Yesterday, I looked at the day on the calendar when players in the 2018 NFL season were born. To do this, I looked at birth rates in 31-day increments. The days from late July to mid August produced a lot of NFL players, while the Geminis of the world — those born from in the window from late May to late June — produced comparably few NFL players.

We also looked at how this compared to the United States as a whole, which tends to have more kids in the summer and few in the winter. In that respect, the Leos of the world — born from late July to late August — didn’t stand out quite as much. June babies, though, looked really low, while January/February seemed to produce more NFL players than you would expect (those are low birth rates for the United States).

Today, let’s examine all players to enter and play in an NFL game since 1970. Once again, we are looking at 31-day periods. Now, that Tom Brady peak has disappeared, and it is players born on or around February 25th that produce the most players. There is another peak in early September, and September 8th is the birthday of the most NFL players since 1970.

Now, let’s compare that to the U.S. birth rate as a whole. Here’s the same graph, but with an orange line (plotted against the right Y-Axis) showing the U.S. birth rate for each date relative to the average date on the calendar. As you can see, summer and early September birthdays are really common, with January/February birthdays being pretty rare. Which makes the high number of NFL players in those months pretty weird:

Some of you may have your mind immediately jump to the story written by Malcolm Gladwell in Outliers about how there were a disproportionately high number of Canadian hockey players born in January and February. However, that reasoning is not applicable here: the thesis behind Gladwell’s story was that in Canada, there is a national cut-off for school is based on a calendar year, with January 1 being that date.

It’s a beautiful example of a self-fulfilling prophecy. In Canada, the eligibility cutoff for age-class hockey programs is Jan. 1. Canada also takes hockey really seriously, so coaches start streaming the best hockey players into elite programs, where they practice more and play more games and get better coaching, as early as 8 or 9. But who tends to be the “best” player at age 8 or 8? The oldest, of course — the kids born nearest the cut-off date, who can be as much as almost a year older than kids born at the other end of the cut-off date. When you are 8 years old, 10 or 11 extra months of maturity means a lot.

So those kids get special attention. That’s why there are more players in the NHL born in January and February and March than any other months. You see the same pattern, to an even more extreme degree, in soccer in Europe and baseball here in the U.S. It’s one of those bizarre, little-remarked-upon facts of professional sports. They’re biased against kids with the wrong birthday.

Two problems here. One, there is no national cut-off date in the United States, with each state having its own guidelines. But more importantly, the majority of states have September 1st, not January 1st, as the cut-off, with most having some cut-off of between September 1 and October 31st, and yes, some having January 1st as a cut-off. In Texas, kindergarten eligibility is based on a child being 5 years old by September 1st.

Let’s group players since 1970 by birth month and also group U.S. births by birth month. To put everything on the same axis, for both sets of data, I am comparing the group to the average for the entire year. So, for example, in January, there are 98% as many births in the United States as in the average make, making it slightly below average; meanwhile, there are 110% as many NFL players from that month as the average month. February, our shortest month, produces just 90% as many children as the average month; however, February almost exactly as many NFL players as the average month.

Meanwhile, in June, the country has an exactly average amount of children born compared to the average month. But for the NFL? That is a very poor month for making NFL players, with June birthdays representing only 92% of the average month.