Relative Age Effects in Male Japanese Professional Athletes: a 25-Year Historical Analysis

Abstract

Background The mechanisms underlying the relative age effect (RAE), a biased distribution of birth dates, in sport events have been investigated for more than two decades. The present study comprised an historical analysis involving the most recent quarter-century (1993–2018) on RAEs among Japanese male professional athletes (soccer, baseball, basketball, and volleyball) to clarify how the RAEs changed over time. Methods Birth data were obtained from 7805 Japanese male professional athletes registered in 1993, 2001, 2010, and 2018. The athletes were divided into four groups based on their month of birth: quartiles Q1 (April–June), Q2 (July–September), Q3 (October–December), and Q4 (January–March of the following year). In addition, based on the data in 1993 for soccer and baseball and in 2010 for basketball and volleyball, the expected numbers of players were calculated in 2001, 2010, and 2018 for soccer and baseball, and 2018 for basketball and volleyball. Results Significant RAEs were observed among soccer and baseball players in 1993, 2001, 2010, and 2018, and strong tendencies of RAEs were found among basketball and volleyball players in 2010 and 2018. The magnitudes of the RAEs in soccer, baseball, and volleyball decreased over time, but not in basketball. Conclusion The exact reasons for the decreasing or unchanging RAEs among these professional players remain unclear, but socio-cultural factors, such as low birthrates and the popularity of sports in Japan, might be related to the changing RAEs.

The New Generation of Professional Soccer Talent Is Born under the Bias of the RAE: Relative Age Effect in International Male Youth Soccer Championships

“…The RAE was first questioned by Grondin et al [ 4 ] and explored shortly after by Barnsley et al [ 5 ]. Since then, this bias effect has been extensively studied in different sports, including team sports such as soccer [ 6 ], basketball [ 7 , 8 ], hockey [ 9 ], rugby [ 10 , 11 ], futsal [ 12 ], handball [ 13 ], baseball [ 14 ], Australian football [ 15 ] water-polo [ 6 ], and volleyball [ 6 ]; and individual sports like athletics [ 16 , 17 , 18 ], alpine ski racing [ 19 ], swimming [ 20 , 21 ], tennis [ 22 ], and even golf, horse racing, sumo, or badminton [ 23 ]. As a consequence, there is a large number of articles that have led to diverse literature reviews and meta-analyses that try to make more accessible all this information [ 1 , 24 ].…”

Trends in Relative Age Effects of Top-Level Female Soccer Players: A Japanese Study

“…In Japan, the RAE of male professional soccer players has been established with data from 1993, 2001, 2010, and 2018 seasons (Nakata & Sakamoto, 2011;Sasano et al, 2020). The Q1/Q4 ratio of player birthdates was 2.…”

“…Following procedures in previous studies (Nakata & Sakamoto, 2011; Sasano et al, 2020), we classified participants’ birth months into four groups: Q1 (April–June), Q2 (July–September), Q3 (October–December), and Q4 (January–March), and we then calculated the proportion of players born in each quarter. We conducted a goodness-of-fit test to examine the difference in ratios from Q1 to Q4.…”

“…Effect sizes ω are considered to be small, moderate, and large, when ω is 0.10, 0.30, and 0.50, respectively (Cohen, 1988). Following procedures in a previous study (Sasano et al, 2020), we compared the proportion of births in Q1 and Q4. In addition, we calculated births in 6-month semesters (S1 and S2) to determine the difference between the sum of births in Q1 and Q2 (S1) versus Q3 and Q4 (S2).…”

“…Dudink (1994) found more soccer players born in months closer to the cutoff date than in months farther from it in both Dutch and English soccer leagues. Since then, the RAE has been reported for soccer players in various countries, including those participating in the World Cup (Delorme et al, 2009; Figueiredo et al, 2021; Götze & Hoppe, 2020; Pedersen et al, 2022; Sasano et al, 2020; Vincent & Glamser, 2006). To our best knowledge, Uchiyama and Maruyama (1996) published the first RAE study of professional soccer players in Japan, and several similar studies followed (Nakata & Sakamoto, 2011; Sasano et al, 2020).…”