When you tell people that you're born on Christmas, they typically react in one of two ways. It's either:
or:
And then there's the internal debate over whether it's worth it to explain that you're Jewish and don't celebrate Christmas.
But you decide to. And there is usually a pretty consistent reaction:
But wait. It gets better. (Or weirder, depending on your outlook.) Your younger brother was also born on Christmas -- the very same day, four years later.
As a Jewish Christmas baby who shares a birthday with her brother, I've experienced this series of events more than one can count. It turns out, being born on a holiday is rare, possibly because mothers schedule cesarean sections to avoid it. According to research published in the New York Times, between 1969 to 1988, the birthrate was...
About 20% lower on weekends.
15% to 20% lower on New Year’s Day and July 4.
About 5% lower on Leap Day, April Fools’ Day and Halloween.
And about 5% higher on Valentine’s Day.
But of all the holidays, Christmas is one of the rarest days to be born. Compared with an average day, there is a 20% diminished likelihood of being born on Christmas, according to research from the same Times study.
Now, let's do the math.
1. The odds of being born on Christmas
[1/365 (the chance of being born on any day)] - [1/365 x 0.2 (the 20% reduction rate)] = 0.0022
2. The odds of sharing a Christmas birthday with my brother
First, some background. The odds of two people sharing a birthday is 1/365, because in this scenario, we don't care what the birthdate actually is. We're just talking about the likelihood of sharing the same day, which acts like one event (1/365) rather than two independent events.
That means, though, we wouldn't be calculating the odds that my brother and I were both born on Dec. 25. We'd just be talking about the odds of having the same birthday -- any random day.
But! We're talking about the odds that my brother and I have the same birthday on a very specific date, which acts as two independent events. So the odds are decreased; it's less likely.
If you assume there's an equal chance of being born on any given day (which we know isn't true -- we'll get to that next), this is the equation for two people having a birthday on the same defined date:
[1/365] x [1/365] = 0.0000075
Then remember that there's a reduced probability of being born on Christmas. So let's replace the 1/365 (or 0.0027) in the equation above with 0.0022, which is the solution from the first step -- the probability of being born on Christmas.
0.0022 x 0.0022 = 0.0000048
So the probability of two people being born on Christmas is 0.0000048 -- or 1 in 208,333.
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The moral of the story is: You just can't plan these things. And if you can -- well, I don't want to think about that.
