This Is Why You Know So Many People Who Share Your Birthday

Happy birthday!

Because there are only 365 days in a year, it's no surprise that almost everyone knows someone with whom they share a birthday. Even those born on the elusive Leap Day, February 29, are able to find a birthday buddy.

That said, there are some birthdays that are just more common than others. One Harvard study surveyed the number of births on any given day over a 26-year period and found that more American babies are born on September 16 than any other day, with fall being the most popular season to give birth. The fewest number of babies were born on February 29 (because of course). In regular years, however, January 1 is the least common birthday, with fewest babies born during the winter months.

This might just sound like fun trivia, but it actually asks some very important questions about human reproductive behavior. Because most babies are born in the fall, they were conceived in the previous winter. The low number of winter babies means that spring is the least common time to conceive a child.

The mystery, of course, is why there is such a stark difference in babies born at different times of the year, instead of being evenly dispersed. Unlike many animals, humans don't have a set mating season. However, there are clearly times when more people successfully conceive, but the answer isn't quite as clear. 

It may be as simple as that people spend more time inside during the winter, with fewer options of how to spend their time. Another possibility is that sperm, which is greatly affected by temperature variations, is of better quality during the winter months, leading to more conceptions. Because the testicles hang outside the body to keep cooler, this could very well play a role. There could also be a connection with the females' ability to carry a healthy baby during certain months as well.

In the future, it could be interesting to compare the statistics from the U.S. and compare them to countries with different climates to see how the pattern holds across the globe, as well as comparing sperm quality from month to month and tracking when miscarriages are most likely to happen.

But beyond figuring out when Americans are most likely to havE the most sex, determining shared birthdays is also useful for some clever statistics.

In honor of the third birthday of It's Okay To Be Smart, host Joe Hanson celebrated the numbers involved in the science of shared birthdays. He breaks down the minimum number of people you need to meet in a room before finding someone who shares your birthday and other interesting facts, like how many Facebook friends you need before having a friend to say "happy birthday" to on every day of the year. 

Happy birthday, It's Okay To Be Smart!

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