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The Recent Trend of Relative Age Effect in Japanese Male Professional Soccer Players [Received March 3, 2023; Accepted August 8, 2023] The details about relative age effect in Japanese male professional soccer players are not readily at hand. 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 were significant 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 effect in 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 not consistently increase or decrease. Keywords: birthday, football, Japan |