Sports are supposed to be great levelers, where results matter more than promise, and what one does is more important than who one knows.
Yet, sports have biases, too, and Nippon Professional Baseball is no exception. Japan's rosters are numerically dominated by players born in a single six-month stretch of the calendar.
Although Yomiuri Giants catcher Shinnosuke Abe said last summer a player's birth month only mattered in little leagues, boys born from April 2 are much more likely to become pros than those born in the weeks and months leading up to April 2.
Over a third of Japanese-born players on NPB rosters are born on one of the 90 dates starting on April 2, while roughly an eighth are born in the quarter year from Jan. 1 to April 1. The 32 dates from March 1 to April 1 are by far the worst for aspiring ballplayers.
Abe, born on March 20, is one of the lucky few to make it from that month.
"By the time you get to be a pro, the month you're born in doesn't matter," he told The Daily Yomiuri.
Maybe not, but it has a massive impact on a player's chances of getting that far in Japan.
April 1 impacts the baseball population because children born on that day begin their school careers in Japan a year earlier than those born on April 2. Although 12 years will pass from the first day of school and the age at which young men often begin pro careers, NPB's ranks are largely shaped by how old children are relative to their classmates.
This phenomenon, known as the relative age effect, is well documented. Studies around the world have shown that the date on which a child is born can influence his life in ways that have nothing to do with astrology.
In a 2006 study on educational and social outcomes, Hitotsubashi University's Daiji Kawaguchi found Japanese boys born in March do not remain in school as long as those born in April. Children born in March (and April 1) are forever competing against older classmates. Children born on the dates following April 1, however, go through their school years with the advantage of being months older than most of their classmates.
In the United States, Aug. 1 is the magic birthday for baseball, the day after the cutoff date for youth league admissions. An American boy born in August has, according to a study by Greg Spira, a 50-60 percent better chance of making the majors.
Here, the cut is even harsher.
From 1998 to 2007, NPB teams drafted 32 players born on the 32 dates from March 1 to April 1. In those years, there were 90 players born in the remaining 29 days of April. In other words, if you want your next son to play in the Central or Pacific league, pray he's born after April 1.
Stephen J. Drubner and Steven D. Levitt (co-authors of the best-selling book "Freakonomics"), in reporting the phenomenon in European soccer, concluded that youth coaches mistake maturity for talent and tend to select older players--who will throughout their youth soccer careers have the advantage of being more physically developed than many teammates and opponents.
If youth coaches and pro scouts are mistaking maturity for talent, one would expect the average March-born player to be better than the average April-born player.
In a Daily Yomiuri study of 901 Japanese players born between April 2, 1965 and April 1, 1977, the 38 players with March and April 1 birthdates had more average value in their prime years than the 116 players born from April 2 to April 30.
While April had bigger stars by far, the April boys were less likely to have careers. The best player among the March group was catcher-outfielder Koichi Sekikawa (born April 1, 1969), but he would rank 10th among April's All-Stars, right behind second baseman Koichi Hori, born the following day.
The biggest April star in the DY study was outfielder Tomoaki Kanemoto, while the player with the best career after turning 25 was Ichiro Suzuki (October), followed closely by fellow major leaguer Hideki Matsui (June). Kanemoto ranks seventh overall.
Although the March players--and the two April 1 boys among them (Sekikawa and pitcher Masumi Kuwata)--tended to have less spectacular results in their primes, they were more likely to reach the top and stay there.
Of 116 April players, 52 (45 percent) had no real playing time after turning 25. Among the March group that figure was 12 (32 percent).
While it is harder for March-born players to become pros in the first place, this did not apply to catchers in the study. Although players born in March comprised just 4 percent of the study, they recorded 18 percent of the total assists and putouts made by all the catchers.
Abe only moved from third base to catcher when his high school coach ran out of alternatives, but it turned out to be a brilliant career move.
In his study of school leavers, Kawaguchi found that while March-born boys do not go as far in school in Japan as those born in April, the extra schooling had no impact on expected earnings or marriage prospects.
A parallel to that in baseball is that it is easier for players born in the six months from April 2 to turn pro, but they do not perform better as pros.
Three of the best four players in the study--Ichiro, Nobuhiko Matsunaka and Michihiro Ogasawara--were born in the last three months of the year.
On average, the 303 players in the study born from Oct. 1 played more often, hit better and pitched better than the 598 other fellows.
These players, who comprised just 34 percent of the pros in the study, accounted for 46 percent of the runs scored and driven in, while pitching as often as the other players with better ERAs and more saves.
These are exactly the results one would expect if players born in the six months after April 2 are overrepresented in NPB.
Players born from October through March probably have to be much better than their rivals if they are to gain admission to the pro ranks, but when they get past the gatekeepers, it's off to the races.
What a difference a day makes
It may be unintentional, but the fingerprints of birth-month bias smudge all of Nippon Professional Baseball's rosters.
A Japanese boy born on April 2 is several times more likely to become a top professional draft pick in NPB than a boy born the previous day. The average batting and pitching results of players born in April, however, are less impressive than those of the players born in March--including April 1.
The Daily Yomiuri conducted a study of the 901 Japanese players born between April 2, 1965, and April 1, 1977, who were drafted 1-8. In the seven seasons after each player turned 25, the average player in the study scored 47 runs and drove in 42, while winning 5.4 games and saving 2.8.
The players born in April, May, June and July won and saved fewer games while driving in and scoring fewer runs. The players born in January, February and March were better than average. The December players were slightly below average in pitching results, but their hitters dominated the study.