In 2006, the New York Times ranked the most common birthdays in the United States between 1973 and 1999. In the graph above, more popular birthdays are represented as darker shades of blue. September was the most common month, September 16 the most common date. The least common birth dates were February 29, December 25, and January 1.
The popularity of September birthdays isn’t happenstance. Given that September slides in nine months after December, one could presume that holiday celebrations are connected to the high number of September births. This is further evidenced by a September 16 birth date corresponding with a December 24 conception date.
A 2017 study conducted at Indiana University took a deeper look into the cultural factors that influence incidence of human sex patterns. The researchers analyzed Google search trends and Twitter data to measure intercourse frequency and mood fluctuations around Christmas and Eid-al-Fitr. They found these holidays to be closely tied to frequency of intercourse, even after adjusting for excess free-time. Higher frequency of intercourse was found to be linked to people’s elevation in happiness in direct response to the holiday season. September trends were consistent across all countries that celebrated Christmas on December 25, and birthday trends in predominantly Muslim countries moved in correspondence with the end of Ramadan.
Graph by Erin Polka