In any group of 23 people, the odds that two of them have the same birthday is 50 percent. This phenomenon, which holds true in any group of randomly selected people, is called the birthday paradox. When 57 people are in the group, the probability is 99 percent, and the percentage rises only slightly as the size of the group increases, until it reaches 100 percent at 367 people. If two people meet randomly, however, the chance of them having the same birthday is only 0.27 percent.
More facts about the birthday paradox:
- The birthday paradox is actually used in mathematics to crack hashing algorithms, and it can be used in cryptography.
- The reason why the birthday paradox works is because of something called the pigeonhole principle, which states that if there are n number of items placed into m number of holes, and n is more than m, at least one hole will have two items in it.
- The birthday paradox seems so surprising because people don't tend to go around asking the date of others' birthdays. If they did, it would quickly become apparent that shared birthdays are relatively common.