The relative age effect (RAE) is a well-documented phenomenon in youth sport (Cobley et al., 2009). This effect exists when the relative age quarter distribution represents a biased distribution among the four quarters with an overrepresentation of athletes born early in the selection year (Musch and Grondin, 2001). For example, among the under-15, under-16, under-17 and under-18 youth soccer players from 10 European countries, 43.4% were born in relative age quarter 1, and only 9.3% in the last relative age quarter (Helsen et al., 2005). Due to the system of classifying athletes by chronological age within one birth year, age differences of up to twelve months are possible between athletes of the same competition category (Helsen et al., 2005). However, often broader age bands (e.g., two-year age bands or two constituent years) are used for classifying athletes in competition categories, which lead to greater age differences between athletes of the same competition category (Steingrover et al., 2017). These relative age advantages have led to the RAE phenomenon, which is present in diverse types of sport (Cobley et al., 2009). Schorer et al. (2013) underlined the necessity to differentiate between varying effects (constituent year vs. within-one-year) to better understand the mechanisms behind talent development. However, most studies in RAE research did not consider diverse age-cohorts.
A recently published review article demonstrated that the RAE is present in several age categories of national and international soccer (Sierra-Diaz et al., 2017). Gutierrez Diaz Del Campo et al. (2010) found a significant RAE among prepubescent youth soccer players from Spain (under-11 till under-18) selected for elite youth teams of clubs belonging to the Spanish Professional Football League; however, no difference was found between the relative age quarter distribution of the amateur players compared to the distribution of the Spanish population. Additionally, no significant influence of the age group the athletes compete in, their position on the pitch and the number of years they have spent in the category (U11-U18) was present. As a consequence, the authors concluded that the main cause of RAE in elite soccer seems to be the talent identification process. (Gutierrez Diaz Del Campo et al., 2010) Similar findings were revealed in prepubescent ice hockey (Hancock et al., 2013). However, at the youngest levels of international soccer, no study has been published, so far. Nevertheless, the existence of the RAE in several age categories in soccer indicates that the talent development system in this sport discriminates against relatively younger athletes and a lot of talents get lost because talent in a sport does not depend on the birth month (Lames et al., 2008).
Talent identification systems are often based on selection biases that confuse maturation for talent (Cobley et al., 2009). In this context, Cobley et al. (2009) proposed the maturation-hypothesis to explain RAE in sport: it can be assumed that the relative age of an athlete is related to his/her cognitive and physical maturation; thus, the favorable selection of relatively older athletes compared to relatively younger athletes is influenced by the maturational differences between them (Baker et al., 2014). The short-term consequences are that relatively older and earlier maturing athletes seem to be more "talented" and are consequently selected, whereas relatively younger and less mature athletes are often excluded and drop out of sport (Muller et al., 2017; Romann and Cobley, 2015). As a consequence, the combination of a relatively older age and an advanced maturation positively influence the selection and thus, might lead to the RAE. Cobley et al. (2009) showed that this is especially true in sports with high demands on strength and power; in such sports, youth athletes who are above average in height and weight compared to non-athletes of the same age are favourably selected (Cobley et al., 2009). Additionally, Gil et al. (2014) revealed that 10-year-old Spanish youth soccer players of the four relative age quarters did not significantly differ in the biological maturity status from each other, which indicates that relatively younger, but nevertheless selected, youth soccer players can counteract their relative age disadvantage by an advanced biological maturity status. Furthermore, Deprez et al. (2013) and Muller et al. (2017) showed that among relatively younger national youth soccer players a high percentage of early maturing athletes were present, which indicates that relatively younger soccer players might only have a chance of selection if they were early maturing whereas relatively older athletes had a selection advantage independent of their maturity status (Deprez et al., 2013). However, it has to be considered that in both studies other extraneous variables that could have influenced the selection (e.g., training age, training quality, ...) were not controlled. Hancock et al. (2013) found a strong RAE in prepubescent female ice hockey players and hypothesized that physical maturation differences might not have as strong an impact on RAE as always thought, whereas cognitive factors might have a stronger impact which could be seen in prepubescence; alternative, small changes in annual growth are significant enough to produce RAE. Towlson et al. (2017) revealed that maturation and anthropometric characteristics appear to bias the allocation of players (especially for key defensive roles) already from an early development stage, whereas physical attributes do not influence the selection until latter stages of talent development. Johnson et al. (2017) showed that maturation status had an even 10-fold stronger influence on selection in elite youth soccer than the relative age. Therefore, the biological maturity seems to play a major role in the selection of youth soccer players. However, these studies were conducted only at national levels of youth soccer and conflicting results were found. It might be hypothesized that at an international level an even greater selection pressure is present which might strengthen the selection bias of favorably selecting relatively older and early maturing athletes. However, this has to be investigated in order to fully understand the mechanisms behind the RAE in top-level international youth soccer with the goal of contributing to a fairer talent development in this sport.
Therefore, the aim of the present study was to investigate the association of the biological maturity status with the selection and consequently, the RAE among high-level international elite U9 soccer players who were selected for a European Youth Soccer Tournament.
In total, data from 222 male participants of the U9 Euro-championship Soccer Tournament in Vienna in 2016 were evaluated in the present study. Athletes of the birth year 2007 and younger were allowed to participate; however, only athletes born in 2007 participated in the tournament. In total, 40 teams from 18 nations participated in the tournament; however, data from 20 teams could be obtained because these teams agreed to participate in the study. The mean age of the study participants was 9.0 years (standard deviation: [+ or -] 0.4) and ranged from 8.5 to 9.5 years. Table 1 presents the anthropometric data (means and standard deviations) of the study participants.