You might be my sixth cousin

I'm a bit behind on my Guardian Science Weekly Podcasts, but I learned this evening that you might well be my sixth cousin (according to Steve Jones). Yes, you - If you're British that is (I'm probably not so closely related to you otherwise). Fascinating stuff.

Incest means having sex with a relative - and we all indulge in it, whether we realise or not. On average, two randomly chosen British people are sixth cousins, which means that they share an ancestor who lived in the year of publication of The Origin of Species (1859).

Update: this can't be correct, surely. Sixth cousins share their great-great-great-great-great grandparents, and I have 128 of those. In order for it to be likely that you are my sixth cousin, these 128 together must have around 60 million descendants. But this means that at each generation there must have been around seven children born, every one of whom would then go on to have another seven children, and so on. But that's surely absurd, since the UK population in 1851 was around 20 million, so there hasn't really been a huge amount of growth. Assuming two fertile children per generation, the probability that you are my sixth cousin is around one in 7000. Assuming four fertile children, it's around one in 60.

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