The theory behind this was described in the American Mathematical Monthly in 1938 in Zoe Emily Schnabel's *The estimation of the total fish population of a lake*, under the name of capture-recapture statistics.

Calculating this probability (and related probabilities) is the **birthday problem**.

Note that, if you enter a room with 22 people, the chance that somebody there has the same birthday as **you** is *not* 50/50, but much lower. This is because the day of the year that must be the joint birthday is already given, namely, by your own birthday.

To compute the approximate probability that in a room of *n* people, at least two have the same birthday, we disregard leap years and twins, and assume that the 365 possible birthdays are equally likely.
The trick is to first calculate the probability that the *n* birthdays are *different*. This probability is given by

By contrast, the probability that someone in a room of *n* other people has the same birthday as you is given by

The birthday paradox in its more generic sense applies to hash functions where the number of N-bit hashes you can generate before probably getting a collision is not 2^{N}, but rather 2^{N/2}. This is exploited by birthday attacks on cryptographical systems.

In his autobiography, Paul Halmos wrote:

- "Hand-held calculators can be good things and they can have bad effects. The birthday problem can be used to exemplify a bad effect. A good way to attack the problem is to pose it in reverse: what's the largest number of people for which the probability is less than 1/2 that they all have different birthdays? .... the problem amounts to this: find the smallest
*n*for which - The indicated product is dominated by
- The asserted domination comes from the celebrated relation between the geometric and arithmetic means; the next inequality comes from the definition of the definite integral, and the last one from 1 −
*x*<*e*^{−x}. .... The reasoning is based on important tools that all students of mathematics should have ready access to. The birthday problem used to be a splendid illustration of the advantages of pure thought over mechanical manipulation; the inequalities can be obtained in a minute or two, whereas the multiplications would take much longer, and be much more subject to error, whether the instrument is a pencil or an old-fashioned desk computer. .... What calculators do not yield is understanding, or mathematical facility, or a solid basis for more advanced, generalized theories. A pity."