For over a quarter of century, the “relative age effect” (RAE) has been investigated as one of the factors influencing sporting success. Relatively older children in a particular age group are more likely to achieve sporting success, compared with relatively younger children. Within the same age category, there can be a difference of almost a full year between the oldest and youngest children. RAEs have been confirmed in many sports, including soccer [1, 2], baseball [3, 4], basketball [5, 6], handball , swimming [8, 9], track and field [10, 11], sumo wrestling , rugby , and alpine ski racing [13, 14]. The attributes of greater height, mass, aerobic power, muscular strength, endurance, and speed provide performance advantages in most sports, giving relatively older children advantages in sporting ability, psychological confidence, instruction, and playing time . To date, a variety of sports contexts differing in age categories and cultures have been assessed to examine RAEs (see a meta-analytical review, ).
In addition to physiological and psychological factors, the competition principle has also been considered as an important factor affecting RAEs. According to Musch and Grondin , “Competition will come from the number of players available for the places, and this number will depend on the popularity of a given sport in a given country (p. 154).” Thus, the level of competition is associated with the popularity of the sport. For example, in Japan, soccer, baseball, basketball, and volleyball are very popular among male elementary and junior high school students ; however, handball, rugby, badminton, and American football are not so popular. Indeed, Nakata and Sakamoto  showed significant RAEs among Japanese players in soccer, baseball, basketball, and volleyball, which were major sports in Japan, but no significant RAEs were observed in handball, rugby, badminton, American football, or golf. Consequently, if a sport does not need a physical advantage and is not so popular in a given country, RAEs may not be observed. In addition, it takes several years or decades for a sport to gain popularity in a given country. Therefore, historical analysis is needed to know the beginning of RAEs in a country and compare differences in the magnitude of RAEs among generations, and to be considered based on socio-cultural factors. To date, there have been several studies examining RAEs from a historical perspective [18,19,20,21,22]. For example, Nakata and Sakamoto  investigated the existence of RAEs in Japanese professional baseball players born in 1911–1980. They reported that significant RAEs were observed among Japanese professional baseball players born in the 1910s and onward, and the magnitude of RAEs increased with time. These studies suggest that the magnitude of RAEs changed with time and that socio-cultural factors, such as international competitions, and media coverage may have markedly contributed to this.
However, after a thorough literature search, we do not know of any study conducting historical analysis of recent generations of RAEs in a given country, rather than the beginning of RAEs [18,19,20,21,22]. Moreover, previous historical studies only examined RAEs on one sporting event. We consider that RAEs on some sports should be evaluated simultaneously in a given country to clarify the socio-cultural factors relating to RAEs. In fact, demographics have clearly changed compared with 50 years ago. For example, in Japan, the number of children is markedly decreasing (Ministry of Internal Affairs and Communications, the Statistics Bureau and the Director-General for Policy Planning of Japan ), and recent RAEs on some sports might change over time. Based on this research background, the objective of the present study was to investigate the characteristics of RAEs over a recent quarter-century (1993–2018) among professional soccer, baseball, basketball, and volleyball players simultaneously in Japan. Japan has applied a unique annual-age grouping for education since 1886, basing group assignment on student birthdates between April 1 (the “new” year) and March 31 of the following year among elementary, junior high, senior high, and university (college) students and in government and company employment. Sports calendars also follow this system, giving Japanese children and adolescents born between April and June a relative age advantage over those born between January and March. We investigated whether the magnitude of RAEs in Japanese professional soccer, baseball, basketball, and volleyball, which are major male-dominated sports in Japan, changed over time.