Paper : Coaching

The Recent Trend of Relative Age Effect inJapanese Male Professional Soccer Players
Shigeki Matsuda* and Hiroaki Ishigaki***Faculty of Education, Shiga University
2-5-1, Hiratsu, Otsu, Shiga 520-0862 Japanmatsuda@edu.shiga-u.ac.jp**Department of Education, The University of Tokyo[Received March 3, 2023; Accepted August 8, 2023]

The details about relative age effect in Japanese male professional soccer players are not readily athand. This study clarifies the recent trend of relative age effect in Japanese male professional soccer
players using data from 2011 season to 2020 season. Our study had two analytical perspectives:
In analysis 1, we examined the relative age effect in all Japanese male professional soccer players
in each season from 2011 to 2020; in analysis 2, we studied the relative age effect in Japanese male
professional soccer players who newly registered in the league in each season from 2012 to 2020.
There were significant differences in the proportion of Q1 (April–June) –Q4 (January–March) of
every season in analysis 1, and Q1/Q4 were 2.0– 2.3. Effect sizes (ω) were 0.23–0.29. There weresignificant differences in the proportion of Q1–Q4 of every season except for the 2013 season in
analysis 2 and Q1/Q4 were 1.6–3.2. Effect sizes (ω) were 0.22–0.46. In conclusion, relative age effectin Japanese male professional players is continuously present in recent years and is equally present
in young soccer players who recently registered for the first time. The strength of the relative age
effects does not demonstrably change from the 2011 to the 2020 season because effect size did notconsistently increase or decrease.
Keywords: birthday, football, Japan[Football Science Vol. 20, 53-61, 2023]

1. Introduction
Relative age effect (RAE) is a variance causedby difference of birthdate within same age group(Pedersen et al., 2022; Musch and Grondin 2001;
Helsen et al., 2012). Roughly a maximum one-year
difference occurs depending on the birthdate among
the same age group and advantages and disadvantages
occur in various aspects due to the differences inphysical and psychological development. RAE has
been examined for many years, with some reports
focusing on schoolwork and sports (Cobley et al.,2009; Smith et al., 2018). Additionally, many studies
have been conducted on RAE of various athletes
who played soccer (Yagüe et al., 2018; Brustio etal., 2018), baseball (Katsumata et al., 2018; Nakata2017), basketball (Delorme et al., 2011), handball(Schorer et al., 2009), swimming (Cobley et al.,2018), track and field (Brazo-Sayavera et al., 2018;
Nakata and Sakamoto 2011), sumo (Nakata andSakamoto, 2011), rugby (Till et al., 2010), and alpine
skiing (Müller et al., 2015; 2017).
Football Science Vol. 20, 53-61, 2023
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Studies regarding RAE of soccer players have been
conducted worldwide (Sasano et al., 2020; Götze
and Hoppe 2020; Figueiredo et al., 2021; Vincent
and Glamser 2006; Delorme et al., 2009), with arecent study focusing on World Cup players (U-17,
U-19, senior) (Pedersen et al., 2022). Pedersen et al.
(2022) study found that the younger the age groupis, the more clearly the RAE appears, that RAE isstronger in males than in females, and that RAE
becomes stronger year by year. In this way, research
has clarified the recent trend of RAE in the world’s
top-level soccer players. Some methods to reduce
the RAE have been suggested in previous studies(Helsen et al., 2012; Mann and Ginneken, 2017).
For example, Helsen et al. (2012) proposed cut-off
rotation so that the disadvantaged players are notconstant. For example, the U-10 generation covers
9-year-old players born from January to Decemberand 10-year-old players born from April to December.
The U-12 age group covers 10-year-old players bornfrom January to March, 11-year-old players born fromJanuary to December, and 12-year-old players born
53

Matsuda, S. and Ishigaki, H.

from June to December. However, the effectivenessof this proposal has not been verified. Mann and
Ginneken (2017) reported that wearing shirts withthe player’s relative age removed selection bias whenscouting soccer players. However, because RAE
remains (Pedersen et al., 2022), these methods havenot been proven to be widely effective or efficient.
The RAE of soccer players firmly exists and, if
anything, is thought to be increasing in strength
worldwide rather than weakening (Pedersen et al.,2022). From the perspective that all soccer players
should be given equal opportunities regardless of
their birthdate, the problem of RAE in soccer players
is an important theme that must be examined further
in the future.
There are some extant studies about the RAE ofJapanese male professional soccer players (J league
soccer players). Uchiyama (1996), the first to study
the topic, reported that RAE was present in Japanesemale professional soccer players in the 1993 season
because the number of the players born from April
to June was 2.2 times more than that of players bornfrom January to March. Incidentally, the cutoff datein Japan is April. In a 2010 study of the RAE ofJapanese male soccer players (Nakata and Sakamoto,2011), the number of players in Q1 (April–June) was
34.7%, which was the highest quarter, and the numberof players in Q4 (January–March) was 14.6%,
the lowest quarter. The latest study about RAE ofJapanese male soccer players analyzed the data of the2018 season (Sasano et al., 2020) and found that the
number of players in Q1 was 33.1%, which was thehighest, and the number of players in Q4 was 16.1%,
the least. These studies show the existence of RAE
as far back as 1993 and still in existence in 2018
(Nakata and Sakamoto, 2011; Sasano et al., 2020;
Uchiyama, 1996). In addition, in a study conducted
more than 20 years ago (Musch and Hay, 1999),
it was reported that RAE of Japanese male soccer
players was stronger than that of Brazil, German, andAustralian professional soccer players. Therefore,
the RAE of Japanese male soccer players may havesome characteristics that differ from that of the othercountry’s soccer players; it is therefore necessary to
focus on RAE of Japanese male soccer players.
As stated above, because the study of the RAE ofsoccer players will be important in the future, it isnecessary to constantly update the latest informationand details to consider appropriate countermeasures.
The RAE of Japanese male soccer players was54

examined (Nakata and Sakamoto, 2011; Sasano etal., 2020; Uchiyama, 1996) in seasons of 1994, 2001,
2010, and 2018; this is a long investigation intervaland does not show the latest detailed trend of RAE.
The trend of RAE in Japanese male soccer players
changes over time, and in fact it was reported that
the RAE in soccer players who participated in theWorld Cup from 2006 to 2018 became stronger in theprevious study (Pedersen et al., 2022). In addition,
a 2003 project invited players born from January to
March to training camp in a search for good players
in Japan (Asahi Newspaper, 2014); these actions may
affect the RAE of Japanese male soccer players. By
investigating the RAE of the recent Japanese malesoccer players in detail, the recent trend of RAE will
be clear among the players who recently registered in
the Japanese male soccer league (J league).
This study aimed to clarify the recent trend of RAE
in Japanese male soccer players from the 2011 to the2020 season. There are two viewpoints of analysis:
Analysis 1 examined the RAE of all players who
registered in the J league in each season (2011–2020
season), while analysis 2 examined the RAE of
players who newly registered in the 2012–2020
season.

2. Methods
2.1. Players
Analysis 1
The players in analysis 1 were 12,858 J league
players who registered in each season from 2011 to2020 (the actual number of the players was 3,133).
There were 4,612 players in the J1 league, 5,681
players in the J2 league, and 2,565 players in the
J3 league. In addition, because the J3 league wasestablished in the 2014 season, J3 players were
included from the 2014 season. Because the players
who came from foreign countries were excluded fromthe analysis, all players were Japanese. The averageage of the players was 26.1 ± 5.0.
Analysis 2
The players in analysis 2 were 2,323 J league
players who newly registered in each season from2012 to 2020. There were 779 players in the J1
league, 849 players in the J2 league, and 695 players
in the J3 league (who were included from the 2014
season). Because the players who came from foreign
Football Science Vol. 20, 53-61, 2023
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The Recent Trend of RAE in Japanese Male Professional Soccer Players

countries were excluded from the analysis, all players
were Japanese. The average age of the players was
22.7 ± 4.4.

2.2. Data-collection methods
The data of the subject’s birthdate were obtained
from the J league official site (https://www.jleague.
jp/). The data of players lacking the birthdate
information and players who came from foreigncountries were excluded from the analysis. The datacollection period was from August 2021 to September
2021.

2.3. Statistical analysis
The same analyses were conducted in both analysis
1 and 2. At first, the number and proportion of theplayers in each month of the subject’s birth month
were calculated. As with the previous study (Muschand Hay, 1999), the Spearman-rank correlation
coefficient was calculated, and the significance was
examined to determine the relationship between the
difference of birth month and the number of players
in each month. Because the cutoff month is April inJapan, April is assigned “1” and March is assigned“12” in this analysis.
Following the previous studies (Nakata andSakamoto, 2011; Sasano et al., 2020), the birth monthof the players was classified into four groups: Q1(April–June), Q2 (July–September), Q3 (October–
December), and Q4 (January–March). The number
and the ratio of the players in each group were
calculated. In addition, a goodness-of-fit test was
conducted to examine the difference in the ratios
among from Q1 to Q4. The expected values were
calculated based on the birth month distribution ofJapanese male people at birth from the demographic
survey conducted by the Ministry of Health, Laborand Welfare of Japan. In analysis 1, the birth monthof Japanese born from 1985 to 1994 was indexed
because the average age of J league players from 2011
to 2020 was approximately 26.1 years old. As a result,the expected values were 24.9%, 26.2%, 24.8%,
and 24.1% in Q1, Q2, Q3, and Q4, respectively. In
analysis 2, the birth month of Japanese born from1990 to 1998 was indexed because the average ageof newly registered J league players from 2012 to2020 was approximately 22.7 years old. As a result,the expected values were 25.1%, 26.2%, 24.7%, andFootball Science Vol. 20, 53-61, 2023
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24.0% in Q1, Q2, Q3, and Q4, respectively.
A multiple comparison test was performed when asignificant difference was observed in the goodnessof-fit test with ω calculated as the effect size. The
effect size ω is judged to be small, moderate, andlarge, when ω is 0.10, 0.30, 0.50, respectively (Cohen
1988). Following the previous study (Sasano 2020),
Q1/Q4 was calculated to examine the differencein the ratios of Q1 and Q4. In addition, S1/S2 was
calculated to examine the difference between the sumof the ratios of Q1 and Q2 (S1) and the sum of theratios of Q3 and Q4 (S2). Statistical analysis software
IBM SPSS Statistics Version 27 and js-STAR wereused for analysis. The statistical significance level
was set at 5%.

3. Results
Analysis 1
Table 1 shows the numbers and ratios of eachmonth for birth month of the players in each season
as well as the Spearman’s rank correlation coefficient,which was −0.84 or higher and significant in allseasons. Figure 1 shows the ratio of each month forbirth month of the players in each season and reflects
part of Table 1. Every line in Figure 1 shows adownward trend.
Table 2 shows the numbers and ratios of Q1–
Q4 in each season, the results of the goodness-of-fit
test, effect sizes, and results of multiple comparison
tests. The goodness-of-fit test showed significant
differences in the ratios of Q1 to Q4 in all seasons.
A multiple comparison test showed significant
differences between Q1 and Q4 in all seasons. In the2014, 2015, 2016, and 2017 seasons, the results ofmultiple comparison tests were Q1 > Q2 > Q3 > Q4,
ω ranged from 0.23 to 0.29. Q1/Q4 was 2.0–2.3, andS1/S2 was 1.6–1.8.
Analysis 2
Table 3 shows the numbers and ratios of eachmonth for birth month of the players who newly
registered to the J league in each season as well as
the Spearman’s rank correlation coefficient. Except
for the 2013 season, the Spearman’s rank correlation
coefficient was −0.84 or higher, which was significant.
Figure 2 shows the ratio of each month for the birth
month of the players in each season and reflects partof Table 3. In general, every line in Figure 2 shows adownward trend.
Table 4 shows the numbers and ratios of Q1–Q4
55

Matsuda, S. and Ishigaki, H.

Table 1 The distribution of the subject’s birth month in each season
April
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020

May

June

July

AugustSeptember

OctoberNovember

DecemberJanuary

February

March

ｒ

p

-0.84*<0.01-0.93*<0.01-0.92*<0.01-0.94*<0.01-0.94*<0.01-0.94*<0.01-0.99*<0.01-0.95*<0.01-0.95*<0.01-0.91*

<0.01

Sum

n

87

105

70

86

76

91

74

54

63

43

45

45

839

%

10.4

12.5

8.3

10.3

9.1

10.8

8.8

6.4

7.5

5.1

5.4

5.4

100.0

n

106

107

89

85

83

100

77

51

72

51

43

46

910

%

11.6

11.8

9.8

9.3

9.1

11.0

8.5

5.6

7.9

5.6

4.7

5.1

100.0

n

108

110

79

83

81

93

73

59

68

56

42

52

904

%

11.9

12.2

8.7

9.2

9.0

10.3

8.1

6.5

7.5

6.2

4.6

5.8

100.0

n

148

148

120

107

104

120

93

80

85

67

61

69

1202

%

12.3

12.3

10.0

8.9

8.7

10.0

7.7

6.7

7.1

5.6

5.1

5.7

100.0

n

187

159

134

133

124

140

107

94

98

80

69

79

1404

%

13.3

11.3

9.5

9.5

8.8

10.0

7.6

6.7

7.0

5.7

4.9

5.6

100.0

n

183

156

130

131

117

134

95

93

86

80

75

78

1358

%

13.5

11.5

9.6

9.6

8.6

9.9

7.0

6.8

6.3

5.9

5.5

5.7

100.0

n

188

167

143

143

136

131

107

100

94

80

79

80

1448

%

13.0

11.5

9.9

9.9

9.4

9.0

7.4

6.9

6.5

5.5

5.5

5.5

100.0

n

186

183

155

172

132

150

102

105

101

83

84

87

1540

%

12.1

11.9

10.1

11.2

8.6

9.7

6.6

6.8

6.6

5.4

5.5

5.6

100.0

n

188

194

154

189

146

154

110

108

99

83

83

75

1583

%

11.9

12.3

9.7

11.9

9.2

9.7

6.9

6.8

6.3

5.2

5.2

4.7

100.0

n

196

183

165

197

168

169

129

112

112

85

82

72

1670

%

11.7

11.0

9.9

11.8

10.1

10.1

7.7

6.7

6.7

5.1

4.9

4.3

100.0

July

AugustSeptember

October

2014

2015

2016

Note. *: p<0.05

(%)

20.0
18.0
16.0
14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.0

April

May
2011

June
2012

2013

November December
2017

2018

January

February

2019

2020

March

Figure 1 The ratio for birth month of the players in each monthin new registered players in each season, test resultsof the goodness-of-fit test, effect sizes, and results ofmultiple comparison tests. The goodness-of-fit test
showed significant differences in the ratios of Q1–Q4
in all seasons except for the 2013 season. A multiple
comparison test revealed a significant difference
between Q1 and Q4 except for 2013; ω ranged from
0.16 to 0.45 Q1/Q4 was 1.6–3.2, and S1/S2 was 1.2–
2.3. Except for the 2013 season, ω was 0.31–0.45,
56

Q1/Q4 was 2.0–3.2, and S1/S2 was 1.6–2.3.

4. Discussion
Because the cutoff date is April in Japan, the resultsof Table 1 and 2 were obtained. It can therefore be
said that RAE was consistently observed in Japanesemale soccer players who registered in each seasonfrom 2011 to 2020. Previous studies have investigatedFootball Science Vol. 20, 53-61, 2023
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The Recent Trend of RAE in Japanese Male Professional Soccer Players

Table 2 The distribution of Q1–Q4 in each season
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020

Q1

Q2

Q3

Q4

Sum

n

262

253

191

133

839

%

31.2

30.2

22.8

15.9

100.0

n

302

268

200

140

910

%

33.2

29.5

22.0

15.4

100.0

n

297

257

200

150

904

%

32.9

28.4

22.1

16.6

100.0

n

416

331

258

197

1202

%

34.6

27.5

21.5

16.4

100.0

n

480

397

299

228

1404

%

34.2

28.3

21.3

16.2

100.0

n

469

382

274

233

1358

%

34.5

28.1

20.2

17.2

100.0

n

498

410

301

239

1448

%

34.4

28.3

20.8

16.5

100.0

n

524

454

308

254

1540

%

34.0

29.5

20.0

16.5

100.0

n

536

489

317

241

1583

%

33.9

30.9

20.0

15.2

100.0

n

544

534

353

239

1670

%

32.6

32.0

21.1

14.3

100.0

χ2

p

ω

Multiple Comparison test

Q1vsQ4

S1vsS2

43.3*

<0.01

0.23

Q1,Q2>Q3>Q4

2.0

1.6

60.0*

<0.01

0.26

Q1,Q2>Q3>Q4

2.2

1.7

48.1*

<0.01

0.23

Q1,Q2>Q3>Q4

2.0

1.6

80.8*

<0.01

0.26

Q1>Q2>Q3>Q4

2.1

1.6

93.3*

<0.01

0.26

Q1>Q2>Q3>Q4

2.1

1.7

90.8*

<0.01

0.26

Q1>Q2>Q3>Q4

2.0

1.7

98.3*

<0.01

0.26

Q1>Q2>Q3>Q4

2.1

1.7

108.5*

<0.01

0.27

Q1,Q2>Q3,Q4

2.1

1.7

129.9*

<0.01

0.29

Q1,Q2>Q3>Q4

2.2

1.8

135.5*

<0.01

0.28

Q1,Q2>Q3>Q4

2.3

1.8

ｒ

p

-0.87*

<0.01

-0.36

0.25

-0.94*<0.01-0.88*<0.01-0.94*<0.01-0.93*<0.01-0.84*<0.01-0.90*<0.01-0.85*<0.01

Note. *: p<0.05

Table 3 The distribution of the subject’s birth month in each season (newly registered players)

2012
2013
2014
2015
2016
2017
2018
2019
2020

April

May

June

July

AugustSeptember

OctoberNovember

DecemberJanuary

February

March

Sum

n

31

23

38

15

18

18

15

8

15

14

4

11

210

%

14.8

11.0

18.1

7.1

8.6

8.6

7.1

3.8

7.1

6.7

1.9

5.2

100.0

n

16

18

7

6

10

10

9

13

7

10

8

7

121

%

13.2

14.9

5.8

5.0

8.3

8.3

7.4

10.7

5.8

8.3

6.6

5.8

100.0

n

48

38

36

32

28

33

24

24

23

15

23

23

347

%

13.8

11.0

10.4

9.2

8.1

9.5

6.9

6.9

6.6

4.3

6.6

6.6

100.0

n

51

31

26

35

29

26

31

22

21

20

12

18

322

%

15.8

9.6

8.1

10.9

9.0

8.1

9.6

6.8

6.5

6.2

3.7

5.6

100.0

n

47

26

33

33

24

23

18

21

8

14

15

7

269

%

17.5

9.7

12.3

12.3

8.9

8.6

6.7

7.8

3.0

5.2

5.6

2.6

100.0

n

31

33

32

25

29

19

21

11

14

10

9

11

245

%

12.7

13.5

13.1

10.2

11.8

7.8

8.6

4.5

5.7

4.1

3.7

4.5

100.0

n

24

39

22

33

18

22

12

10

17

13

13

9

232

%

10.3

16.8

9.5

14.2

7.8

9.5

5.2

4.3

7.3

5.6

5.6

3.9

100.0

n

34

40

28

38

28

22

15

16

18

17

10

8

274

%

12.4

14.6

10.2

13.9

10.2

8.0

5.5

5.8

6.6

6.2

3.6

2.9

100.0

n

37

27

33

32

38

32

27

22

17

18

10

10

303

%

12.2

8.9

10.9

10.6

12.5

10.6

8.9

7.3

5.6

5.9

3.3

3.3

100.0

Note. *: p<0.05

the RAE of Japanese male soccer players in 1993,
2001, 2010, and 2018, and confirmed the existenceof RAE (Sasano et al., 2020, Nakata and Sakamoto,2011). The results of this study were consistent
with those of previous studies. However, previous
studies have only investigated four seasons withlong intervals, so it is significant that this study has
clarified the trends of RAE for the past 10 years,
including the latest information up to 2020. Soccer
is a very popular sport, and there are competitions
and selections from an early age. Competition is asFootball Science Vol. 20, 53-61, 2023
https://www.jssf.net/home.html

fierce in Japan as it is in many countries. In young
age groups, physiques differ greatly depending on
the month of birth, so players born between January
and March have some disadvantage at the selection.
A clear RAE has been seen for soccer players who
belong to high-level academies overseas (Carling,2009; Kelly et al., 2020), and it has also been seenfor players enrolled in J league academies (Hirose,2009). Thus, players born between January andMarch often miss the chance to play at a high level in
their youth. Differences in playing opportunities and57

Matsuda, S. and Ishigaki, H.
(%)

20.0
18.0
16.0
14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.0

April

May

June

2012

July

2013

August2014

September
2015

October

2016

November December

January

February

2017

2019

2020

2018

March

Figure 2 The ratio for birth month of the players in each month (newly registered players)
Table 4 The distribution of Q1–Q4 in each season (newly registered players)
2012
2013
2014
2015
2016
2017
2018
2019
2020

Q1

Q2

Q3

Q4

Sum

n

92

51

38

29

210

%

43.8

24.3

18.1

13.8

100.0

n

41

26

29

25

121

%

33.9

21.5

24.0

20.7

100.0

n

122

93

71

61

347

%

35.2

26.8

20.5

17.6

100.0

n

108

90

74

50

322

%

33.5

28.0

23.0

15.5

100.0

n

106

80

47

36

269

%

39.4

29.7

17.5

13.4

100.0

n

96

73

46

30

245

%

39.2

29.8

18.8

12.2

100.0

n

85

73

39

35

232

%

36.6

31.5

16.8

15.1

100.0

n

102

88

49

35

274

%

37.2

32.1

17.9

12.8

100.0

n

97

102

66

38

303

%

32.0

33.7

21.8

12.5

100.0

χ2

p

ω

Multiple Comparison test

Q1vsQ4

S1vsS2

42.6*

<0.01

0.45

Q1>Q2，Q3，Q4

3.2

2.1

5.37

0.15

0.16

1.6

1.2

22.7*

<0.01

0.33

Q1>Q3，Q4

2.0

1.6

19.7*

<0.01

0.31

Q1，Q2>Q4

2.2

1.6

41.8*

<0.01

0.45

Q1，Q2>Q3，Q4

2.9

2.2

38.4*

<0.01

0.43

Q1，Q2>Q3，Q4

3.2

2.2

28.5*

<0.01

0.37

Q1，Q2>Q3，Q4

2.4

2.1

39.5*

<0.01

0.43

Q1>Q2>Q3，Q4

2.9

2.3

30.0*

<0.01

0.38

Q1，Q2>Q3>Q4

2.6

1.9

Note. *: p<0.05

environments among youth because of differences inbirthdays are some of the factors that result in RAE in
J-League players.
In this study, Q1/Q4 was 2.0–2.3 (Table 2). In theprevious study investigating the RAE of Japanesemale professional soccer players (Sasano, 2020),
Q1/Q4 was 2.2 in 1993, 2.5 in 2001, 2.4 in 2010,
and 2.1 in 2018. Although the Q1/Q4 values wereslightly low compared to 2001, no remarkable Q1/
Q4 differences were observed between this study and58

a previous study (Sasano 2020). In this study, S1/
S2 was 1.6–1.9. The previous study (Sasano, 2020)
found 2.0 in 1993, 2.0 in 2001, 1.7 in 2010, and 1.7
in 2018. Although the S1/S2 values in this study wereslightly lower than in 1993 and 2001, no substantial
S1/S2 differences were observed between this study
and a previous study (Sasano, 2020). Sasano (2020)
analyzed Japanese male soccer player RAEs in 1993,
2001, 2010, and 2018 and found that the RAE had
weakened in recent years because the effect size had
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The Recent Trend of RAE in Japanese Male Professional Soccer Players

decreased. Since there is no major difference between
the results of Q1/Q4 and and S1/S2 obtained in thisstudy and those found in a previous study (Sasano,2020), it would be difficult to state that this result
supports the results of the previous study (Sasano,2020), which found that the RAE had weakened inrecent years. Looking at the effect size from 2011
to 2020 in Table 2 of this study, the effect size did
not change significantly, ranging from 0.23 to 0.29.
Moreover, as shown in Table 4, looking at the data
for newly registered Japanese male soccer players
from 2012 onwards, although the effect size fluctuatesslightly from 0.22 to 0.46, there is no consistent
trend of decrease or increase. There is no perceptiblechange in the magnitude of the RAE over the 10 years
after 2011.
In this study, the RAE of newly registered Japanesemale soccer players from 2012 to 2020 was examined
to further clarify the recent trends in RAE. Except forin 2013, RAE was observed even in newly registeredplayers. Regarding the results of the multiple
comparison tests, most of the results for all registered
Japanese male soccer players were Q1 > Q2 > Q3 >Q4 and Q1, Q2 > Q3 > Q4 (Table 2), but the resultsof the multiple comparison test for newly registered
Japanese male soccer players slightly varied, and no
significant difference was found between Q3 and Q4(Table 4). The statistically significant differences
are affected by the number of players. Because thenumber of newly registered players was lower than
the total number of registered players, the differencein the number of total registered players and newly
registered players was considered one of the factorsthat caused the differences among the results of themultiple comparison tests. However, Q1/Q4 for newly
registered Japanese male soccer players is between
2.0 and 3.2 (excluding 2013), and there are several
seasons when it is close to 3.0. Because the Q1/Q4 of
all registered players was 2.0–2.3, it can be asserted
that the Q1/Q4 of newly registered Japanese malesoccer players is slightly higher. S1/S2 is 1.6–2.3
for newly registered Japanese male soccer players(excluding the 2013 season), and there are values
greater than 1.6–1.9 for all registered Japanese malesoccer players. From these results, it can be said that
there is a clear RAE even for relatively young players
who newly registered between the 2012 and 2020
seasons.
Although the exact reason why there was no
significant difference in the proportions from Q1 toFootball Science Vol. 20, 53-61, 2023
https://www.jssf.net/home.html

Q4 in 2013 season is unclear, it may be due to theimplementation of the club license system in the J
League from February 1, 2012. It is possible that it
was more difficult than usual to contract new players
because each club prioritized efforts to meet thelicense standards. In fact, there were only 121 new
players who were registered to the J League in 2013,
which was less than half of most other seasons. This
small number of players may have influenced theresults of the 2013 season.
Aside from the 2013 season, RAE is clearlypresent in the other seasons, but as mentioned above,
when looking at the effect size, there is no constanttrend of increase or decrease. It can therefore be
asserted that there is no change in the strength of the
RAE in the nine seasons from 2012 to 2020 even for
newly registered Japanese male soccer players. In
the previous study that analyzed the RAE of soccer
players (U-17, U-19, seniors) who participated in
the World Cup (Pedersen at al., 2022), the effect size
gradually increased (1997–2019 for U-17 and U-19,
and 2006–2018 for seniors). It is considered that
RAE is becoming more pronounced year by year,
due to the fact that many teams, mainly top clubs,
are actively scouting and selecting players from ayoung age. Based on the analysis of the effect size,
no trend toward a stronger RAE for Japanese malesoccer players was observed in the present results.
Therefore, the tendency of the RAE of Japanese malesoccer players may be slightly different from that of
the world as a whole.
Since the 2003 recruiting project in Japan thatspecifically invited players born in January to March
to a camp (Asahi Newspaper, 2014), attention has
been paid to the bias in the birth months of soccer
players worldwide, including in Japan. Such actionand efforts may affect the results of this study.
Alternatively, January is used as a cutoff date inaccordance with international standards for theplayers of the Japanese national team. As a result,because of the April cutoff date, the RAE may have
weakened. It is not clear why there is no change in the
RAE, but in any case, it is important that it remains
present in Japanese male soccer players. Although
the trend of the RAE of Japanese male soccer players
may be different from that of the world at present, it
may change in the future. This study analyzed only
up to the 2020 season. It is necessary to investigate
the actual detailed situation of the RAE in the future.
Regarding the relative age effect, it has been
59

Matsuda, S. and Ishigaki, H.

noted that later-born players among the same agegroup selectively drop out (Williams and Reilly,2000), and research has primarily been conductedfrom the perspective of unequal opportunities
for children to play sports. In addition, from theperspective of improving competitiveness, there isconcern that excellent soccer players who are far
from the cutoff date may be overlooked. For thesereasons, several proposals have been made to reduce
the RAE of soccer players (Helsen et al., 2012;
Mann and Ginneken, 2017). To also identify good
players in Japan, a 2003 project invited players bornfrom January to March to a training camp (Asahi
Newspaper, 2014). Although research on the RAE ofsoccer players has been conducted for many years andseveral countermeasures considered, there is still an
RAE in the top-level football countries of the world,which is stronger in more recent studies (Pedersen
et al., 2022). Pedersen et al. (2022) concluded thatthese findings were because of the increased physicalburden being placed on soccer players and thedevelopment of stronger scout systems to acquire
young players, especially in top soccer clubs. As
it is expected that the playing intensity of soccer
will continue to increase in the future (Nassis et al.,2020), there is a high possibility that competitivenesswill increase further. Therefore, the competition
for young soccer players with good physiques isunlikely to reduce; it is unlikely that RAE will
go away any time soon. It would be necessary toinvestigate the reality of RAE in soccer players in
the future. From the standpoint of providing equal
opportunities for children to play sports (soccer),
developing soccer players, and spreading soccer, it isimportant to discuss the development of appropriate
countermeasures for RAE. The results of this study
could be useful in guiding future soccer player RAE
research and developing RAE countermeasures.
In conclusion, the RAE of Japanese male
professional soccer players (players registered ineach season) has been observed continuously fromthe 2011 to the 2020 seasons. The proportion in Q1
has ranged from 2.0 to 2.3 times compared with theproportion in Q4. For players newly registered after
the 2012 season, except for the 2013 season, RAE
has been seen in the 2012–2020 season. Except for
2013, the Q1 rate has ranged from 2.0 to 3.2 times
compared with the proportion ion Q4. From 2011
to 2020, because there is no constant increase or
decrease in the seasonal changes of effect size for60

both registered players in each season and newly
registered players, it can be concluded that there is no
significant change in the strength of the RAE.
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Name:
Shigeki Matsuda
Affiliation:
Faculty of Education, Shiga University

Address:
2-5-1 Hiratsu, Otsu, Shiga 520-0862 Japan
Brief Biography:
2006-2010 Graduate School of Natural Science & Technology,
Kanazawa University
2007-2013 Lecturer, Gifu Shotoku Gakuen University
2013-2014 Associate Professor, Gifu Shotoku Gakuen University
2014-2019 Associate Professor, Shiga University
2019- Professor, Shiga University
Main Publications:• M atsuda, S., Demura, S., Uchiyama, M. (2008). Centre ofpressure sway characteristics during static one-legged stance of
athletes from different sports, Journal of Sports Sciences, 26(7):775-779.
• M atsuda, S., Demura, S., Nagasawa, Y. (2010). Static onelegged balance in soccer players during use of a lifted leg.
Perceptual and Motor Skills, 111(1): 167-177.
• Matsuda, S., Nagasawa, Y., Ishihara, T., Demura, T., Komura,
K. (2015). Is it possible to improve collegiate soccer players’
jump ability? ―A comparison of soccer and volleyball players’
jump height, arm swing, and body crouch in vertical and header
jumps, Football Science, 12: 1-10.
• Matsuda, S., Ishigaki, H. (2023). Trends in relative age effects
of top-level female soccer players: A Japanese study, Perceptualand Motor Skills, 130(3): 984-998.
Membership in Learned Societies:• Japan Society of Physical Education, Health and Sport Sciences• J apanese Society of Test and Measurement in Health andPhysical Education• Japanese Society of Science and Football• Japan Society of Human Growth and Development61