Birthdays by Month

While we're on the topic of random stuff I've always wondered about... which is the most common month for birthdays? I feel anecdotally that I meet/know a lot of people who, like me, have a November birthday, but maybe I just notice them more? What's the real deal?

It turns out that the Internet does indeed know the answer to this question (are we surprised?) but it's actually kind of hard to dig up. Every search I tried with any combination of the words 'birthday' and 'probability' automatically gave me a gazillion links to the Birthday "Paradox", which is a pretty interesting statistical oddity in its own right and not entirely unrelated, but also not quite what I was looking for. As these things go, it's always a matter of finding the right google phrase -- in this case "seasonal variation of births" did the trick. In the United States from 1978 to 1987 the distribution of birthdays looked like this:

Data: U. S. Dept. of Health and Human Services, 1989

So September is the most popular birthday month, and January the least. November is actually a bit below average (so much for my subjective impression). Notice that my plot is slightly deceptive in that the y-axis doesn't go down to zero. This means that the 'big' difference between January and September isn't really that big - the variations are only about 5% of the average. It seems, people tend to make babies just about any old time of year.

Perhaps most interestingly, this seasonal variation of births is not the same across cultures and nations. The U. S. peaks in late summer whereas Northern Europe has more spring birthdays. No doubt the variations are very different elsewhere in the world and probably have many interesting implications for the sociologists to untangle.