Barnsley was at a Lethbridge Broncos hockey game in southern Alberta, a team that played in the same Major Junior A league as the Vancouver Giants and the Medicine Hat Tigers. He was there with his wife, Paula, and their two boys, and his wife was reading the program, when she ran across a roster list just like the one above that you just looked at.

"Roger," she said, "do you know when these young men were born?"

Barnsley said yes. "They're all between sixteen and twenty, so they'd be born in the late sixties."

"No, no," Paula went on. "What month."

"I thought she was crazy," Barnsley remembers. "But I looked through it, and what she was saying just jumped out at me. For some reason, there were an incredible number of January, February, and March birth dates."

Barnsley went home that night and looked up the birth dates of as many professional hockey players as he could find. He saw the same pattern. Barnsley, his wife, and a colleague, A. H. Thompson, then gathered statistics on every player in the Ontario Junior Hockey League. The story was the same. More players were born in January than in any other month, and by an overwhelming margin. The second most frequent birth month? February. The third? March. Barnsley found that there were nearly five and a half times as many Ontario Junior Hockey League players born in January as were born in November. He looked at the all-star teams of eleven-year-olds and thirteen-year-olds-the young players selected for elite traveling squads. Same story. He looked at the composition of the National Hockey League. Same story. The more he looked, the more Barnsley came to believe that what he was seeing was not a chance occurrence but an iron law of Canadian hockey: in any elite group of hockey players-the very best of the best-40 percent of the players will have been born between January and March, 30 percent between April and June, 20 percent between July and September, and 10 percent between October and December.

"In all my years in psychology, I have never run into an effect this large," Barnsley says. "You don't even need to do any statistical analysis. You just look at it."

Look back at the Medicine Hat roster. Do you see it now? Seventeen out of the twenty-five players on the team were born in January, February, March, or April.

Here is the play-by-play for the first two goals in the Memorial Cup final, only this time I've substituted the players' birthdays for their names. It no longer sounds like the championship of Canadian junior hockey. It now sounds like a strange sporting ritual for teenage boys born under the astrological signs Capricorn, Aquarius, and Pisces.

March 11 starts around one side of the Tigers' net, leaving the puck for his teammate January 4, who passes it to January 22, who flips it back to March 12, who shoots point-blank at the Tigers' goalie, April 27. April 27 blocks the shot, but it's rebounded by Vancouver's March 6. He shoots! Medicine Hat defensemen February 9 and February 14 dive to block the puck while January 10 looks on helplessly. March 6 scores!

Let's go to the second period now.

Medicine Hat's turn. The Tigers' scoring leader, January 21, charges down the right side of the ice. He stops and circles, eluding the Vancouver defenseman February 15. January 21 then deftly passes the puck to his teammate December 20-wow! what's he doing out there?!-who shrugs off the onrushing defender May 17 and slides a cross-crease pass back to January 21. He shoots! Vancouver defenseman March 12 dives, trying to block the shot. Vancouver's goalie, March 19, lunges helplessly. January 21 scores! He raises his hands in triumph. His teammate May 2 jumps on his back with joy.

4.

The explanation for this is quite simple. It has nothing to do with astrology, nor is there anything magical about the first three months of the year. It's simply that in Canada the eligibility cutoff for age-class hockey is January 1. A boy who turns ten on January 2, then, could be playing alongside someone who doesn't turn ten until the end of the year-and at that age, in preadolescence, a twelve-month gap in age represents an enormous difference in physical maturity.

This being Canada, the most hockey-crazed country on earth, coaches start to select players for the traveling "rep" squad-the all-star teams-at the age of nine or ten, and of course they are more likely to view as talented the bigger and more coordinated players, who have had the benefit of critical extra months of maturity.

And what happens when a player gets chosen for a rep squad? He gets better coaching, and his teammates are better, and he plays fifty or seventy-five games a season instead of twenty games a season like those left behind in the "house" league, and he practices twice as much as, or even three times more than, he would have otherwise. In the beginning, his advantage isn't so much that he is inherently better but only that he is a little older. But by the age of thirteen or fourteen, with the benefit of better coaching and all that extra practice under his belt, he really is better, so he's the one more likely to make it to the Major Junior A league, and from there into the big leagues.*

Barnsley argues that these kinds of skewed age distributions exist whenever three things happen: selection, streaming, and differentiated experience. If you make a decision about who is good and who is not good at an early age; if you separate the "talented" from the "untalented"; and if you provide the "talented" with a superior experience, then you're going to end up giving a huge advantage to that small group of people born closest to the cutoff date.

In the United States, football and basketball don't select, stream, and differentiate quite as dramatically. As a result, a child can be a bit behind physically in those sports and still play as much as his or her more mature peers.* But baseball does. The cutoff date for almost all nonschool baseball leagues in the United States is July 31, with the result that more major league players are born in August than in any other month. (The numbers are striking: in 2005, among Americans playing major league baseball 505 were born in August versus 313 born in July.)

European soccer, similarly, is organized like hockey and baseball-and the birth-date distributions in that sport are heavily skewed as well. In England, the eligibility date is September 1, and in the football association's premier league at one point in the 1990s, there were 288 players born between September and November and only 136 players born between June and August. In international soccer, the cutoff date used to be August 1, and in one recent junior world championship tournament, 135 players were born in the three months after August 1, and just 22 were born in May, June, and July. Today the cutoff date for international junior soccer is January 1. Take a look at the roster of the 2007 Czech National Junior soccer team, which made the Junior World Cup finals.

Here we go again:

No. Player Birth Date Position

1 Marcel Gecov Jan. 1, 1988 MF

2 Ludek Frydrych Jan. 3, 1987 GK

3 Petr Janda Jan. 5, 1987 MF

4 Jakub Dohnalek Jan. 12, 1988 DF

5 Jakub Mares Jan. 26, 1987 MF

6 Michal Held Jan. 27, 1987 DF

7 Marek Strestik Feb. 1, 1987 FW

8 Jiri Valenta Feb. 14, 1988 MF

9 Jan Simunek Feb. 20, 1987 DF

10 Tomas Oklestek Feb. 21, 1987 MF

11 Lubos Kalouda Feb. 21, 1987 MF

12 Radek Petr Feb. 24, 1987 GK

13 Ondrej Mazuch Mar. 15, 1989 DF

14 Ondrej Kudela Mar. 26, 1987 MF

15 Marek Suchy Mar. 29, 1988 DF

16 Martin Fenin Apr. 16, 1987 FW

17 Tomas Pekhart May 26, 1989 FW

18 Lukas Kuban Jun. 22, 1987 DF

19 Tomas Cihlar Jun. 24, 1987 DF

20 Tomas Frystak Aug. 18, 1987 GK

21 Tomas Micola Sep. 26, 1988 MF

At the national team tryouts, the Czech soccer coaches might as well have told everyone born after midsummer that they should pack their bags and go home.

Hockey and soccer are just games, of course, involving a select few. But these exact same biases also show up in areas of much more consequence, like education. Parents with a child born at the end of the calendar year often think about holding their child back before the start of kindergarten: it's hard for a five-year-old to keep up with a child born many months earlier. But most parents, one suspects, think that whatever disadvantage a younger child faces in kindergarten eventually goes away. But it doesn't. It's just like hockey. The small initial advantage that the child born in the early part of the year has over the child born at the end of the year persists. It locks children into patterns of achievement and underachievement, encouragement and discouragement, that stretch on and on for years.

Recently, two economists-Kelly Bedard and Elizabeth Dhuey-looked at the relationship between scores on what is called the Trends in International Mathematics and Science Study, or TIMSS (math and science tests given every four years to children in many countries around the world), and month of birth. They found that among fourth graders, the oldest children scored somewhere between four and twelve percentile points better than the youngest children. That, as Dhuey explains, is a "huge effect." It means that if you take two intellectually equivalent fourth graders with birthdays at opposite ends of the cutoff date, the older student could score in the eightieth percentile, while the younger one could score in the sixty-eighth percentile. That's the difference between qualifying for a gifted program and not.