## 1 Introduction
Their findings revealed that, alongside the RAE, the probability of reaching the professional level significantly decreased with increasing birth order. In fact, players born later in the birth order were less likely to make it to the top level of competition. The study also highlighted an interesting nuance: second-born players with an older brother tended to have higher salaries than those without siblings, suggesting that birth order may play a role in the development of certain skills or characteristics that contribute to success in sports. Similarly, [PERSON] et al. [16] examined the role of family characteristics, including birth order, in psychomotor development and found that children with siblings exhibited greater motor and affective development than only children. This research supports the notion that family dynamics, including birth order, also may influence the developmental trajectories of athletes. The RAE has been the subject of numerous studies across a wide range of sports disciplines, being observed in both youth and adult sports [17; 18; 19]. However, soccer and ice hockey stand out among team sports, with the latter serving as the starting point for research on this phenomenon in the sports field [20]. The RAE in ice hockey has been widely studied, particularly in the United States and Canada, with the study by [PERSON] et al. [7] being one of the first to address this issue. The RAE has been identified both in professional leagues [7; 21] and in youth competitions [22; 23]. Although the literature consistently supports the existence of the RAE in this sport, several factors can influence its magnitude, including age, competition level, season, and playing position. Specifically,data from [PERSON] et al. [7] show a significant decrease in the likelihood of reaching higher competitive levels for athletes born in the last months of the year, a trend observed from youth categories to professional leagues. This trend is also evident in soccer, one of the most widely studied sports in relation to the RAE due to its global reach. Research across different countries and competition levels consistently shows a significant overrepresentation of players born in the first months of the selection year. For instance, [PERSON] [24] found that 55-60% of professional players in Belgium, France, and the Netherlands were born in the first semester, while [PERSON] and [PERSON] [25] reported similar patterns in Japan (66%) and Germany (56%). In youth international tournaments, [PERSON] et al. [26] observed that up to 79% of players in U-17 and U-20 World Cups were born in this period. The RAE is particularly pronounced in youth categories, where selection processes favor early-born players [3]. Studies indicate that this advantage is more pronounced at younger ages and is directly influenced by the cut-off date used for age group classification. Beyond talent selection, the RAE has also been linked to competitive success [27] and long-term player retention, with dropout rates increasing for those born later in the year [14]. Overall, soccer exemplifies how the RAE shapes player development pathways, influencing both short-term opportunities and long-term career progression. Similarly, the RAE has also been analyzed in individual sports, such as swimming [28; 29; 30; 31], tennis [32; 33; 34], shooting sports [35; 36; 37], fencing [38; 39], badminton [40; 41], chess [42], wrestling [43], e-sports [18], canoeing [44] or weightlifting [45], although the results show considerable variability. In general terms, there is a tendency to favor athletes born in the early months of the year, although with less intensity compared to team sports [20]. Moreover, differences in the manifestation of this effect appear to be linked to the characteristics of each sport [46]. On the other hand, the influence of the RAE varies by sex, showing a greater impact on male athletes [9] and a more heterogeneous response in female athletes [1].