"To avoid large age differences in sports, children and youth are organized into annual-age groups. These age-specific groups are organized by a selected cut-off date, which in most countries is January 1. Thus, in the same team there may be a member born in January and another member born in December, implying an almost 12-month age difference. A person’s age relative to that of his/her peers within the same annual group is referred to as relative age [1, 2], and its variations are the relative age differences [1, 2].
One of the consequences of these relative age inequalities is the relative age effect (RAE), which describes an overrepresentation of athletes born a few months after the cut-off date (i.e., January, February, and March, when the cut-off date is January 1) as well as underrepresentation of those born at the end of the year [1, 2]. This phenomenon has been demonstrated in a variety of sports [1, 2], mainly in team sports such as soccer [3, 4], ice hockey [5], basketball [6], rugby [7], and handball [8, 9], but also in individual sports such as athletics (track and field) [10, 11], skiing [12], tennis [13, 14], and swimming [15]. Some of these studies included youth [3, 5–7, 10–15] and others senior [4, 6–9, 13] participants. For example, Brustio et al. [16] conducted a large study that included 2064 Italian elite soccer players and observed that 43.3% of players had been born in the first quarter of the year, in contrast to 10.7% born in the last quarter of the year.
The most supported hypothesis explaining the causes for RAE is based on the selection hypothesis [1–16]. This hypothesis suggests that chronologically older children/youth have physical or anthropometrical advantages, which are associated with better performance, and thus are more likely to be identified as more talented and selected into higher-level teams—generating and perpetuating bias toward relatively older players. Likewise, the chances of the relatively younger being selected are thus reduced. The relevance of physicality is confirmed by observation of the RAE particularly in sports where body size and strength are important, as opposed to its nonexistence in weight-categorized sports (taekwondo [17], judo [18], boxing [19] and shooting, in which a large body size is not particularly advantageous [20].
Apart from these physical and physiological advantages, there are also relevant psychological and social influences that may explain RAE in sports [1, 2]. According to Hancock et al. [5], parents may have a relevant role in the genesis of RAE (Mathew effect). One possibility is that parents do not enroll their relatively younger children in sports where physical demands are high [5]. In addition, both athletes’ and coaches’ expectations (Galatea and Pygmalion effects, respectively) toward relatively older athletes may further contribute to this phenomenon [21].
Once players have been included in high-level teams, they have access to high-quality training, equipment, installations, medical staff, etc., thus enhancing their skills consequently, the RAE is established, maintained, and reinforced [1, 2], reaching its highest prevalence at the most prestigious competition levels. Hence, most studies have confirmed the existence of the RAE in the highest-level elite sports, such as World [3, 9, 10] and European Championships [4, 22, 23] and National selections [6]."
