Relative Age Effects in Japanese Athletes

Home , Baseball in Japan

J Phys Fitness Sports Med, 3(5): 467-476 (2014) DOI: 10.7600/jpfsm.3.467 JPFSM: Review Article Relative age effects in Japanese athletes

Hiroki Nakata1* and Kiwako Sakamoto2

1 Department of Health Sciences, Faculty of Human Life and Environment, Nara Women’s University, Kitauoya-Nishi Machi, Nara City, Nara 630-8506, Japan 2 Department of Integrative Physiology, National Institute for Physiological Sciences, 38 Nishigo-Naka, Myodaiji, Okazaki, Aichi 444-8585, Japan

Received: September 26, 2014 / Accepted: October 21, 2014 Abstract ‘Relative age effect’ (RAE) is considered a factor in successful sporting activities. That is, older children in a particular age group are more likely to achieve sporting success than younger ones. Many studies on RAEs have been conducted for the sports systems in North American and European countries. In this article, we elaborate on the RAEs in Japanese athletes of both male and female sports, the association of RAEs with number of years in school and with legal-gambling sports, and give a historical analysis of Japanese baseball. Since 1886, Japan has applied a unique age-grouping for education, which groups students with birthdays from April 1 to March 31 of the following year into the same age category. Therefore, April 1 is the beginning of each ‘new year’, and this specific calendar applies to the education system including elementary, junior high and senior high schools, university (college), as well as to government and private companies. Sports calendars also follow this system, and many professional sports such as baseball, soccer, basketball, and volleyball start the new season in April. Therefore, players born in April, May, and June should have a relative age advantage. We in- troduce the characteristics and phenomenon of RAEs in Japanese athletes, and discuss several problems that should be addressed in future studies. Keywords : relative age effect (RAE), Japan, sports, baseball

formance advantage in most sports11-13). Introduction Several factors contribute to becoming an elite athlete, RAE in Japanese male sports including genetics, training, nutrition, family background, and socio-cultural influences. For over two decades, the In our first study, we focused on the RAE in Japanese ‘relative age effect’ (RAE) has been considered a factor male sports14). Japan employs a unique annual age-group- in successful sporting activities. That is, older children in ing policy in sports and education, which is from April a particular age group are more likely to achieve sporting 1 to March 31 of the following year. To our knowledge, success than younger ones. RAEs have been confirmed however, only one study has examined RAEs in Japanese in some male sports such as soccer1,2), ice hockey3), sports. Hirose2) reported the existence of an RAE in elite baseball4), cricket5), handball6), basketball7), tennis8), and youth soccer players. We investigated RAEs in 12 dif- NASCAR9). The difference is greatest at almost one year, ferent male sporting events in Japan simultaneously (i.e. which is typical of the public school system in most coun- baseball, soccer, basketball, volleyball, handball, golf, tries, associated with significant differences in the cogni- horse riding, rugby, American football, sumo, Ekiden, and tive development of children10). badminton). Sumo originated in Japan, the only country According to Musch and Grondin10), several possible where it is practiced professionally. Ekiden is a long- factors and mechanisms contribute to RAEs, including distance relay running race on roads, and also originated competition. ‘Competition will come from the number in Japan. Professional horse racing was included in the of players available for the places, and this number will study, because it was assumed that the skew (distribution) depend on the popularity of a given sport in a given coun- of the RAE among jockeys would be different from that try’. Thus, the level of competition is associated with the in other sports, owing to the weight restrictions of becom- popularity of the sport. A second factor is physical devel- ing a jockey in Japan. The birthdates of the athletes were opment. Attributes of greater height, mass, aerobic power, collected from official websites and published books. muscular strength, endurance and speed do provide a per- Table 1 presents the results of chi-square tests in each sport, and the distribution of the general population in Ja- *Correspondence: [email protected] pan. Significant RAEs were observed in baseball, soccer, 468 JPFSM: Nakata H and Sakamoto K volleyball, Ekiden, basketball, sumo, and horse racing. (April-June) was the quarter with the most athletes’ birth- The percentage of relatively older players in the first quar- dates, there were no significant RAEs in handball, rugby, ter (Q1) was higher in baseball, soccer, volleyball, bas- badminton, American football, or golf, indicating that not ketball, and sumo. By contrast, the skew among jockeys all sporting events involve an RAE effect. These results was clearly different from that in any other sport, showing were similar to the findings of Côté and colleagues15), that the percentage of relatively older riders was lower in who investigated the sports of ice hockey (National Hock- Q1 and higher in Q4. There were no statistically relevant ey League), basketball (National Basketball Association), differences in handball, rugby, badminton, American foot- baseball (Major League Baseball), and golf (Professional ball, or golf. The general population in Japan showed a Golfers Association) in the USA and Canada, and report- symmetrical distribution in each quarter: Q1: 24.9%, Q2: ed significant RAEs for ice hockey and baseball players, 26.3%, Q3: 24.7%, and Q4: 24.1% (Table 1). but not for golfers and basketball players. A significant RAE was found not only in major sports Jockeys also showed a significant difference in the played in many countries, such as soccer, baseball, bas- distribution of birthdates, but the skew was clearly differ- ketball, and volleyball, but also in uniquely Japanese ent from that of other sports. That is, many jockeys were sports such as sumo and Ekiden. Our findings indicated born in Q4 (January-March), a disadvantage to sport- that even though in Japan RAEs were observed, and Q1 ing success in terms of the RAE for other sports. This is

Table 1. Distribution of birth dates in Japanese male athletes

Q1 (Apr-Jun) Q2 (Jul-Sep) Q3 (Oct-Dec) Q4 (Jan-Mar) Total X2 p Cramer's phi

N effect size

Baseball 240 (32.8%) 212 (29.0%) 175 (23.9%) 104 (14.2%) 731 57.53 < .001 0.281

182 183 183 183

Soccer 331 (34.7%) 273 (28.6%) 212 (22.2%) 139 (14.6%) 955 86.07 < .001 0.300

238 239 239 239

Volleyball 39 (29.3%) 43 (32.3%) 28 (21.1%) 23 (17.3%) 133 8.44 < .05 0.252

33 33 33 34

Ekiden 79 (24.6%) 83 (25.9%) 99 (30.8%) 60 (18.7%) 321 10.08 < .05 0.177 80 80 80 81

Basketball 48 (30.4%) 44 (27.8%) 35 (22.2%) 31 (19.6%) 158 14.02 < .001 0.298

59 59 60 60

Handball 41 (31.3%) 32 (24.4%) 32 (24.4%) 26 (19.8%) 131 4.08 0.176

32 33 33 33

Sumo 181 (28.3%) 164 (25.7%) 163 (25.5%) 131 (20.5%) 639 8.46 < .05 0.115 159 160 160 160

Rugby 116 (24.2%) 142 (29.6%) 121 (25.3%) 100 (20.9%) 479 7.45 0.125

119 120 120 120

Badminton 40 (27.4%) 36 (24.7%) 39 (26.7%) 31 (21.2%) 146 1.53 0.102

36 36 37 37

American Football 73 (22.0%) 102 (30.7%) 83 (25.0%) 74 (22.3%) 332 6.53 0.140

83 83 83 83

Golf 35 (25.4%) 34 (24.6%) 37 (26.8%) 32 (23.2%) 138 0.4 0.054

34 34 35 35

Horse Racing 19 (12.3%) 32 (20.6%) 49 (31.6%) 55 (35.5%) 155 19.88 < .001 0.358

38 39 39 39

(N = 4318)

General population 2,250,909 (24.9%) 2,381,282 (26.3%) 2,235,355 (24.7%) 2,178,901 (24.1%) 9,046,447

The numbers in each second row show the predicted number of players obtained using the chi-square tests. Adapted from Nakata & Sakamoto (2011)14). JPFSM: RAEs in Japanese athletes 469 a novel finding, suggesting an advantage for relatively RAEs in French professional champion-level female and younger children. Similar cases were reported for dance male players in three sports (handball, soccer, and basket- and gymnastics, where physical attributes are not impor- ball). They found no statistically significant RAEs in the tant, or even possibly disadvantageous, and late puberty female or male athletes, but their research methods, which is an advantage16). This phenomenon was defined as the examined the RAEs in female and male sporting events maturation-selection hypothesis12). simultaneously in a given country, are useful to clarify the The strength of this study was the inclusion of multiple mechanisms of RAEs, including the degree of popularity samples from 12 different sports in the same country si- of the sport and the prevalence of physical attributes for multaneously, which has not been reported previously. success. Firstly, we investigated the relationship between popularity and RAEs in sports in Japan. If popularity is a RAE in Japanese female sports major factor in the prevalence of RAEs, we expected Several recent studies have focused on gender differ- statistically significant RAEs in badminton, volleyball ences in RAEs, but the results remain a matter of debate. and basketball among females, and in baseball, soccer Some researchers reported that RAEs for females were and basketball among males, because these sports are not statistically significant in tennis17), dance18), ice hock- the most popular among teenage girls and boys in Japan, ey19), basketball20), handball20), shooting16), and soccer21), respectively26). Secondly, we expected the involvement of while others suggested significant RAEs in tennis8), soc- other factors, including height and body composition, if cer22), and volleyball23). Possible explanations for differ- RAEs were not found in these sports. ences in RAEs between female and male athletes might Table 2 presents the frequency of birthdates and chi- be related to less competition among girls and differences square tests in each sport, and the distribution of the gen- in physical development24). If the popularity of a sport is eral population in Japan. Significant RAEs for females greater among boys than girls, strong RAEs would be ob- were observed in volleyball, but not in the other sports. served among the males, compared to the females. More- A tendency for a biased distribution was found in basket- over, since girls mature faster than boys physically and ball, with a rate of less than 20% for Q4 and a phi effect biologically, the prevalence of RAEs would be different size of 0.193. among female and male athletes. In this research, since a statistically significant RAE for Our second study focused on the RAE for Japanese female athletes was found only in volleyball, popularity females in six sporting events (i.e. softball, soccer, bas- would not be directly associated with a biased distribu- ketball, volleyball, track and field, and badminton)25). tion of birthdates. The RAE in female athletes differed Delorme and colleagues24) researched the prevalence of from that in male athletes (Table 1). In female athletes, it

Table 2. Distribution of birth dates in Japanese female athletes Q1 (Apr-Jun) Q2 (Jul-Sep) Q3 (Oct-Dec) Q4 (Jan-Mar) Total X2 p Cramer's phi

N effect size

Softball 136 (25.7%) 152 (28.7%) 131 (24.7%) 111 (20.9%) 530 3.67 0.083

131 139 132 128

Soccer 65 (27.3%) 61 (25.6%) 62 (26.1%) 50 (21.0%) 238 1.93 0.090

59 63 59 58

Volleyball 46 (33.3%) 48 (34.8%) 22 (15.9%) 22 (15.9%) 138 16.14 < .001 0.342

34 36 34 33

Basketball 47 (27.3%) 54 (31.4%) 42 (24.4%) 29 (16.9%) 172 6.44 0.193

42 45 43 42

Track & Field 32 (25.8%) 36 (29.0%) 25 (20.2%) 31 (25.0%) 124 1.50 0.110

31 33 31 30

Badminton 24 (18.0%) 41 (30.8%) 34 (25.6%) 34 (25.6%) 133 3.64 0.165

33 35 33 32

(N = 1335)

General population 2,113,573 (24.7%) 2,251,738 (26.3%) 2,127,473 (24.9%) 2,068,122 (24.2%) 8,560,906

The numbers in each second row show the predicted number of players obtained using the chi-square tests. Adapted from Nakata & Sakamoto (2012)25). 470 JPFSM: Nakata H and Sakamoto K is likely that other factors are involved in volleyball and, also revealed biased distributions of birthdates in high to a lesser extent, in basketball. Volleyball and basketball school and college baseball players. The effect size of the require players to be tall in the components of the game RAE among baseball players was larger in high school that encourage jumping. It seems here that two combined than college graduates. factors, popularity and the need for height, might explain We consider the differences in the RAE between high the RAEs of Japanese female athletes. school and university (college) graduates to constitute im- In a future study, it would be interesting to examine portant information with which to clarify the mechanisms whether RAEs in Japanese female athletes change with of the RAE effect. In Japan, most sports, including soccer time. Japan won the women’s soccer World Cup in 2011 and baseball, start from April and end in March of fol- and the AFC Women’s Asian Cup in 2014. We infer that lowing year. Thus, we expected the April quarter groups if female soccer becomes popular, RAEs will occur in the to have an advantage over those born later in the year. future. To test this hypothesis, a longitudinal study will be Grondin & Koren28) reported that the RAE for baseball is needed. much more important in Japan than in the United States, because large numbers of Japanese players were born during Q1 (April-June). We replicated this result, and showed Association of RAEs with number of years in school that the skew was stronger among those who graduated To our knowledge, little research has been conducted on from high school at age 18 than among players who grad- how long RAEs last into adulthood, since most previous uated from university (college) at age 22. Indeed, while studies focused on junior players. In our third study, we the presence of RAEs was consistent, the effect sizes examined the relationship between RAEs in sports and were smaller for college than for high school graduates academic background, by categorizing Japanese profes- in both soccer and baseball (Table 3). Schorer and col- sional soccer and baseball players by academic back- leagues6) analyzed the influence of competition level on ground (high school vs. university)27). RAEs before adulthood in German handball players, and Table 3 shows the distribution of birthdates in soccer showed that effect sizes for RAEs were largest early in and baseball players divided into quarters. Chi-squared player development. Their findings were consistent with tests revealed biased distributions of birthdates in high our results. school and college soccer players. The effect size of the To interpret these findings, one hypothesis is the de- RAE among soccer players was larger in high school than crease in RAEs in adulthood. Relatively older players college graduates. In baseball players, chi-squared tests should be naturally heavier, taller, stronger, faster, have

Table 3. Distribution of birth dates in soccer and baseball players, categorized by academic background

Q1 (Apr-Jun) Q2 (Jul-Sep) Q3 (Oct-Dec) Q4 (Jan-Mar) Total X2 Cramer's phi

N effect size

(Soccer)

High school 227 (38.0%) 168 (28.1%) 118 (19.8%) 84 (14.1%) 597 78.75* 0.36

149 149 149 150

University 104 (29.1%) 105 (29.3%) 94 (26.3%) 55 (15.4%) 358 19.19* 0.23 89 89 90 90

Total 331 (34.7%) 273 (28.6%) 212 (22.2%) 139 (14.6%) 955 86.07* 0.30

238 239 239 239

(Baseball)

High school 108 (35.5%) 99 (32.6%) 61 (20.1%) 36 (11.8%) 304 44.45* 0.38

76 76 76 76

University 115 (31.8%) 97 (26.8%) 91 (25.1%) 59 (16.3%) 362 18.74* 0.23

90 90 91 91

Total 223 (33.5%) 196 (29.4%) 152 (22.8%) 95 (14.3%) 666 57.38* 0.29

166 166 167 167

The numbers in each second row show the predicted number of players obtained using the chi-square tests. *: p < 0.001 Adapted from Nakata & Sakamoto (2012)27). JPFSM: RAEs in Japanese athletes 471 more endurance, and better coordination than younger the motorcycle speedway. ‘Kyotei’ is the racing of hydro- players during childhood3), attributes that provide per- plane boats, referred to in Japan as ‘Boat Race’. formance advantages in most sports12). However, the Table 4 presents the results of chi-square tests in each advantages would weaken toward adulthood, as physical sport. Significant RAEs were observed in Keirin and maturity evens out. A second hypothesis is that talented Keiba. The percentage of relatively older players in the players, who have had early success in sports, tend to be first quarter (Q1) was higher in Keirin. By contrast, the drafted for professional teams after graduating from high skew in Keiba was clearly different, with the percentage school. A third explanation is that these phenomena are of relatively older riders lower in Q1 and higher in Q4. specific to Japan, because many activities, sports-related There were no significant RAEs in Auto Race and Kyotei. or academic, are based on a unique cutoff date (April 1st), These results indicate the existence of significant RAEs which is not the case in other countries. Of course, more not only in major sports, such as soccer, baseball, bas- than one of these explanations may apply. However, it ketball, and volleyball (Table 1), but even in gambling- is difficult to clarify why an RAE persisted among those related sports, such as Keirin (cycling) and Keiba (horse over 22 years of age, when theoretically there should racing). Since Keirin is not a major sport among teenage be no physical advantage in older players. We inferred boys in Japan26), popularity would be excluded from the that greater opportunities for selection and experience in factors producing the significant RAE in Keirin. Thus, childhood for the relatively older players might lead to physical development in relatively older men might be a longer-term advantages in adulthood. major factor for the significant RAE in Keirin. To become a professional player of Keirin, there are two ways. The first is to pass a physical fitness test for cycling college. RAEs for legal-gambling sports Another pathway to enter a cycling school is obtaining Many studies have investigated the existence and mech- successful results in international competitions or equiva- anisms of RAEs in major sports, which was shown in the lent events for cycling or other competitive sports (such Introduction. In our fourth study, we focused on RAEs in as swimming, athletics, and speed skating). Japanese male gambling-related sports, which have not been investigated in other countries. In Japan, there are Historical analysis of RAEs in Japanese baseball four sports for which pari-mutuel betting is legal: Keirin (cycling), Keiba (horse racing), Auto Race (motorcycle In general, it takes several years or decades for a sport racing), and Kyotei (motorboat racing). ‘Keirin’ is a form to gain popularity in a given country. Thus, historical of cycling involving sprints on a track. The sport origi- analysis is needed to clarify the beginning of RAEs in a nated in Japan, and the first Olympic competitions were country and compare differences in the skew of RAEs held in 2000. In our previous study on ‘Keiba’, the skew among generations. In addition, this analysis should be was clearly different from that in any other sport, showing considered based on socio-cultural factors. To date, there the percentage of relatively older riders to be lower in Q1 have been several studies examining the RAE from a his- (April-June) and higher in Q4 (January-March)14). How- torical perspective. ever, the data only involved jockeys competing in Tokyo. Daniel & Janssen29) analyzed National Hockey League The current research investigated the RAE in jockeys players and found no significant RAE in the 1961/62 and from all over Japan. ‘Auto Race’ is a Japanese version of 1972/75 seasons, but a significant effect in later samples

Table 4. Distribution of birth dates in Japanese male athletes in legal-gambling sports Q1 (Apr-Jun) Q2 (Jul-Sep) Q3 (Oct-Dec) Q4 (Jan-Mar) Total X2 p Cramer's phi N effect size Keirin 1015 (28.6%) 918 (25.9%) 837 (23.6%) 779 (21.9%) 3549 35.75 < 0.001 0.221 887 887 887 888 Kei ba 82 (15.0%) 140 (25.7%) 130 (23.9%) 193 (35.4%) 545 44.71 < 0.001 0.2 16 136 136 136 137 Auto Race 102 (21.5%) 121 (25.5%) 113 (23.8%) 138 (29.1%) 474 5.58 0.20 5 118 118 119 119 Kyo tei 38 (23.0%) 48 (29.1%) 38 (23.0%) 41 (24.8%) 165 1.66 0.07 2 41 41 41 42 (N = 473 3)

The numbers in each second row show the predicted number of players obtained using the chi-square tests. 472 JPFSM: Nakata H and Sakamoto K taken from 1985 to 1986. They suggested that the RAE 1955#2. The Little League involves elementary school was only a recent phenomenon in the National Hockey children (ages 7-12), while the Boys League involves League. elementary (ages 7-12) and junior high school (age 13- Grondin & Koren28) focused on RAEs throughout the 15) students. The Japan Rubber Baseball Association for history of Major League Baseball. They found no signifi- children was founded in 1970#3. cant RAE among players born in the 1920s and 1930s, The objectives of our fifth research were to clarify when but a significant effect among those born in the 1940s, the significant RAE began in Japanese professional base- 1950s, and 1960s. They discussed the inception of the ball, and to compare differences in the skew of the RAE Little League organization in 1939, and the American among decades. We formulated four hypotheses. The first Legion Baseball for teenagers, founded in 1926, and sug- was that the significant RAE started among players born gested that the growth in popularity of the Little League in the 1940s, since the previous study showed a signifi- gave rise to the significant RAEs in professional players. cant effect among American baseball players born in the Wattie and colleagues19) reported a significant RAE in 1940s, 1950s, and 1960s28). If the RAEs in both Japanese male Canadian ice-hockey players born after 1955, specu- and American baseball players were based on similar so- lating that a combination of socio-cultural events special and cultural backgrounds, the starting points of the cific to Canada, such as population expansion, increased RAEs may also be similar. The second hypothesis was participation in ice-hockey, and international challenges, based on players born in the 1950s and later. The growth influenced the present structure and trends. of television should have played an important role in ex- Cobley and colleagues30) performed a historical analy- panding the baseball population, and television in Japan sis on RAEs in professional German soccer players, and started broadcasting professional baseball games in the found significant effects among players born in the pre- 1950s. The third was based on players born in the 1970s 1935 group onwards. and later. The international stage associated with height- Abel and colleagues31), who used an online database, ened competition may be one of the factors producing the found that the distribution of birth months was statisti- RAE. The National Japanese team joined the Baseball cally significant for male, but not female baseball players World Cup in 1972 and the Olympics in 1984 for the first born between 1914 and 1938. time. The fourth hypothesis was based on players born In our fifth research, we examined the RAE in Japanese in the 1910s. There are two national tournaments of high professional (elite) baseball players born between 1911 school baseball in Japan, in the spring and in the summer. and 198032). Many studies on RAEs have been conducted These tournaments started in 1915#4. High school baseball for the sports systems in North American and European is as popular as professional baseball, and to be able to countries. However, it is necessary to confirm RAEs in go to Koushien, where the national competition is held, other countries if universal factors are truly related to is an honor for all young baseball players and schools. the effects. In other words, it needs to be clarified when To participate at Koushien, schools have to first win their a significant RAE starts in a country (e.g. in the 1940s), regional competition. because the popularity and system of a given sport dif- As already mentioned, Japan has applied a unique an- fers among countries and changes with time. Grondin & nual age grouping for sports and education since 1886. Koren28) and our previous studies14,25,27) already researched Another unique social attribute might be its defeat as a the prevalence of RAEs in Japanese baseball players. nation in World War II (WWII). After WWII, the politics However, when the significant RAE in Japanese profes- and economy of Japan changed from imperialism to de- sional baseball began remains unclear from a historical mocracy. Furthermore, regarding the social structure of standpoint. workers in Japan before WWII, most Japanese workers Interestingly, since 1886 Japan has applied a unique an- in primary industries were categorized as rice farmers. As nual age-grouping for education, which runs from April the nation’s economy grew stronger, tertiary industries 1 to March 31 of the following year. Therefore, April 1 began to expand rapidly. In our fifth research, we inves- is the beginning of the new school year, and this specific tigated the relationship between these unique social and calendar applies to the whole education system from el- cultural attributes and the RAE in Japanese baseball play- ementary through university (college), as well as to the ers. recruiting of new employees in government and private Table 5 presents the results of chi-square tests for each companies. Sports calendars also follow this system, and decade. Significant RAEs were observed in the 1910s many professional sports such as baseball, soccer, basket- through 1970s. The extent of RAE was greater in the ball, and volleyball, start the new season on April 1. In addition, there are several baseball leagues for chil- #1: Japan Little League Baseball Association (http://jllba.com/) dren in Japan, the category of which is based on the type #2 of ball, hard or rubber. There are two major hardball : Japan Boys League (http://www.boysleague-jp.org/) leagues for children, Little League and Boys League. The #3: Japan Rubber Baseball Association (http://jsbb.or.jp/) Little League started in 1970#1, and the Boys League in #4: Japan High School Baseball Federation (http://www.jhbf.or.jp/) JPFSM: RAEs in Japanese athletes 473

1940s, 1950s, 1960s, and 1970s than in the 1910s, 1920s, RAE in the United States. However, this notion is not and 1930s. This result was also supported by the data of directly consistent with our findings in Japan, because Q1 in each decade, showing that the percentage of Q1 the hardball and rubber-ball leagues only started in the was 28.1% in the 1910s, 25.1% in the 1920s, 28.0% in the 1950s and 1970s, respectively, and a significant RAE for 1930s, 35.6% in the 1940s, 32.4% in the 1950s, 34.1% in Japanese baseball players was already observed in the the 1960s, and 36.8% in the 1970s. However, it is neces- 1910s. Thus, the establishment of these leagues for chil- sary to interpret these results carefully. Table 6 shows the dren would not be a direct factor in the RAE in Japanese distribution of the general population for each decade in baseball players, though it surely contributed to the strong Japanese males. A biased distribution was found in the effect in the 1950s, 1960s, and 1970s. 1910s, 1920s, 1930s, 1940s, and 1950s, indicating that As previously mentioned, we proposed four hypoth- the birth percentage was more than 29% for Q4. eses on the start time of a significant RAE in Japanese According to Grondin & Koren28), the popularity of professional baseball, and the fourth hypothesis seems to baseball among children could have contributed to the be supported by the results of this study. The popularity

Table 5. Distribution of birth dates in Japanese baseball players according to birth year groups

Q1 (Apr-Jun) Q2 (Jul-Sep) Q3 (Oct-Dec) Q4 (Jan-Mar) Total X2 Cramer's phi

N effect size

1910s 97 (28.1%) 89 (25.8%) 72 (20.9%) 87 (25.2%) 345 25.34 ** 0.271

67 76 85 117

1920s 116 (25.1%) 112 (24.2%) 111 (24.0%) 123 (26.6%) 462 15.27 * 0.182

91 104 109 158

1930s 258 (28.0%) 260 (28.2%) 196 (21.2%) 209 (22.6%) 923 68.02 ** 0.271 190 210 220 303

1940s 271 (35.6%) 215 (28.2%) 128 (16.8%) 148 (19.4%) 762 109.86 ** 0.380

168 187 180 227

1950s 167 (32.4%) 171 (33.2%) 103 (20.0%) 74 (14.4%) 515 79.57 ** 0.393

120 123 120 152

1960s 227 (34.1%) 204 (30.7%) 128 (19.2%) 106 (15.9%) 665 71.00 ** 0.327 161 166 161 177

1970s 216 (36.8%) 177 (30.2%) 114 (19.4%) 80 (13.6%) 587 70.81 ** 0.347 147 153 142 145

The numbers in each second row show the predicted number of players obtained using the chi-square tests. *p < .01; ** p < .001. Adapted from Nakata & Sakamoto (2013)32).

Table 6. Distribution of birth dates in the general population according to birth year groups

Q1 (Apr-Jun) Q2 (Jul-Sep) Q3 (Oct-Dec) Q4 (Jan-Mar) Total

n (%) n (%) n (%) n (%) n

1910s 1791767 19.4 2025205 22.0 2275111 24.7 3123909 33.9 9215992

1920s 2059032 19.6 2372155 22.6 2485030 23.7 3579334 34.1 10495551 1930s 2206393 20.6 2422245 22.7 2541991 23.8 3515386 32.9 10686015

1940s 2773900 22.1 3081322 24.5 2971644 23.6 3751951 29.8 12578817

1950s 2115867 23.4 2155607 23.8 2105256 23.3 2674998 29.6 9051728

1960s 2172512 24.2 2233566 24.9 2171743 24.2 2388225 26.6 8966046

1970s 2387059 24.9 2499533 26.1 2316398 24.2 2368142 24.7 9571132

Adapted from Nakata & Sakamoto (2013)32). 474 JPFSM: Nakata H and Sakamoto K of high school baseball could be a factor in the RAE. As agriculture, forestry, and fishery, and 23.7% in tertiary for the start of professional leagues, Nippon Professional industries such as information-communication technol- Baseball was founded in 1936#5, which was after the ogy, finance, service, real estate, and education. In 1960, founding of the national high school tournament. Conse- 32.7% of workers were in primary industries and 38.2% quently, high school baseball in Japan would have been in tertiary industries. In 1980, 10.9% of workers were in more popular than professional baseball among players primary industries and 55.4% in tertiary industries. Great- born in the 1910s and 1920s, and enthusiasm for baseball er changes were not seen in the percentage of workers in was more likely to be linked to being a national symbol secondary industries including mining, construction, and rather than a mere sport. Thus, Japanese baseball popular- manufacturing over time: 20.5% in 1920, 29.1% in 1960, ity during those years could have been related to the RAE and 33.6% in 1980#6. The definition of industry was based among players born in the 1910s and 1920s. on the Japan Standard Industrial Classification. The Empire of Japan, which strengthened rapid milita- As the nation’s economy grew stronger, tertiary in- rization and industrialization, existed from the Meiji Res- dustries began to expand rapidly. Before WWII, most toration in 1868 to the end of WWII in 1945. During the Japanese workers in primary industries were rice farmers. war, the use of English was banned in Japan. Also during Rice was the staple food in Japan for breakfast, lunch, WWII, especially between 1942-1945, all baseball games and dinner, and Japanese sake, which is an alcoholic in high school were cancelled, and all professional base- beverage, was made from fermented rice. In general, ball games were also cancelled in 1944 and 1945. Some Japanese farmers plant rice in late May, and harvest it famous Japanese baseball players (e.g. Eiji Sawamura) in late September. Therefore, June, July, and August are died as soldiers during WWII, and there may have been busy months for cultivating rice, but March, April, and many players who were injured both physically and men- May are not. It may be inferred that the time pressures of tally, even if they did survive the war. Thus, it is likely daily life are related to birthrates, and that the less busy that Japanese baseball was affected more than American period during March, April, and May can be associated baseball during WWII. After WWII, the army of the Unit- with higher birthrates in January, February, and March. A ed States occupied Japan, and changed Japanese politics previous study on Japanese lifestyle, which investigated and economy from imperialism to a democracy. However, sexual relationships in 1,468 Japanese people, reported the impact of political and social changes after WWII that the highest percentage of people did not have such a may not have strongly influenced the RAE in Japanese relationship because of work-related exhaustion33). The baseball. second explanation was that the season for giving birth Regarding the strong skew in the distribution of birth- in humans has been associated with temperature. Accord- dates from the 1940s onwards, social-cultural factors ing to Cogwill34), the number of births might be related should be considered. Two studies suggested that popu- to temperature. Even higher primates including humans lation growth and increased participation in sports, as have an annual breeding season in the spring because well as the international stage associated with heightened many European countries such as Norway, Sweden, Fin- competition in youth sports, were linked to media cover- land, France, England, and the Netherlands show a major age and the growth of television19,29). Since the percentage peak in births between January and April. In our results, of Q1 markedly increased from the 1940s onwards (Table this was consistent with the birth distribution from the 5), their suggestions were confirmed by the RAE in the 1910s to the 1950s, but not from the 1960s onwards (Table history of Japanese baseball. Firstly, media coverage, 6). Cogwill34) assumed that as the standard of living in all especially television, started broadcasting professional these countries improved, the chance of being born alive baseball games in the 1950s. Secondly, a Japanese team during the “off-season” became greater, which may have joined the Baseball World Cup in 1972 and the Olympics had some effect on birth patterns. in 1984 for the first time. The general population showed a biased distribution of Conclusion birthdates from the 1910s to the 1950s (Table 6); many Japanese were born in Q4 (January – March). However, In this paper we showed RAE in Japanese athletes, and it is difficult to explain this finding. We suggest several introduced parts of its characteristics and mechanisms. possible explanations. The first explanation is related to The present review can be summarized as follows: the social structure of workers in Japan. In 1920, 53.8% 1. The distribution of birthdates in each male sport of workers were engaged in primary industries including showed significant RAE in baseball, soccer, volleyball, Ekiden, basketball, sumo, and horse racing, but not in all sports. #5: Nippon Professional Baseball (http://www.npb.or.jp/) 2. The distribution of birthdates in each female sport #6 : Ministry of Internal Affairs and Communications, the showed a significant RAE only in volleyball, suggest- Statistics Bureau and the Director-General for Policy ing that the determinants of RAEs in sports may differ Planning of Japan (http://www.stat.go.jp/index.htm) between males and females. JPFSM: RAEs in Japanese athletes 475

3. The skew of birthdates was stronger among players soccer: Japan Women’s Football League; basketball: who only graduated from high school than those who Women’s Japan Basketball League (WJBL); volley- graduated from university or college. This phenom- ball: V-League; track and field (long distance): Japan enon was confirmed in both baseball and soccer play- Industrial Track & Field; badminton: Badminton Nip- ers, suggesting RAEs in professional sports are related pon League). Therefore, the data for young female to years of age and years in school. athletes under 18 years old should be important to 4. RAEs can be found in not only major sports, such clarify the mechanisms of RAEs in female sports. as soccer, baseball, and volleyball, but also legal- 4. In the historical analysis, we showed data only for gambling sports including cycling (Keirin) and horse Japanese professional baseball players32). RAEs in racing (Keiba). other sports should be investigated, because the mag- 5. There were statistically biased distributions of birth- nitude of RAE would be associated with time and dates among baseball players born in the 1940s and socio-cultural factors such as international competi- subsequent decades (medium effects), and similar (but tion and media coverage. small) RAEs were observed among players born in 5. A few studies have focused on data regarding sports the 1910s, 1920s, and 1930s. The magnitude of RAEs dropout in relatively young athletes7,35,36), suggesting changed with time, and socio-cultural factors such as the existence of a relationship between motivation international competition and media coverage may among relatively young athletes and RAE in the sport. have contributed greatly to this effect. Indeed, relatively younger players may experience failure and frustration and be more likely to withdraw However, we still need to clarify the detail mechanisms from the sport rather than continue voluntary partici- of RAE in Japan. These issues are discussed in the final pation37). The motivation and dropout mechanisms section. associated with RAE in Japanese sports should be 1. Indeed, it is impossible to change the cut-off day of investigated. April 1, which is the beginning of the ‘new academic 6. Most of the studies have examined RAEs in elite ath- year’, and this specific calendar applies to elementary, letes. Recently, one study reported data on RAE in a junior high and senior high schools, and university general school sample38), suggesting that age alone (college) as well as to government and private compa- appeared to account for RAEs in their sample, with nies, and to sports calendars. This indicates that RAE no effects for age relative to peers or month of birth. can occur in many sports in many countries. However, Further studies in a general school sample are needed sports governing bodies, parents, coaches, and athletic to determine explanations of RAEs in other contexts. trainers should understand this phenomenon, because ‘talented’ or ‘skilled’ attributes of young athletes Conflict of Interests might be merely based on RAE. If so, it is possible that truly ‘talented’ young athletes, who are relatively The authors declare that there is no conflict of interests younger athletes, could get overlooked. Cobley and regarding the publication of this article. colleagues12) also suggested that countries and/or sport governing bodies employing intensive early talent identification and development systems in sport may, References ironically, be achieving the opposite effect by con- 1) Dudink A. 1994. Birth data and sporting success. Nature 368: straining and reducing their talent pool through early 592. selection processes and generating RAEs. 2) Hirose N. 2009. 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