The relative age effect is widespread among European adult professional soccer players but does not affect their market value

PLOS ONE RESEARCH ARTICLE The relative age effect is widespread among European adult professional soccer players but does not affect their market value Eduard Bezuglov1,2,3, Ryland Morgans1, Mikhail Butovskiy4, Anton Emanov3,5, Larisa Shagiakhmetova ID3*, Bekzhan Pirmakhanov6,7, Zbigniew Waśkiewicz8, Artemii Lazarev9 a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 1 Sechenov First Moscow State Medical University (Sechenov University), Moscow, Russian Federation, 2 High Performance Sports Laboratory, Moscow Witte University, Moscow, Russian Federation, 3 Academy of Talents, Moscow, Russian Federation, 4 FC Rubin, Kazan, Russian Federation, 5 Smart Recovery Sports Medicine Clinic, Moscow, Russian Federation, 6 Faculty of Medicine and Health Care, Department of Epidemiology, Biostatistics and Evidence-Based Medicine, Al-Farabi Kazakh National University, Almaty, Kazakhstan, 7 FC Kairat, Almaty, Kazakhstan, 8 Jerzy Kukuczka Academy of Physical Education in Katowice, Katowice, Poland, 9 Mount Sinai Hospital, Chicago, IL, United States of America * Abstract OPEN ACCESS Citation: Bezuglov E, Morgans R, Butovskiy M, Emanov A, Shagiakhmetova L, Pirmakhanov B, et al. (2023) The relative age effect is widespread among European adult professional soccer players but does not affect their market value. PLoS ONE 18(3): e0283390. https://doi.org/10.1371/journal. pone.0283390 Editor: Rafael Franco Soares Oliveira, Instituto Politécnico de Santarém: Instituto Politecnico de Santarem, PORTUGAL Received: August 1, 2022 Accepted: March 8, 2023 Published: March 23, 2023 Copyright: © 2023 Bezuglov et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: The data underlying the results presented in the study are available from www.transfermarkt.com. Funding: The authors received no specific funding for this work. Competing interests: The authors have declared that no competing interests exist. Background The relative age effect (RAE) is most prevalent in highly competitive youth soccer and persists to a lesser extent in senior soccer. However, it is known that soccer players born in the second half of the year are as successful at senior level, indicating that they are equally talented although under-represented at youth level due to bias during the selection process, in which the emphasis is on more pronounced physical qualities in a specific period of time. Examining the prevalence of the RAE among professional soccer players depending on the level of competition and playing position, as well as analyzing the relationship between the player’s birth quarter and market value, are of scientific interest. Methods The dates of birth, playing position, and market value of all adult male soccer players included in the final rosters of teams from the top-division of 54 European countries, listed on www.transfermarkt.com on August 15th, 2020, were analyzed (18,429 soccer players in total). All players were categorized into four groups according to the quarter of birth (Q) and playing position. All teams were further sub-divided in groups depending on the soccer clubs’ level of representation in the UEFA Champions League. Results Of 18,429 players, 30.9% were born in Q1, 25.7% in Q2, 23.8% in Q3 and 19.6% in Q4. The number of soccer players born in Q1 was lower in less competitive leagues. The number of players born in Q1 decreased as the level of competition decreased; the highest percentage of these players was observed in clubs that are among the top 50 ranked in UEFA or PLOS ONE | https://doi.org/10.1371/journal.pone.0283390 March 23, 2023 1 / 18 PLOS ONE The RAE is widespread among European adult professional soccer players but does not affect their market value compete in the most prestigious European championships. The RAE was less pronounced in teams that participate in lower competitive championships. There was no significant difference in market value between players playing position and level of competition when born in different quarters. Although, the most expensive soccer players in the European championships were late-born forwards. Players of various groups differed in their market value. Conclusions The RAE is currently prevalent in all the most competitive senior soccer leagues and teams in Europe regardless of playing position. There are no significant differences in market value between players of the same playing position and level of competition when born in different quarters. The most expensive soccer players in the European championships are forwards born in Q4. These findings may indicate that the under-representation of “late-born” soccer players in youth, and then consequently in adult soccer, is not associated with lower talent, but with other factors, possibly based on physiological characteristics and socio-cultural factors. Further measures are needed to mitigate the discriminatory effects of selection bias based on the RAE. Introduction The relative age effect (RAE) refers to the over-representation of athletes born before a specific date for age grouping in various sports comparing to those born after this date [1]. Athletes born relatively late in the selection year potentially suffer a disadvantage during the selection process [2]. In sports where the selection cut-off date is January 1st, the number of athletes born between January and March may be several times higher than the number of athletes born between October and December [3]. Thus the RAE is widespread among young male athletes (age 15–18 years) performing competitively in soccer, athletics, and basketball [1,3–6]. Many studies have examined the wide spread of the RAE among soccer players of different ages and levels of competition [7–10]. Williams et al. showed that in a cohort of FIFA U17 World Cup players approximately 40% were born in the first quarter of the year while only 16% were born in the last three months of the year [11]. Rada et al. further demonstrated that the number of players born in the first month of the calendar year is twice as many of those born in the last month of the year, while the RAE is also widely prevalent in second-tier players [12]. More recent studies also showed that the RAE is evident in elite German [13] and Scottish soccer players [14]. Although, the prevalence of the RAE in older soccer players is not as high when compared to younger players [15]. The main negative factor of the RAE is the "discrimination" of late-born athletes likely related to them being less physically developed (as they are younger) and therefore considered less mature. For example, a study by Romann et al. showed that the difference in 60-meter sprint performance between “early-” and “late-born” athletes aged 8 to 15 years born in one year ranged from 5– (...truncated)