The birthday paradox explains that it is very likely that you share your birthday with a friend, but not if this date is December 25thas it is less likely to be born.
“The 25th 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, after December 26, when the British celebrate Boxing Day,” economist Jay Zagorsky explained in a 2019 article for The Conversation.
Using information from US birth records from 1978 to 1992, researchers Stacy Dickert-Conlin and Amitabh Chandra found that the least popular day is definitely December 25th. Scientists have found (in part) biological reasons, a number of factors that link our bodies to the changing world around us.
“Hypotheses include deterioration of sperm quality during summer, seasonal differences in anterior pituitary-ovarian function caused by changes in daylight length, and variation in ovum quality or endometrial receptivity“explains a 2001 article in the journal Obstetrics and Gynecology.”Increased sexual activity associated with the year-end holiday season has also been postulated as a possible explanation.“
However, to be born at Christmas, conception should take place in March and, despite climate change and rising temperatures, it’s not so hot in this period. The reasons, therefore, seem to be more practical: no children are born because it is a day off and doctors and midwives prefer to deliver sooner or later.
“All the least favorite days in the US are related to holidays, whether it’s Christmas, New Year’s, 4th of July or Thanksgiving”, writes Zagorsky. ”Depending on the year and location, between 30 and 40 percent fewer babies are born during December 25.“