The Surprising Math of Shared Birthdays

You only need a group of 23 people for there to be a 50 percent chance that two people share a birthday. This feels wrong because we intuitively compare our own birthday to others, rather than looking for any pair.

While it seems like you would need hundreds of people to find a match, the math relies on the total number of possible pairings in the group. With 23 people, there are 253 unique pairs, which provides plenty of opportunities for a shared date. This phenomenon is called a paradox because human instinct drastically underestimates how quickly probabilities grow. It remains one of the most famous examples of how our brains struggle with complex probability.