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AnthonyC comments on 2012 Survey Results - Less Wrong
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"Siblings and Older siblings have correlation 0.48 (61 degrees), which isn't too surprising , but makes me wonder: do we expect this correlation to be 0.5 in general?"
In every sibling relationship, there is one older and one younger sibling, so half of all siblings are older siblings - a line with slope 0.5.
A correlation coefficient is not a slope. (A slope changes if you multiply one of the variables by a constant, whereas a correlation doesn't.)
EDIT: I think the slope is the correlation times the standard deviation of y divided by the standard deviation of x.
How would twins reply to these questions?
All the twins I've known have regarded the first-born as "older" (and one has been first-born).
In Italy traditionally it's the other way round. (Don't ask me why.)
So apparently, according to tradition the twin that is conceived first is believed to be born last, and there are folk explanations like "The first conceived attaches to the uterus first, so is more firmly stuck". And so the later-born is considered oldest due to having been conceived first, even though that is not even a thing that can happen in the case of identical twins.
Still tracking down a decent history of the phenomenon, but it's an interesting start.
That sounds very counterintuitive. Do you have a citation? I can't find information online.
It's something I heard from my uncles (a pair of twins) and their mother. I can find stuff online, but it's in Italian. Googling for gemello più vecchio (Italian for ‘older twin’) does turn up relevant stuff, so it's not something my grandma made up. EDIT: apparently there was a myth about the first to be conceived is the last to be born (which for identical twins is Not Even Wrong). Someone answered on Google Answers, "if you went in a phone booth with a friend, the first of you to get in would be the last to come out, wouldn't she?"
My Italian should be good enough for that. Grazie!