The Birthday Paradox is a probability theory that says in a random sample of people there will be 2 people with the same birthday in a group of only 70 people. The assumed logic would be that you would need 367 people to guarantee that 2 people share the same birthday. The number is much lower than you might expect because every time a new person is added, they add to the number of options available - you are only looking for someone to have the same date as anyone else in the group rather than looking for one specific day.
It is unclear when the Birthday Paradox was first discussed, but it seemingly dates back to the 50s. Martin Gardner - skeptic and mathematician - first wrote about it in 1957. Surprisingly this theory also has real-world applications: the maths behind it can be used to help with cryptography, specifically breaking them.