Dr. Phil Plait has them all figured out. But you know, this could be made into a flowchart, with people being born flowing into the right circle, a trickle of people flowing from the right to the left, and because of the death rate, there would be outflow from both circles. Read plenty more geeky jokes in the comments of the post at Bad Astronomy. Link
I believe that was the joke.
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As mentioned in the comments, this is not even a valid Venn diagram.
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