Behind the Science: The Birthday Paradox

In a room of 40 people, what are the odds that two have the same birthday?

If you’re like me, or most people, you’ll do some basic math. You think, OK, there are 365 days in a year, and so we should divide that by 40 and get around 9. So, the odds are 1 in 9, right?

And of course that’s wrong, or this would be a boring post. The answer is about 90%--nearly 100%.

How can that be? This is a famous puzzle known as the Birthday Paradox. It plays directly into one of our major blind spots as humans.

Step back and look at the question again. I wrote “what are the odds that two have the same birthday?” but nearly everyone reads that as “If I were in that room, what are the odds someone would have the same birthday as me?” That answer would indeed be about 1 in 9. But now the same thing is true for the next person, and the next person, etc. 

It’s a paradox to us because we are egocentric, and blind to how others matter to others. But that’s not how the world works. The relationships between other people are real too! This is what nearly every business is blind to, and it’s part of what we are solving for at Social Value. We measure the impact between people, usually customers. It sits right in that blind spot, it’s huge, and it’s very valuable.

Did you know, or do you have the same blind spot I did?