Relationship Between the Relative Age Effect and Lengths of Professional Careers in Male Japanese Baseball Players: a Retrospective Analysis
Nakata Sports Medicine - Open
Relationship Between the Relative Age Effect and Lengths of Professional Careers in Male Japanese Baseball Players: a Retrospective Analysis
Hiroki Nakata 0
0 Department of Health Sciences, Faculty of Human Life and Environment, Nara Women's University , Kitauoya-Nishi Machi, Nara city 630-8506 , Japan
Background: The mechanisms underlying the relative age effect in sport events have been investigated for more than two decades. The present study focused on the relationship between the relative age effect and lengths of professional careers among professional male Japanese baseball players. Methods: The birth dates of players and lengths of professional careers were collected from an official publication, and data were divided into four quarters (Q1: April-June; Q2: July-September; Q3: October-December; Q4: January-March of the following year) grouped by 3 years. Based on the data for Q4, the expected numbers for the lengths of professional careers were calculated for Q1, Q2, and Q3. Results: The number of players with professional careers of more than 19 years was significantly smaller in Q4 than in Q1, Q2, and Q3. Conclusions: The relative age effect among professional male Japanese baseball players was associated with the lengths of professional careers. Relative age appears to be a very important factor for the development of expertise among male Japanese baseball players and involves long-term disadvantages after becoming professional players.
RAE; Baseball; Japan; Relative age effect
The relative age effect is regarded as a contributing
factor to sporting success. For example, the Federation
Internationale de Football Association (FIFA) uses a
system for youth soccer with January 1 as the cut-off
date to establish its age groups. Within the same age
category, a difference of almost one full year may exist
between the oldest and youngest participants. Therefore,
relatively older children within a particular age group
are more likely to achieve sporting success. This
phenomenon has been called the relative age effect.
Relatively older children have advantages in growth,
biological maturity, and cognitive development . In
addition, relatively older children (athletes) have a greater
opportunity to participate in competitions and,
consequently, may enhance their psychological, technical, and
tactical abilities, thereby supporting greater athletic
development . The relative age effect has been confirmed in
many types of sports, including baseball [3, 4], soccer [5–8],
tennis , cricket , basketball [4, 11], NASCAR ,
sumo wrestling , rugby , judo , ice hockey
[14–17], and winter sports [18–21].
© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
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the Creative Commons license, and indicate if changes were made.
The birth dates of players and lengths of professional
careers were collected from an official publication .
Data from professional male Japanese baseball players
(N = 4218 males) who played in Nippon Professional
Baseball (NPB) and were born between 1911 and 1980
were analyzed. Baseball players were divided into four
groups based on their month of birth: Q1 (April–June),
Q2 (July–September), Q3 (October–December), and Q4
(January–March of the following year). Chi-squared tests
were applied to each group according to the four
quarters in order to assess the significance of deviations from
the expected number of births in each quarter. The
Table 1 Number of players divided into lengths of professional
careers based on quarters of the birth year
Moreover, several studies have examined the relative
age effect from a historical perspective [8, 16, 22–25]. It
generally takes several years or decades for a sport to
gain popularity in a given country. Thus, historical
analyses are needed in order to clarify the beginning of the
relative age effect in a country and compare differences
in the skew of this effect among generations.
The present study focused on how long the relative
age effect continues into adulthood because most studies
have focused on junior players, while, to the best of our
knowledge, only a few studies have examined this topic.
We previously reported that the relative age effect
persisted among players older than 22 years of age when,
theoretically, no physical advantage is expected for older
players . The relative age effect has been
demonstrated in professional athletes who graduated university
(college) at 22 years old; however, this relationship was
weaker than that among those who graduated high
school at 18 years old. Steingröver and colleagues 
recently investigated whether relative age influenced
career lengths in the National Basketball Association
(NBA), National Hockey League (NHL), and National
Football League (NFL). They showed that the number of
matches played was significantly larger in relatively
younger players than in relatively older players in the
NHL. No significant differences were observed in career
lengths in the NBA or NFL between relatively younger
and older players.
The present study examined the relationship between
the relative age effect and lengths of professional careers
among professional male Japanese baseball players.
Steingröver and colleagues  reported significant
differences in career lengths between relatively younger
and older players in the NHL; however, this relative age
effect needs to be confirmed in other countries if
universal factors are truly related to this effect. In other words,
even if a significant relative age effect is observed in a
country, the popularity and system of a sport differ
among sports and countries. In Japan, a unique
annualage grouping has been applied since 1886, which is
between April 1 and March 31 of the following year.
Therefore, April 1 is the beginning of the “new year”
(i.e., cut-off date), and this specific calendar follows an
education system including elementary, junior high, and
senior high schools and university (college), government,
and companies. Sports calendars also follow this system.
Thus, players born in April, May, and June are expected
to have a relative age advantage. Grondin and Koren
 reported that the relative age effect for baseball was
more important in Japan than in the USA because large
numbers of Japanese players were born during Q1
(April–June). Based on these backgrounds, a relative age
effect was hypothesized to exist on the lengths of
professional careers among Japanese professional male players.
Table 2 Distribution of all players and the general population divided into quarters
The number in the second row shows the expected number of players obtained using the chi-squared test based on the number of the general population.
n number of players, X2 chi2 value, w effect size
expected distribution was calculated based on national
birth statistics for males during 1911–1980 in Japan
(Ministry of Internal Affairs and Communications, the
Statistics Bureau and the Director-General for Policy
Planning of Japan). Professional male Japanese baseball
leagues start in April and finish in October, with players
being drafted in November. Foreign players were
excluded because they had not passed through the
Japanese school system.
Data for each quarter were then grouped by 3 years
and categorized into seven groups (i.e., 1–3, 4–6, 7–9,
10–12, 13–15, 16–18, and 19 years). The reason for
grouping by 3 years was that the sample size comprised
more than 100 baseball players in each career year
group. If data were grouped by 5 years (ex. 1–5, 6–10,
11–15, 16–20, and 21 years), the sample size for 21 years
was less than 100 (Table 1). After grouping, based on
the data of Q4, the expected numbers for the lengths of
professional careers were calculated for Q1, Q2, and Q3.
In this analysis, chi-squared tests were applied to Q1,
Q2, and Q3 according to the lengths of professional
careers in order to assess the significance of deviations
from the expected number for the lengths of
professional careers in Q4. Statistical tests were performed
using computer software (SPSS for windows ver. 22.0).
Significance was set at p ≤ 0.05.
Table 2 shows the birth date distribution of male
baseball players and the general population. Chi-squared
tests revealed significant relative age effects, indicating
that the percentage of relatively older players in Q1 was
Table 3 shows the results of chi-squared tests for the
lengths of professional careers in seven groups of 3 years.
The distributions of Q1, Q2, and Q3 were significantly
different from that of Q4 (Q1: p < 0.001, effect size 0.168;
Q2: p < 0.001, effect size 0.156; Q3: p < 0.001, effect size
0.147). In addition, the distribution in six groups (i.e., from
1–3 to 16–18 years) was similar among Q1, Q2, Q3, and
Q4. On the other hand, this difference was more
prominent in those with a professional career spanning more
than 19 years, showing that the number of baseball players
with professional careers of more than 19 years was
markedly smaller in Q4 than in Q1, Q2, and Q3 (Q1: X2 = 19.8;
Q2: X2 = 8.9; Q3: X2 = 14.0).
The present study investigated the relative age effect on the
lengths of professional careers among male Japanese
professional baseball players. The results revealed that the
number of players with professional careers of more than
19 years was markedly smaller in Q4 than in Q1, Q2, and
The number in the second row shows the expected number of players calculated from the number of players in Q4 (Jan–Mar)
X2 chi2 value, w effect size
Table 3 Number of players grouped by 3 years for lengths of professional career
Q3. In addition, relative age was found to be a very
important factor for the development of expertise among Japanese
male baseball players and may involve long-term
disadvantages after becoming professional players. In other words,
even if relatively younger players became professional
players, their talent may not be sufficient to continue for a
long career such as more than 19 years, or they may be
more likely to drop out of a professional career. This result
was in contrast to previous findings showing the absence of
a relative age effect in the NBA and NFL, and the favoring
of relatively younger players in the NHL .
It is difficult to explain why relatively older baseball
players have longer professional careers. One explanation
is disadvantages in childhood. Relatively older players may
have greater opportunities for selection and experience in
childhood because they are naturally heavier, taller,
stronger, and faster; have greater endurance; and are more
coordinated than younger players during childhood , all
of which translate into performance advantages in most
sports . This may lead to more long-term advantages
for relatively older players in adulthood because of the
development of self-confidence in childhood; however, these
advantages are expected to become less apparent towards
adulthood when physical maturity evens out. A second
explanation is that this phenomenon is specific to Japan
and Japanese professional male baseball players because
many activities related to sports and academics are based
on a unique cut-off date (April 1), which is not the case in
other countries. Furthermore, as described in the
“Background” section, the relative age effect for baseball is more
important in Japan than in the USA . These
possibilities may interact. Further studies are needed in order
to elucidate the mechanisms responsible for this
phenomenon in more detail.
As a limitation of the present study, the lengths of
professional careers may be associated with many factors
other than the relative age effect. Baker and colleagues
 reported that career lengths in Major League
Baseball (MLB) were longer for infielders than for outfielders
and catchers. Koz and colleagues  also showed a
significant negative relationship between the draft round
and games played in the NHL, NBA, and NFL and
fielding players in MLB. However, the present study did
not focus on the playing position or draft round. These
factors need to be examined in more detail in future
studies. In addition, in the present study, the significant
relationship between the relative age effect and lengths
of professional careers was only observed in Japanese
male baseball players. Thus, this relationship needs to be
examined in other sports including soccer, volleyball,
Ekiden (a long-distance relay running race on roads),
basketball, and sumo wrestling because these sports
include a significant relative age effect among Japanese
male athletes .
The results of the present study provide additional
information for elucidating the mechanisms underlying the
relative age effect in professional sports. Our results
suggest that the relative age effect in professional sports
may be related to the lengths of professional careers.
Hiroki Nakata declares that no financial support was received for the conduct
of this study or preparation of this manuscript.
Hiroki Nakata declares that he has no conflict of interest.
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
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