A recent discussion in the “Hey Bill” feature in billjamesonline discussed why some players do better than others and brought up the topic of relative age effects. I did a study about 10 years ago about the effects of NPB players’ birth months that was published in the Daily Yomiuri, which means it’s disappeared from the web. The upshot of that study was that players born from April 2 to June 30 are over-represented in the NPB amateur draft and, on average, have less valuable careers than player born from July 1 to April 1–the cutoff date for school admissions.

Children born on April 1 will enter school in Japan a year before a child born the following day.

Overview

I replicated the study using every domestic player signed by an NPB team from the end of the 1965 season through the start of the 1997 season. Omitting four players I don’t have birth dates for, that remaining group of 2,160 players contains two active players, Ichiro Suzuki and Kazuya Fukuura. And whatever they produce in 2019 is not going to affect anything one way or another. The starting point of the study was set by the introduction of NPB’s first draft in 1965.

Breaking down each quarter of a year by birth month — with April 1 counting as March — and draft round. The most populous cell is the 127 signed first-round picks signed who were born from April to June. The second most is the 121 players born in those months taken in the second round. As expected, the 341 “haya umare” or early-born players whose birthdays go from Jan. 1 to April 1, make that quarter the least populous.

The table below gives the career win shares produced by players born in each quarter and the total number in each group, without reference to draft round.

The last thing that needs to be mentioned is the problem of value in the major leagues. Major League win shares are given 20 percent more weight in the calculations. It’s just a guess. They could be 50 percent more valuable for all I know.

Distribution of domestic players by birth-month quarters

	Apr-Jun	Jul-Sep	Oct-Dec	Jan-Mar
Avg WS	210.0	230.5	306.8	223.0
Number	754	651	414	341
Percent of total	34.9	30.1	19.2	15.8

The favoritism in the draft show players born in the April-June quarter is exacerbated by an even higher share of those players taken in the first two rounds, and by the performance of those players.

Value rank of birth-month quarter by round

Round	Quarter starting	Avg WS	Best career
1st	July	68.8	Kazuhiro Kiyohara, 1B
1st	January	60.2	Masaki Saito, P
1st	October	59.7	Koji Yamamoto, CF
2nd	October	55.6	Taira Fujita, SS
3rd	October	54.7	Hiromitsu Ochiai, 1B
2nd	January	48.1	Hiromitsu Kadota, DH
1st	April	44.6	Hideki Matsui, CF
2nd	July	44.5	Keishi Suzuki, P
4th	October	39.0	Ichiro Suzuki, RF
3rd	January	38.8	Yoshihiko Takahashi, SS

Discussion

Another thing that needs to be mentioned is that the birth-month quarter starting in January is largely populated by pitchers and catchers. In my previous study, I found that more than a quarter of the players drafted as catchers were born between Jan. 1 and April 1.

When I first did this study, a number of people gave me what I’d snarkily call “baseball announcer explanations” for why players born from October to April 1 outperform the players who are chosen more often by pro teams. The most popular one of these was, “Oh, they’re used to overachieving, so they try harder.”

All these guys try hard. I think there are three things going on.

1. Accessibility
2. Age bias
3. Burnout

Accessibility

Players who are born after April 1 are larger and physically more developed than players months younger than they are. This gives them more time to play, more time to stand out and be noticed by coaches, who select them to play so that they can be seen by scouts.

Age bias

Because players born from October to April 1 are less physically developed than the players they are competing against, they are less likely to dominate competitions when scouts are watching.

Burnout

This is something that hadn’t occurred to me until recently. According to people who know a lot about how youth baseball functions in Japan, many of the players who eventually turn pro in Japan are not the best in their age groups when they are young. Amateur sports in Japan are intense, year-round, meat-grinding wars of attrition.

The best players typically become pitchers, and because competition (with the exception of university baseball) is in single-elimination tournaments, those aces throw game after game until their bodies break down. They are then surpassed by those who were a step behind them a year or two earlier.

Many of Japan’s best pitchers were not aces in elementary school or junior high. Masahiro Tanaka was a catcher until high school. Koji Uehara ran track in junior high and was an outfielder until his senior year in high school, when his school’s ace, Yoshinori Tateyama began to break down from injury.

It is not that much of a stretch, then, to see many of those players born from April to June as being at the end of their physical tethers by the time the pros call on them.

I know I’ve talked about this before

If we make a top-25 of players in NPB’s draft era, the best single draft round was the first round of the 1968 draft, Hall of Famers Hisashi Yamada, Koji Yamamoto, a player who has curiously been overlooked for the Hall, Michio Arito, and another who will eventually make it, Koichi Tabuchi.

The second best group are three from the Fab 4, the fourth round of the 1991 draft, Ichiro Suzuki, Tomoaki Kanemoto and Norihiro Nakamura.