Why Do So Many People Have Birthdays Today?

OK, so I don’t actually know if a lot of people have birthdays today compared to other days.  But among my friends and acquaintances (including my daughter), there is a very large cluster of birthdays today, December 19.

Like any serious researcher, I took the question to Facebook.  Here’s what my Facebook friends had to say about the reason for this cluster of birthdays:

  • St. Patrick’s Day.
  • December birthdays are from March conceptions.
  • People like to have their birthdays on Friday so they can party through the weekend.
  • All the birthdays tomorrow mean lots of babies have been born on that day, historically.
  • Math. Odds are a disproportionate number would have birthdays on *some* day.
  • Global warming.
  • Springtime fertility rituals.
  • Induced labor a week before Christmas.
  • Spring break.
  • It’s called Birthday Concentrationism. During a relatively stagnant Christmas season in 1947, a number of Keynesians gathered to centrally plan everybody’s birthday for a single day of gifting stimulus. Individuals without birthdays on the 18th of December were reassigned said day based upon Pareto efficiency second welfare function something or other.  But this would not explain people born after that date, to which I respond “just shut up and enjoy it.”
  • I doubt March Madness is to blame!
  • They are all angling for Star Wars themed parties. Take the hint and buy them all vintage Star Wars lunch boxes.
  • Disproportion syndrome.
  • For younger people, weekday birthday prevalence is partially explained by the increasing % of C-sections which skews away from weekends. But afaik December/January have a low rate of births compared to other months, though the days could still be clustered for whatever reason. I wouldn’t be surprised if doctors were more likely to induce prior to the holidays, whether the birth were by c-section ultimately or not.
  • Only a small percentage of babies are born on their technical due dates, though I don’t know how many come earlier vs. later. I think first babies are more likely to come late, so if you have a lot of newlywed couples attempting to conceive their first child shortly after the wedding, and wedding season is like May-June, then that could explain a December cluster for instance. But many members of the cluster would still just be there by chance.

Good enough for me!  Social media comes through again.

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