Today is February 29th. Given that it comes only once every four years, that would make it a rare birthday. But it is even rarer. As Andrew Leigh and I discovered in our research, the birthrate on this day is about 10% lower than if it was an ordinary Friday. Why? Parents like to move their children’s birthdays off days like this and on to something more regular. Of course, today they might get some resistance from their doctors who don’t want those births pushed on to the weekend, so it isn’t as rare as ‘normal’ leap days. [Thanks to Andrew for the reminder].