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X writes:

Sur France Inter ce matin, le 23 septembre est le jour en France avec le plus de naissances (+5%). Have you done the same analysis on births for France than you did for the US?

I replied that we (Aki, really) have not analyzed any French data, but if you have the numbers, we could take a look. From the U.S. data and the cover of BDA, I recall that late Sept is the peak of births. It’s not a day-of-year effect, it’s a seasonal effect, I assume it reflects when people get pregnant, 9 months earlier. Or it could reflect differential rates of miscarriages, but I’m guessing it’s variation during the year of when people get pregnant.

X replied:

I found this rather fascinating paper by Gérard Calot who founded INED, and a more recent paper [by Arnaud Régnier-Loilier and Jean-Marc Rohrbasser] about the historical moves for most frequent periods, but no daily data so far.

This is great stuff. I love birthday data. I guess I should collaborate with some demographers! In the meantime, above and below I’ve reproduced some of the fun graphs from the papers that X pointed us to.

Again, I have no idea how much of this comes from what time of year people do that thing that makes babies, and how much comes from babies conceived at different times of year not making it all the way to birth. But I guess the experts know.


  1. Terry says:

    People who think they see patterns in birthdays over the year are obviously succumbing to the hot hands fallacy. Yet another example of human irrationality and the human tendency to see patterns where non exist.

  2. The decline in births on Sunday or holiday (“Dimanche ou fête”) in “Figure 1. – France. Evolution des coefficients journaliers des naissances de 1946 à 1978” (Evolution of daily coefficients of births from 1946 to 1978) might reflect a progressive increase in births by C-section, as those would tend not to be scheduled on a Sunday or holiday.

  3. Bill Spight says:

    Do birthdates in the temperate zone of the southern hemisphere show a similar seasonal pattern? If so, they should show a peak in late March, eh?

  4. Jeremy Fox says:

    What’s up with the big peak in April/May back in the 1970s and ’80s? Did lots of women get pregnant during August vacations back then? If so, has that become less of a thing over time?

  5. jim says:

    I’m sticking with August weddings until Terry comes up with data to shoot it down :)

    Makes perfect sense: declines in ’85 and more ’95 as sexual norms change and more people engage in premarital sex.

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