If you kill a person 100 years ago, how many people would cease to exist by present day?

None. Other children would be born to other parents. Nature would replenish itself when it is allowed to.

I don’t mean in the grand scheme of things. I mean if I killed one person historically, how many of the people that were born because of that person’s existence would now be gone?

The number fluctuates between 0 and thousands, because it depends on how many spawns that person has and how many spawns those spawns will spawn and so forth.
Some never spawn a spawn, some do nothing else but spawning spawns.

It depends how young these people have kids. If they have a baby at 16, and then their kid at 16 has a baby, there is a grandma at the age of 32. Some people have their first child at 32. There is more to consider than just how many kids each generation has.

Interesting question, @Zone36. I think you’re going to have to specify whether you want an answer in theoretical or practical terms.

In theoretical terms, we would be talking average number of offspring to survive until reproductive age, average age of childbearing, etc., over the past four to six generations. Social customs have a bearing on it, too, so it matters which four to six generations. Age of marriage, size of families, etc., in society at large have changed a lot in the past hundred years. And—what part of the world are you talking about? the U.S., China, India, Europe? That makes a difference too.

In practical terms, of course, it’s all going to depend on the person you kill. After all, many people never become parents at all. If you killed one of them, it would make no difference. And some are prolific reproducers. If you killed one who bore or fathered 20 children rather than 2, and whose offspring would have done the same, it could make a noticeable difference in the population of a sizeable city. Five generations of ten children each is 100,000.

I want a hypothetical answer. Just what you might assume in general. I’m having trouble even imagining how much they would branch out in 100 years IF each child was guaranteed to have 2 children of their own. I’m not looking at how this question can’t be answered. Just how you think it would turn out.

I just want to leave the question at no more than 2 children from each previous generation. I want to assume that each generation always 1 if not both children reproduce. So no ends of line in any branches. We can even say every other year it goes 1 and the next 2. So imagine that for 100 years and then for the longer spans of time as well.

I think you mean to ask.. on average how many people would not be born.
at present the average american family is (i think) at 2.5 children. So, you could do the math from there.. but the average family size would be different 100 or 200 or 500 years ago I’m sure.

so theoretically, the generations are born 1900, 1925, 1950, 1975, 2000. so 1 +2 +4+8+16= 30 people plus the one you “killed”.

Thanks Zenvelo. That’s a good starting point.

Another way you could ask/figure this out would be to ask: If your great great grandparent died before having kids.. how many of your family would never have been born.

@Zone36: Using your simplification above that each person is guaranteed 2 children, you can say that the number of descendants is Σ from k=0 to n of 2^k = 2^(n + 1) – 1. Since a generation is roughly 25 years (source), n is about 4. This means that killing someone born 100 years ago (with no children yet), assuming the family tree “regularly” forks and each person is guaranteed two children, will cause 31 people to not exist.

If the average age of having children is 25, then couple one has 2 kids, at 25 they have 4 kids, at 25 they have 8 kids, and at 25 they have 16 kids. So 2+4+8+16=30 not including the guy killed. If the average age they start having kids is 20, you add another generation which gives you 30+32=62 in 100 years, at 200 years the numbers increase fast.

@Vortico That’s why I wanted to add that every couple years only one child had kids. So it might be 26.

With such a basic equation that one person seems to have so little impact over 100 years.

Well to go back in time 100 years, the year would be the closing of 1910. Suppose you killed my grandfather who was born in 1893. He had four children, including my father. His four children produced 14 children, including me. These 14 children have produced 11 grandchildren for my father and his siblings. These grandchildren so far have birthed 14 children.

If you killed my grandfather 100 years ago, you would stopped the birth of 43 of his ancestors to date.

I would have to say it wouldn’t make the least bit of difference. If you killed my Grandpa, then Grandma would have married someone else, had three kids, and they would still have expanded from there. In fact, it might have resulted in far more children, because she might have loved him more than she loved grandpa, and had six – eight children instead of just my Mom, Aunt and Uncle.

The story is that instead of getting a divorce after Mom was born, they just lived in the same house as strangers for 20 years until she fell in love with someone else, and finally got the divorce.

…of 43 of his descendants, you mean, @bkcunningham, not of his ancestors.

@Jeruba yes, that is what I should have written. Thank you.

There is a Jewish saying (which I couldn’t find) about when you kill someone you kill many universes (or many worlds) or something like that. Maybe someone here knows the saying. The gist is, that the death of one person is a powerful thing for many generations, for ever.

This reminds me of a story I read about a man going on a time machine in which he was instructed to stay on the path and never disrupt or touch anything around him while he was in the past. He accidentally veered off the path and stepped on a twig that ended up killing a butterfly, but that was it. He was sure that wouldn’t cause any harm, but when he returned to present-time the whole world was different, down to the spelling of the English language.

They also say that the flutter of a butterfly’s wing can cause a tsunami halfway across the world.

So, who knows? No one, that’s who.

I’d also like to point out that the year 1900 wasn’t 100 years ago. :)

@lbwhite89, you’re alluding, I think, to the famous short story by Ray Bradbury called “The Sound of Thunder,” described here. The story was published in 1952. Interestingly, articles like this one about “the butterfly effect” don’t seem to make a link between the popular name for this phenomenon and the Bradbury story.

Kill one man, and you are a murderer. Kill millions of men, and you are a conqueror. Kill them all, and you are a god. Jean Rostand, Thoughts of a Biologist (1939)

This is an interesting probability problem. In an ideal situation after just 700 years of 28 generations (each 25 years long) generating 2 offspring each we are looking at 67,108,864 possible people.

Since the number of unborn people is infinite (I don’t have the space to list all of them here, but Henry J. Thorncake, for example, is among them), any finite increase in that number leaves it unchanged.