The present study is one of the most comprehensive on the relative age effect in sports, also investigating birthdates in relation to sports performance. The objective was to determine the magnitude and prevalence of RAEs in different individual sports, by age groups and sex, and in relation to physical performance. Our results show that RAEs are consistently prevalent in most individual sports in Sweden, both physically demanding and cognitive/skill-based. In general, no correlation between birthdate and performance was observed, neither in elite nor in recreational athletes. Findings of skewed birthdate distribution in all athletic populations should be viewed from both sports and health perspectives, and in light of the United Nations’ CRC.
Subgroup Analyses
When stratified by age group, greater magnitude (distribution skewness, expected/observed ratio, and effect sizes) of RAEs was seen in the youngest (≤ 6 years; V = 0.31) and lower in older (≥ 40 years; V = 0.06) age categories. These findings are in line with previous research [35, 43], proposing a decline in the influence of maturation and growth with increased age, and must be a result either of participants born early in the season leaving sports at a later age, or a re-entry of participants with later birthdates.
In older adults (≥ 60 years), and even stronger in ≥ 65 years, an inversed RAE was found in male cross-country skiing2 and orienteers of both sexes. This can potentially be an effect of late-born individuals returning to sports later in life, after not having an equal chance against their early-born peers during childhood and adolescents. In the youngest age groups (< 10 years), multiple environmental constraints may influence children’s participation in sports [48], including parents’ reluctance to enroll later-born/maturing children [16, 49], more so with higher socioeconomic status [50]. It can be a fair assumption that among the youngest, other factors than the children’s will causes the pronounced RAE [16, 48,49,50]. At this young age, parents most likely decide when, where, and how the child participates in activities. In adolescence, selection is more often done by coaches.
When pooling sport types (Table 3), RAEs are significant in both physical sports and skill-based sports (V = 0.23 and 0.09, respectively) but with an important difference: The RAE in skill-based sports is inversed, with a slight overrepresentation in T3 (ratio = 1.02, supplementary material 4). This is mostly explained by male adults (21–39 years), which is by far the largest cohort in the skill-based sport types, and the only subgroup with an inversed RAE. Even distributions or inversed RAEs have been found in other non-physical skill-based sports, such as shooting [42, 51]. While not investigated in this study, sports with a high degree of technical skills and esthetics (such as gymnastics, dance, and figure skating) have been found to have no, inversed, or atypical RAEs [39, 52] discussed in detail by others [39, 48, 52, 53]. One possibility is that relatively younger individual’s dropout from physical sports, to focus more on studies or other hobbies, is a parental decision when the child lags behind in performance. Potentially, this could influence the inverse RAE in some E-sports cohorts. Meanwhile, E-sports today is possibly more competitive than 10–20 years ago, and in youths, being relatively older could be a competitive advantage. Speculatively, this could influence the RAE in youths compared to the inversed RAE in adults.
In the present study, RAEs were most prominent in physical sports such as cross-country skiing (both recreational and elite) and athletics, where T1 constituted of up to 51% of the athletes in some sub-samples, with large effect sizes, for instance, cross-country skiing boys ≤ 8 years (T1 = 51%, V = 0.43) for 14-year-old athletes in athletics, boys (T1 = 51%, V = 0.45) and girls (T1 = 48%, V = 0.41). In orienteering, RAEs were significant, but less skewed compared to cross-country skiing and athletics. In the relatively small sample of elite alpine skiers, a RAE was seen only in adult males, in line with previous research demonstrating RAEs in males only [54] or in both sexes [39] (Supplement 2).
It is generally assumed that the magnitude of the RAE in female sports is smaller due to less intense competition among young girls [42, 55]. Some have suggested that female sports are less strength-related than male sports, making the maturation-related developmental lead less decisive [56]. In the present study, RAEs were consistently seen in both sexes, and we did not observe a smaller overall RAE magnitude in females compared to males, as reported in some previous studies [20, 35, 51]. It has been suggested that relatively older female athletes may be at a greater risk of dropping out of sports [48], possibly due to early maturation that has been associated with increased negative psychosocial outcomes [57]. Further, some propose that a stereotyped definition of femininity could discourage early maturing females from participating in sports, in order to conform to socially constructed gender roles [58]. However, this is not reflected in our results were RAEs are consistently seen in both sexes.
Because most samples included a mix of recreational, competitive, and elite athletes, no sub-analysis was done on the level of competition. However, alpine skiers and cross-country skiing dataset 3 constituted of elite athletes only. As mentioned, alpine skiers showed no RAE when both sexes are pooled, in either junior or senior athletes. Only in adult males, a significant RAE was seen, with a large effect size but a limited sample size (N = 70, V = 0.39). In elite cross-country skiers, however, RAEs were prevalent in both junior and senior athletes (Fig. 4) as also seen in previous studies [39].
Performance
Our results show that earlier born children sometimes perform better and are higher ranked than later-born peers. This trend is not seen in adult athletes where, in general, the average performance is the same, regardless of when you are born. There are just not as many athletes born late in the year. There is no surprise that children born earlier in the season also perform better—they are up to a year older than those born at the end of the season. In athletics, there is in general no difference between tertiles in performance (Figs. 11 and 12, Supplement 4). In some events and age groups, T3 and/or T2 actually performed better than T1. This is contrary to the general concept of RAE [59]. However, the result can be explained by the sample population. It is inevitable that the average level will be roughly the same for all three subgroups; all have been selected according to the same criteria (to place in the top 30). While the performance is equal, the distribution is nevertheless skewed. Hence, it is reasonable to say that the late-born adolescents that make the top 30 cut are early physically developed for their age in addition to being talented, while the early-born adolescents are overrepresented due to RAEs.
In this context, it has been shown that early sport success is not an adequate predictor of top-level performance [60]. Rather, entering competitions later is linked to better performance during adulthood [61]. The “underdog hypothesis” should be addressed in this context. The underdog hypothesis contends that, because they are less (physically) mature, relatively younger players must possess or develop superior technical, tactical, and physiological skills to be competitive [62]. In early adulthood, when differences in physical maturity are attenuated, this could in turn be in favor for the relatively younger athletes [63]. Importantly, for this hypothesis to be realized, later maturing/relatively younger athletes must be retained within the sport system. Todays’ early talent selection system, present in too many situations, is counter-productive when the goal is to find the very best future athletes.
General RAE Discussion
Early talent selection, self-determined dropouts [64], or parents’ decision not to enroll relatively younger children in sports [48] all contribute to observed RAEs. With an increasing proportion of physical activity taking place in organized forms, such as in sports clubs [31] and the known relationship between childhood and adult physical activity levels [65], RAEs will impact long-term health. Also, being omitted from sports participation due to any of the above reasons violates the CRC, stipulating that the best interest of every child should always be in focus. Coaches’ and parents'selection of future elite athletes, annual age-grouping, systems, and adults’ administration of competition results must not be an excuse for the facilitation of sports dropout. Early selection and specialization in sports as young creates a requirement for children’s performance but is to a large extent a reflection of maturation, not talent and future performance. For those who do not live up to the requirements, there is a risk, or even guarantee, of being excluded from their team.
Should the RAE here seen in individual and previously in most team sports be disregarded because of the survival of the fittest? Nolan and Howell [66] state that the RAE will continue to exist and raise the question: “why shouldn’t it?” Why should it not be a Darwinian selection in sport at the highest level, and what says that the system of elimination must be fair? However, our results clearly demonstrate the lack of predictive power in early selection as athletes born early in the year do not perform better compared to their later-born peers. Most importantly, the current system results in discontinuation of physical activity in late-born children. In addition to the health benefits of sports participation, children who drop out of sports experience more social and emotional problems [67].
It has also been appreciated that not everyone has the physical and mental capacities to make it to the top [68]. A fair organization in competitive sports is important for children to develop individually [7], but also for the very best senior athletes to-become not to be negatively selected at an early age in favor for individuals who do not have the future capacity to succeed as elite athletes. A skewed distribution based heavily on birthdate will eliminate future senior elite athletes while passing along the near elite. This can be viewed as the number of individuals above and below the expected distribution (Fig. 13). Assuming that talented children are born all year around, and not more likely in the spring, some individuals constituting the bars reaching above the expected distribution have made it to an elite level based on skewed selection due to birthdate. Other individuals who should have become elite athletes and contributed to the bars not reaching the expected distribution ever continued their athletic careers due to negative selection in younger years. At no time are all the physiologically and psychologically most suited athletes given the chance to reach their potential. Thus, it can be concluded that “survival of the fittest” in sports is skewed by less-than-optimal selection system and “fittest of the selected” a more appropriate term.
Moesch et al. [69] found that elite athletes pass important steps in their career at a significantly older age than their near-elite peers. Only after the age of 18 years will elite athletes complete more training hours in their sport than near-elite athletes, with a significant difference shown only after the age of 21 years. Also, elite athletes spend significantly fewer years on junior national teams, but more years on senior national teams. This, in combination with the conclusion made by Williams and Reilly [70], where it was seen as extremely difficult to identify those who will eventually reach the top, provides evidence that applying Darwinism on youths will inevitably lead to lower elite performance, as a group, among adult athletes.
To make competitions fair, to ensure that children are treated equally and ultimately develop optimal elite athletes, coaches and parents must be aware of the risk for skewed and inappropriate selections at young age. Loss of talent may be avoided if parents, coaches, and sport federations would allow prediction of success to occur at a later age [7, 27, 71]. Also, Martindale [68] concluded that de-emphasizing age-group success is a crucial concept to implement in talent development environment and that problems in selection and coaching will continue to exist if not a change in the stress on age group success is eliminated. The current system used in most countries may also disfavor early maturing children in the sense that they live under the impression that success can be reached without effort.
Strengths and Limitations
A key strength of this study is that the distribution analyses are executed with the true parental distribution as a reference [46]. Also, the large sample size, often lacking in previous examinations, together with the multiple sports included results in a broad and robust examination of the topic. In line with similar research, multiple chi-squared tests have been executed, with no correction for multiple tests. A more stringent approach would reduce type I errors, i.e., false positives. Meanwhile, adjustment for multiple comparisons is not always desirable [72, 73]. The general result and conclusion would not be different, with a few less rejections of the null hypothesis (equal distribution of athletes).