If someone asks you to guess their birthday, without giving any additional information, you might find yourself a bit lost. After all, there’s a 1 in 365 chance you’ll get it with a random guess, right? And who would take odds like that?
Well, it turns out you might be able to play around with the system a bit. There’s one day of the year when people are much less likely to be born than any other: December 25, also known as Christmas Day.
This is no joke – nor is it a probabilistic paradox like the birthday problem. It’s a true statistical fact, originally found for a 1999 paper exploring a quirk of the US tax system.
“Dec. 25 is the least popular day in the United States, Australia and New Zealand to give birth. In England, Wales and Ireland, it is the second least popular, behind the 26 December, when Britons celebrate Boxing Day,” economist Jay Zagorsky explained in a 2019 article for The Conversation.
Using information from US birth records spanning from 1978 to 1992, researchers Stacy Dickert-Conlin and Amitabh Chandra were able to list each day of the year in order of the likelihood of a birth falling on it – and in 2006 , these data were collected in a list published in the New York Times.
Aside from February 29 – as it only appears once every four years, it has somewhat of an advantage in this regard – there was a clear winner in terms of popularity. Or rather, a clear losing: Of all available days of the year, none was less likely to be a birthday than December 25.
But it’s weird, right? If it was a random date – a date with no particular cultural significance – then it wouldn’t be really interesting. But Christmas Day is different. So, is it just a coincidence that has made this traditional holiday the only day of the year when people don’t want to give birth? Is there a biological variable that comes into play? Or is it something completely different?
Well, researchers have found biological reasons for the annual ebb and flow in birth rates. Late summer and fall babies are more common than any other birthday, for example, and this is thought to be due to a range of factors that bind our bodies to the changing world around us.
“Hypotheses include deterioration of sperm quality during the summer, seasonal differences in anterior pituitary-ovarian function caused by changes in daylight duration, and variation in ovum or cell quality. endometrial receptivity,” explains a 2001 article in the journal Obstetrics and Gynecology.
“The increase in sexual activity associated with the holiday season has also been postulated as a possible behavioral explanation for the December conception spike,” the paper adds, although “the exact reasons remain unknown.”
But while that may explain why there are so many babies in September, it doesn’t really account for the lack of Christmas birthdays. After all, step back nine months from the holiday season and you’re at the end of March – barely the middle of summer, extremely hot and overwhelming with cum. So what gives?
In fact, the reason why it’s particularly unlikely that you’ll give birth on Christmas Day is almost disappointing. Far from being a wild coincidence, the reason people aren’t born on a vacation is, well, because it’s a vacation.
“All of the least favored days in the United States are tied to the holidays, whether it’s Christmas, New Years, July 4, or Thanksgiving,” Zagorsky wrote. But “depending on the year and location, between 30 and 40 percent fewer babies are born on December 25 than the peak day of the year.”
Although you may think that your date of birth is totally out of human control, in this modern world the truth is that it is often much more scheduled than we tend to realize. “Almost no caesareans are scheduled by doctors on holidays or weekends,” Zagorsky said. “About one in three American babies is born this way.”
Even among babies born vaginally, more than a quarter of births in the United States are medically induced. This, too, is less likely to happen when doctors prefer to celebrate holidays with family than at work — or, for that matter, when pregnant women themselves would rather receive gifts than episiotomies.
So if you’re one of the few people who’s had to spend your life giving everyone other present on your birthday, you can at least take comfort in knowing that you are a rare breed. Up to 40% rarer, in fact, than those September dime births.
And the fact that your special birthday was probably just because your mom’s OB/GYN got doused in eggnog? Well, we won’t tell if you don’t.