# Probability after 2000 years

• Jan 30th 2013, 03:38 PM
Sabinak
Probability after 2000 years
Hi! I'm new to this forum but I thought that I should give it a shot to post a question.

So I got this odd question about the possibility to be a relative from a direct line with a person who lived about 2000 years ago (like jesus or something). By direct line I mean going directly down from parent to child and so on, so no cousins or anything is involved. Now to be a little bit more specific so if for instance that every couple would to have 2 children remaining the population at an even level. So i was thinking 2 to the power of 80 (estimating that it would be 80 generations from the year 0 to now). Hear is were I got just too confused and thought to seek help. I got the number like 1.2 trillion or something. But looking at prb.org there have been roughly 100 billion people who has walked the earth. Yeah, here i just didn't know what to do. Anyone care to help? And yes I do know that the numbers I've taken are not close to the actual reality. And I don't consider location of ones own relatives important. Just like in general what would be the probability of one being related in a direct line to a person born 2000 years ago if every couple have 2 children? Any response would be appreciated :) Thanks in advance!
• Jan 30th 2013, 06:18 PM
chiro
Re: Probability after 2000 years
Hey Sabinak.

For this question you will have to make a few assumptions: are the probability of people meeting up (and then getting married/having a kid) the same for all people?

I think that this problem is a lot more complicated than you think because the way that people meet and marry is not as random as the above suggestion would have.

People tend to mingle in their own social circles which makes this problem very difficult since you now have to classify people according to their "circle" and do so in a specific way.

All these sort of things need to be factored into the model to make it even half useful as a prediction tool.

How badly do you need this answered?
• Jan 31st 2013, 10:16 AM
Sabinak
Re: Probability after 2000 years
Hey chiro!

I'm well aware that there is gonna be a lot of assumptions, and that is kinda what im dealing with as well. Let's say that every man and woman would to have 2 children, a boy and a girl. and they would have respectively a boy and a girl with their partner (i know this does not represent the world). There wouldn't be any child mortality or any death until you would to have 2 children. There probability of meeting someone as u mentioned would be the same. So i am really looking for a quite simple answer that does not depict the world.

How badly do i need this answer? Well it's not like an academic essay im writing here. It's a brainteaser from my dad... so quite badly! :)
• Jan 31st 2013, 04:06 PM
chiro
Re: Probability after 2000 years
To be honest, I don't know how many assumptions and variables you will need for this.

The first thing you will have to do is to decide what attributes are the most important for the model. Once you do this, we can work through the model gradually.

The first thing you might want to do is to look at how you can model the proportion of people that will be "likely" candidates for meeting a partner to have children. For this you will need to like at .the demographics and come up with some "rough" probability.

Once you do this, the rest involves applying this multiple times, one per generation.