I was in a discussion about horoscopes and how they’re wrong, when a possibly crazy claim came up: most people are born in one half of the year instead of another.
Because it’s trivially true if all you need is 50% + 1 to make it “most” — I pressed the guy making the claim for some numbers behind it. He claimed that the six months that has the highest birthrate had 10% more people born than the six months that has the lowest birthrate. This sounded way too large, so we made a $20 bet.
Turns out it is simple to find the month that most people are born in — but getting month-by-month data was kinda tricky. You can find 1995-2002 data for the US birthrate by month here.
I’ve embedded the analysis below — turns out it was a closer bet than I was expecting.
Turns out there’s roughly a 6% difference between the most … birthy half of the year relative to the least birthy half of the year. If birthy isn’t a word, I’m inventing it now. I would have guess about half of the real answer.
Comments on this entry are closed.
Nice crunching. I concede the bet.
Stumbled on your blog as I was looking for info re: which part of the month is the "birthiest"… Thanks for the interesting info and if you have an answer for question, please let me know.
The analysis does not take into account the fact that the number of days in months is vary. The average births per day by month is given in the ABC News report referred to the article. The difference between the peak month September and the low month January is 9.7% — very close to 10%.
But it's not the highest variation between the most extreme two months; it's the highest variation over a contiguous 6-month period. I could look for contiguous 183-day periods, and if I had day-to-day variation I would have used it.
I did the next best thing I could think: I normalized births/day within each month (dividing by respective number of days in each month). Turns out the maximum 183 day period compared with the minimum 183 day period decreases the difference from 6.1% to 4.6%.
I wonder if it has anything to do with the amount and quality of fresh vegetables and fruits that are consumes in annual cycles. For example: the peak in birthiness in August relies on conception in December. December is right after harvest season and people would conceivably be at their healthiest (more likely to conceive) all year due to the abundance of fresh and healthier domestically produced produce. I suspected this before I saw the chart on your site.