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Comment by bsimpson | Hacker News Reader

It's interesting how uniform the birthday distribution at the end looks. I'd expect more seasonality (e.g. more babies conceived in the cold, dark months).

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Remember that the seasons are inverted on the other side of the equator. (Granted, the population splits something like 90/10 but still.)

There are relatively few people living south enough to have "cold and dark winters," anyway. The northern hemisphere is much more concentrated towards the north.

Most people live above 35°S where, at the most extreme, winter days are about 10 hours and a half long (plus about an hour of decent twilight). Temperatures obviously vary depending on region but they don't really get much below 10°C as far as I know.

So really, it's more like mostly bright and somewhat chilly.

Yes, it's also extremely unlikely.

The actual distribution in developed countries is not uniform: there is a spike at the end of September (because many more people make babies at or around New Year's Eve) and a considerable drop on Dec. 25th (because people will avoid that date and provoque the birth some days before in case it might happen).

Also, on the site there is a huge spike on Nov. 15 which, incidentally, is the birthday of the author: maybe they tested it many times?

Why do people not like having their child be born on Xmas?

It's that some small percentage of births are intentionally induced for various medical reasons. Sometimes because the baby is full term but labor is not starting on its own yet. Other times an OB will attempt to rotate a full term baby that's in breach position to avoid needing a C-section surgery. Successful or not, this rotation procedure has a chance of triggering labor (which is why they wait until the baby is at or close to full term before trying it).

Bottom line: hospitals are short staffed on Xmas so they set scheduled procedures which may induce labor for the day before or the day after whenever possible which preserves their limited capacity on Xmas for unscheduled births and emergencies.

My sister’s birthday is a few days after Christmas. She hated this as a child because once her birthday was over there was almost a whole year without any prezzies

15th november are valentine's babies, I'm not surprised about this spike

Any such spike would be over weeks, not heavily concentrated on one day. It's clearly people taking the quiz twice to see what it says if you match the creators birthday.

> The actual distribution in developed countries is not uniform

This is a nice story I've heard many times, but is it actually true? Like what are your statistical sources here?

US births by day data is available here:

https://github.com/fivethirtyeight/data/tree/master/births

For the period 2000-2014 the number of births by month, divided by the number of days in a month, is:

   1  31   5 072 588     163 631.87   
   2  28   4 725 693     168 774.75   
   3  31   5 172 961     166 869.71   
   4  30   4 960 750     165 358.33   
   5  31   5 195 445     167 595.00   
   6  30   5 163 360     172 112.00   
   7  31   5 450 418     175 819.94   
   8  31   5 540 170     178 715.16   
   9  30   5 399 592     179 986.40   
  10  31   5 302 865     171 060.16   
  11  30   5 008 750     166 958.33   
  12  31   5 194 432     167 562.32   
     365  62 187 024     170 375.41

The difference isn't huge but it's significant. There were on average 180k birth in Sept. vs 165k in April and 170k in December.

Edit: also, here, a graph per day:

https://www.reddit.com/media?url=https%3A%2F%2Fi.redd.it%2Fm...

(The source is mentioned but I did not verify it directly).

The scale on that plot is cut off to emphasise small-ish variations. It is practically uniform.

The point of the plot is to show the drop on Dec. 25th; but the numbers are unrelated to the plot and do show actual variations from month to month.

I wonder if modern living has pretty much made this a non-factor. We have things to entertain ourselves regardless of the duration of light outside and so we're no longer left sitting at home in front of the fire bored, we're sitting at home in front of the fire playing with our phones. Less likely to bow-chicka-bow-wow out of boredom.

Weird how, coincidentally, 3x as many people are born on Nov 15 as any other day

Nope, Jan 1st has about the same amount. So the lazy liars and those wondering what the dialogue says if they share a birthday with the prompt.

is that US Nov 15th?

Cause I was born in Australia on Nov 14th :-D

Wouldn't that be US Nov 13th?

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