've seen estimates of the world populate in 1500 being 450 million people. There are almost 7 billion now. So, all things being average: 15.5 descendants. Maybe?

Way more than that. My grandfather, who was born in the 20th century, has 39 (living) descendants. Go back another generation, and his father has something in the neighborhood of 75 descendants. I can't really give you an estimate for my great-great grandfather, because that's not the the kind of genealogy that I do. But I can tell you, even for your average Joe Farmer, not the Genghis Khans of the world, the number is going to be big.

If I had to guess, jelly-beans-in-a-jar style, I'd guess something like 75,000.
posted by donajo at 12:59 PM on February 15, 2010 [1 favorite]

The problem is the most successful breeders back then would have out bred the less successful. A lot of lines just die out. If you buy into the hype, I'm "descended" from the High Kings of Ireland... 9 million other people and I, or so. I'm not sure how you'd work that sort of thing into an equation. Obviously a lot of folks weren't outbred, I'm just saying it isn't a strict reduction. It's certainly been looked at. You might have some success looking at the number of European noble families and extrapolating from there. A lot have gone extinct.
posted by jwells at 1:22 PM on February 15, 2010

I've seen estimates of the world populate in 1500 being 450 million people. There are almost 7 billion now. So, all things being average: 15.5 descendants. Maybe?

That math only works if people can't share descendants.

I would be pretty surprised if the answer were under a thousand.
posted by dfan at 1:34 PM on February 15, 2010

I've seen estimates of the world populate in 1500 being 450 million people. There are almost 7 billion now. So, all things being average: 15.5 descendants. Maybe?

If everyone in every generation marries and has two kids exactly, then the population stays stable. Each person, then has 2^n descendants after n generations. So, after 15 generations, thats 32k descendants. Since the population also went up by a factor of 15 then each generation had incrementally more than 2 descendants. In this case, 2.4.

So, that brings us to: The average person in 1500 has 500k living descendants.

However, that model has one serious flaw. In each generation, many people end up with no descendants which is something you cant recover from.

So, if your question is: Of the people with living descendants, how many descendants do they have? The answer is probably closer to half a billion
(further reading)
(simple mathematical model)
posted by vacapinta at 1:37 PM on February 15, 2010 [11 favorites]

This question also a raises paradox: as vacapinta points out, a person who lived in the 1500s is either likely to have some large number of living descendants - or none.

A line either goes extinct, or tends to propagate through the population.

The extreme case of this is described in this paper (Warning: pdf) On the Common Ancestors of All Living Humans, which postulates that ALL of us had a common ancestor (possibly) as recently as only 2,000 years ago.

The logic is similar for "500 years ago".
posted by AsYouKnow Bob at 4:32 PM on February 15, 2010

(Whoops - sorry, vacapinta, I should have followed your links first.)
posted by AsYouKnow Bob at 4:42 PM on February 15, 2010

And no, you are not like Charlemagne.

Ah, but probably, you are.

The math is covered by others above. For a more comprehensive view (including math), I have long enjoyed Mark Humphrys' take on this:

1. The MRCA [most recent common ancestor] of the West is in historical times, quite possibly as recent as 1000 AD.

2. Quite likely everyone in the West descends from Charlemagne, c. 800 AD.

We pick him as an example because he is the proven ancestor of some people alive today (for example, he is a proven ancestor of my children). Hence probably the ancestor of all people alive today. By the same reasoning, as well as from Continental/pre-Norman figures like Charlemagne, quite likely everyone in the West descends from figures like:

* The English/Saxon/pre-Royal Cerdic, c. 500 AD.
* The Irish/Celtic Niall of the Nine Hostages, c. 450 AD.

Genealogy is usually about the fishing expedition for famous ancestors or cousins, but Humphrys shows that it's more likely than not that you'll find them. The only question is how much research you need to do to get there. It's been found, for instance, that Barack Obama and Dick Cheney are tenth cousins. Indeed, probably all US Presidents are related not much more distantly than that. Essentially, unless you are a child of recent immigrants, you probably can trace your lineage back to the earliest colonists and thus back again to any number of other Americans.

Fascinatingly, we may all -- 90% of us, anyway -- be related to Confucius and Muhammad as well:

Most people are probably descended from the Prophet Muhammad anyway

* If we accept the argument above that, even if the exact lines aren't known, most of the prominent families of Islam would have soon ended up descended from the Prophet, then most of modern Islam would descend from the Prophet.
* And only a tiny amount of intermarriage with Christians and Jews in medieval Spain would yield the result that, even if all the descents above are false, quite likely almost every Muslim, almost every Jew, and almost everyone in the West today is descended from the Prophet.

* Simply put, if anyone at Muhammad's time left a reasonable number of descendants, they are probably the ancestor of most of the West today, even if no specific descent can be proved. See full discussion on Common ancestors of all humans.

So, while half a billion may be some median number, I'd say that thinking in terms of actual descendant count are less meaningful than the realization that any surviving line from the 1500s is highly likely to be ubiquitous, at least if you mean someone in Europe in the 1500s, and the population of the West today.
posted by dhartung at 5:21 PM on February 15, 2010 [5 favorites]

This is all fascinating stuff. Thanks everyone for the help.
posted by ryecatcher at 5:55 PM on February 15, 2010

The family trees I've worked on show that most people do not leave descendants. It seems counter-intuitive, but that's because when we think of people in the past we tend to recall either our ancestors or wealthy and famous people. Lots of people back then couldn't afford to marry or raise kids. Infant mortality was high. Childhood mortality was high. I'd estimate that fewer than fifty percent of people actually had children, and I've seen many families struck by an illness or something that wiped most of them out.

The family trees I've worked on are characterised by a few families with many children (say, ten or more) propagating the bulk of the population a couple of generations later. Families with two or three kids very often die out. So the mean number of descendants is like taking the mean wealth of Buckingham Palace: it averages out the wealth of the Queen and the wealth of her cleaning staff to give a very misleading figure.
posted by Joe in Australia at 3:03 AM on February 16, 2010

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