# Assuming the average length of time between familial generations is approximately 25 years, how many people had to have had sex since the year 1 A.D. in order for you to be here to read this question?

Asked by rojo (24176) October 19th, 2016

From what I found the average amount of time between generations in developed countries is in the high 20’s to low 30’sand in less developed countries it is in the high teens to low 20’s which is why I assumed 25 years figuring that no matter how developed they were in 1 A.D. they would be considered third-world by todays standards.

And assuming no parent went on to have sex with their offspring.

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Well the math is pretty easy, my year of birth divided by 25 years works out to 78 generations times two people, so 156 people.

zenvelo (36387)

“Had to” and “average” don’t go together to provide a logical question.

You also seem to be assuming I am not very very old.

Zaku (26407)

ZEPHYRA (21639)

Assuming 25 years for a generation is very unrealistic. For most of history until quite recently, people we would think of as boys and girls were getting married (so to speak, nobody cared if peasants had a formal wedding) at the age of around 14 or 15, maybe 16. They immediately got busy having children because it was a lucky man or woman who made it very much past the age of 30. Margery Kempe, an English woman born in the 14th century and who lived until 1438 (possibly) beat the odds, and lived to the age of 65. She and her husband had 14 children. (We know this because late in life she made the pilgrimage to Jerusalem, and dictated her life’s story when she got back to England.) That sort of thing killed most women, however.

Short life spans have been common until quite recently in human history. Emma Lazarus, who wrote the poem on the Statue of Liberty, died at the age of 38. William the Bastard of Normandy, who became William the Conqueror, was just seven years old when his father died. It took him until the age of 32 to fight his way into power in Normandy. Edward III Plantagenet was just 15 when his father, Edward II, was horribly murdered by his wife and her lover. He got an early education in politics and warfare in the process of claiming his throne. Edward III’s son, Edward of Woodstock,whose oldest son died when his second son was just three years of age, died before his father. So when his father died, his son Richard became king at ten years of age. Louis XIV of France was all of five years old when he became the king of France in 1643. He had four older siblings, all of whom died in infancy or early childhood.

As one might well imagine, life was considerably worse for peasants,who were the majority of the population. I’d say that on average, you should calculate a generation over the last two thousand years as being 16 years.

Setanta (1680)

You can have sex without it resulting in pregnancy and birth.

Multiply the birth number by a factor of 600.

elbanditoroso (30020)

Historians, physicians, sociologists and anthropologists all count a generation to be 20 years. I’ll go with that. It took 98 generations for me to get here from 1 ce. and 15,000 generations since the advent of the Homo Sapiens.

@zenvelo is that correct? it took two people to have you, four people to have your parents, eight people to have your grandparents, etc.. That would be 14 people just to get back to your great-grandparents.

@Zaku not so. People “had to” have sex and successfully conceive in order for you to be here and your ancestors to have been there. Some of your ancestors had children when they were “approximately” 20 years old while some waited until they were in their 30’s before they had offspring. For the purpose of the question I had to select a point somewhere in the middle. Also, your age does not enter into it unless you want to use your birth as the starting point and work backwards.

@ZEPHYRA Correct, nobody asked them to but they did anyway, how many?

@Setanta 25 years may be high, I think it may be, but I know from looking into my own background all through the early 1900’s and beginning back to 1765 many of my ancestors were in their mid-twenties before they began having children. They were, for the most part, farmers, farm hands, publicans and the like living in small rural villages in England and Ireland. Yes, there were exceptions, earlier births, later births but for the most part it was mid 20’s. In this instance we are looking at a time frame that encompasses the industrial revolution. Even my wife and I were 27 before we had kids. Another thing to consider is that while a couple may start having children at an early age, they have about a 10 – 15 year breeding age span. Finally, not all of a generation survived so I don’t believe a 25 year average is that unreasonable.
In a hundred years the difference between a 15 year and 25 year average generational span is the about 2.5 generation so yes, it makes a difference, a big difference; 80+ generations vs 135+ generations in the time frame I am asking about. This is why I picked a average and asked for an answer for that number.

@elbanditoroso yes your ancestors had sex without resulting pregnancy but these times a don’t matter since they did not directly relate to your existence. And, if we get down to it, there are those that did conceive whose offspring did not live to reproduce themselves but again, these people are irrelevant to your existence.

@Espiritus_Corvus I have usually assumed 18 years myself. So what is the number of people based on 20 years and what is the difference in the number of people having sex that resulted in conception between the 81 generations based on 25 years and 98 generations at 20 years?

rojo (24176)

This is not life or death data to me. So, I’ve never bothered to find out why science decided upon 20 years per generation. But I do know that if you were to write a paper on the subject, that is the accepted formula and 20 years is universally accepted worldwide as a generation. Makes my life easy, because if I were to thoroughly investigate why all the professions use the parameters they use, I wouldn’t have any time to sail, fuck, ride my horse, play with my dogs or go to the grocery store.

@elbanditoroso it took two people to have you, four people to have your parents, eight people to have your grandparents, etc.. That would be 14 people just to get back to your great-grandparents.

By this calculation, and assuming that none of your ancestors were related (which is certainly false), the answer is approximately 2 power 90 = 1,237,940,039,285,380,274,899,124,224, i.e. more people than ever lived on this planet.

olivier5 (3084)

@rojo …it took two people to have you, four people to have your parents, eight people to have your grandparents, etc.. That would be 14 people just to get back to your great-grandparents

Perfect example of fallacious mathematical argument. Your logic would mean that the Earth is now at its smallest population ever.

It only takes one coupling to make a generation, the parents.

My mother is an only child, but is the grandmother of 8. How does she become 14 people?

zenvelo (36387)

@olivier5 I got 80 generations and 1,208,925,819,614,630,000,000,000 inividuals and yeah I suppose we could use a percentage to account for all the sibling matings but still, more people than ever lived on this planet. Where is the error?

@zenvelo but it takes two couplings to make the parents and four couplings to make the grandparents

mom = mom grandma(3) + momgrandad(4)

Since we receive DNA from both parties in a mating, the DNA that makes you you is from 14 people and 28 individuals one generation further up.

And, the issue that couples have multiple children that reach breeding age themselves explains why the earth population is not the smallest. but except for cases of incest as @olivier5 mentions, which does change the individual count but does not affect how many sexual interactions had to occur for you to be here today. Perhaps this is the answer or a part of the answer to the the question of how can we possibly have that many ancestors in just 80 generations.

Where is the fallacy?

rojo (24176)

I think it is just that many of our ancestors are related to the nth degree. It doesn’t need to be incest. E.g if your parents have any common ancestors, those are counted multiple times in the 2 power X computation.

olivier5 (3084)

I was going to do the math and then realized your question had a very specific caveat: ”...in order for you to be here to read this question?” That means I need to include the families of Ben and Andrew. The programmers who worked on the site. The geniuses who developed the transistor and integrated circuits, The coders who thought of ASCII and HTML, BAL, FORTRAN, PL/1, C, C+ C++, LISP, ALGOL and other. The engineers who developed electricity and power distribution. The metallurgists who designed the keyboard and wall sockets, wires and plugs, The plastics and material scientists and engineers who developed the parts that go in my laptop, The people who assembled the parts, The miners who pulled the ore out of the ground. Marconi, and Faraday and Couloub and a myriad of others who played parts in the development fo radio and wifi .
And, of course, Al Gore.

So the answer is: a whole lot – way more than just the people who carried the same mitochondrial DNA as me..

LuckyGuy (39463)

@rojo You don’t seem to have understood what I meant. I was quibbling about the wording “had to”.

If you say “had to” and “average” as you did, the question is not strictly clear. “Had to” signifies necessity. Since you’re asking about one person, me in fact, “average” doesn’t apply in a “had to” kind of way, as the actual people and dates don’t “have to” have been anything. All my particular ancestors could have had children at around 40 years old, or around 16 years old. I could be 15 years old, or 115 years old, or 1115 years old, or 2115 years old, in which case the answer would be zero.

Sorry, I was abused by teachers judging me based on my refusal to submit to inaccurately-worded math story problems as a child.

Zaku (26407)

@Zaku I did understand but I did ask that we “assume” that it was 25 years.

rojo (24176)

@rojo Ok, but I’d just think it should be worded “average… in your line of descent” and “would have” etc

Zaku (26407)

If we make it ”...how many people would have to had to have had sex since the year 1 A.D. in order for you to be here.” will that help?

rojo (24176)

For the last 2000 years, most reproductive couples started in their mid-teens. Talking about what a family did in the 18th, 19th and 20th centuries means your family was affluent enough that they kept such records, which means they enjoyed more wealth and privilege than the vast majority of the population. Poor people don’t keep such records, and even as recently as the early 20th century in industrial England, most reproductive couples were not married, and there usually was not a record of their lives, other than gross population figures. See The Classic Slum: Salford Life in the First Quarter of the Century, Robert Roberts, University of Manchester Press, 1971.

Setanta (1680)

@rojo No, the problem is the way you leapt from average to personal and didn’t account for the person’s own age. I’d word it more like:

“Assuming the average length of time between familial generations in a 25-year-old person’s line of descent is approximately 25 years, how many people had to have had sex since the year 1 A.D. in order for that person to be here to read this question?”

“Assuming the average length of time between familial generations in your line of descent is approximately 25 years, how many people had to have had sex in the 2000 years before you were born in order for you to be here to read this question?”

Zaku (26407)

or