Week 4 Discussion Response

Taylor & Francis Taylor & Francis Group http://taylorandfrancis.com

9 SPORT ACTIVITY IN CHILDHOOD

Early specialization and diversification

Paul R. Ford and A. Mark Williams

Sport activity in childhood takes place in formal and informal settings. Formal settings involve adults planning, organizing and leading activity for children, such as in coaching or physical education sessions and in competitions. Informal settings involve children leading the sport activities themselves without adult intervention, such as in playgrounds, parks, streets, gardens, car parks, homes, and beaches (Ford, 2016). In some sports in certain countries, children who are considered proficient or with potential are identified to engage in formal, competitive and systematic talent development programmes. In these programmes, children often specialize solely in the sport which has become known as early specialization (for a review, see Malina, 2010). Early specialization is defined as starting in the primary sport during childhood (5 to 12 years of age), starting in a talent development programme in childhood, and during that period engaging in one sport only, or at least primarily (Baker, Cobley, & Fraser-Thomas, 2009). Moreover, early specialization involves engaging in a relatively high volume and intensity of training in that sport, as well as in competition in that sport through tournaments, matches and/or leagues (Baker et al., 2009).

Many children engage in sport outside of these formal programmes, such as in informal activity in a single sport, perhaps with some engagement in other sports and/or some formal recreational activity sport, which is known as early engagement. Informal playful sports activities are self-directed and are engaged in with the intention of fun and enjoyment (Côté & Hay, 2002). This activity is hypothesized to lead to benefits such as increasing intrinsic motivation (cf. Hendry, Crocker, & Hodges, 2014; Imtiaz, Hancock, & Côté, 2016) and acquiring skills, such as decision-making (e.g., Roca, Williams, & Ford, 2012). Other children engage in sport mainly in formal, recreational settings. At the opposite end of the continuum from early specialization, is a pathway termed early diversification. Early diversification is defined as engage- ment in a number of different sports during childhood in mainly informal, but also formal settings, with late or delayed specialization into formal activity in a single primary sport in adolescence (Côté, Baker, & Abernethy, 2007). In adolescence, these pathways appear to converge to contain increasing specialization for talented athletes in a single sport through a high volume and intensity of training and competition. The predicted characteristics of the early specialization, engagement, and diversification pathways are shown in Table 9.1. The predicted and confirmed outcomes associated with and following early specialization, engage- ment, and diversification can be found in Table 9.2.

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Researchers appear to have generally decided that, to paraphrase the renowned English author George Orwell in his book Animal Farm (1946), ‘early diversification good, early specializa- tion bad’. The latest incarnation of this idea was forwarded in an International Olympic Committee Consensus Statement on youth athletic development (Bergeron et al., 2016). It states that ‘youth should avoid early specialisation, as diverse athletic exposure and sport sampling enhance development and athletic capacity, reduce injury risk and increase the oppor- tunity for a child to discover the sport(s) that he/she will enjoy and possibly excel at’ (p. 845). Several other key review papers published in scientific journals have taken a similar position (Baker, 2003; Baker et al., 2009; Côté, Lior, & Hackfort, 2009; Côté & Hancock, 2016; Jayanthi, Pinkham, Dugas, Patrick, & LaBella, 2012; Mostafavifar, Best, & Myer, 2013; Wojtys, 2013). In contrast to this position, coaches and practitioners appear to have decided in Orwell’s terms that while ‘early diversification is good, early specialization is better’. In some countries a large

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Table 9.1 The predicted characteristics of early specialization, engagement, and diversification

Early specialization Early engagement Early diversification

Childhood Start age in primary sport Childhood Childhood Not specified Start in ‘talent programme’ Yes No No Play in primary sport Low High Low to medium Practice in primary sport High Low to medium Low or no Competition in primary sport High Low or no Low or no Other sports Low Low to medium High

Adolescence Play in primary sport Decreases to zero Decreases to zero Decreases to zero Practice in primary sport High Increases to high Increases to high Competition in primary sport High Increases to high Increases to high Other sports Decreases to zero Decreases to low Decreases to low

Table 9.2 The predicted and confirmed outcomes associated with and following early specialization, engagement, and diversification

Early specialization Early engagement Early diversification

Positive outcomes in primary sport Performance improvement Yes Yes Possibly through

transfer Expert performance Yes Yes Yes

Other positive outcomes Continued participation Some Yes Yes Intrinsic motivation Lower Increased Increased Enhanced social skills Lower Possibly medium Increased

Negative outcomes Overuse injury incidence Higher Possibly medium Lower Burnout and dropout Higher Possibly medium Lower

industry has been built around youth development programmes that employ thousands of experienced adults to work with children (and adolescents) who specialize in a single sport in well-resourced and expensive facilities.

In this chapter, we present a critical review of research, theory and practice on sport activ- ity in childhood. We support the views of others that childhood sport activities should contain positive features and lead to several positive outcomes (for reviews, see Côté et al., 2009; Côté & Hancock, 2016; Fraser-Thomas, Côté, & Deakin, 2005; Vierimaa, Erickson, Côté, & Gilbert, 2012). However, in this chapter, we mainly limit assessment of the evidence to the develop- ment of skill or talent, as is the focus of this book. From this perspective, we seek to identify youth sport activities that cause the development of expert performance at the highest levels of professional sport without the occurrence of any negative consequences, such as overuse injuries or burnout. First, we review evidence for and against early specialization and diversifi- cation, which we focus on because these are the two main developmental activity pathways identified by researchers. To our knowledge, there is no single research study that contains children in controlled groups who either specialize or diversify, which measures between-group differences in variables and outcomes of interest at key time points from their current engage- ment in childhood into adulthood. Such longitudinal research is difficult to undertake for many reasons. Scientists have tended to use case-control and cross-sectional studies from which the quality of evidence is lower because of decreased internal validity when compared to cohort studies or randomized control trials (Petticrew & Roberts, 2006). Nevertheless, in the second part of the chapter, we forward evidence-based recommendations seeking to resolve research and practical issues in this area.

Assessing evidence for early specialization

The main rationale used to support early specialization is the positive relationship repeatedly found between amount of time spent practising a task or domain and level of competence. In this rationale, an earlier start age in a sport coupled with specialization enables a larger accu- mulation of time spent practising and, thus, higher levels of competence and achievement at a later date when compared to later start ages or not specializing. In addition, the term practice is usually used in this argument as a general term to describe engagement in formal sport-specific activities during childhood, including training or coaching sessions. It is beyond the scope of this chapter to review all of the research showing that time spent in practice or training leads to skill acquisition and improved competence, as well as improved physical fitness, but these are generally considered to be scientific laws within certain limits (e.g., Schmidt & Lee, 2011; Kenney, Wilmore, & Costill, 2015).

One of the most influential scientific theories on practice is often cited (e.g., Bullock et al., 2009) as support for early specialization. Deliberate practice theory was introduced by Ericsson, Krampe and Tesch-Römer in 1993. A central part of the theory is the monotonic benefits assump- tion. It holds ‘that the amount of time an individual engages in deliberate practice activities is monotonically related to that individual’s acquired performance level’ (Ericsson et al., 1993, p. 368). As such, they state that ‘individuals who start early and practice at higher levels will have a higher level of performance throughout development than those who start later’ (p. 392). There is no mention of the term ‘early specialization’ in the 1993 paper or in Ericsson’s subse- quent writings on the theory (1996; 2003; 2006; 2007; Ericsson & Towne, 2010). The theory covers all domains of achievement, such as musicians, surgeons or law enforcement, so the earlier start age it refers to does not specify childhood. They do state that ‘expert performance is not reached with less than 10 years of deliberate practice’ (Ericsson et al., 1993, p. 372). In

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sport, of course, where the attainment of expert performance is most often required in late adolescence or sometimes earlier, such as in gymnastics (Law, Côté, & Ericsson, 2007), the 10- year rule infers a start age in childhood.

One of the main methods used to examine the question of early specialization or diversifi- cation was introduced by Ericsson et al. (1993). The retrospective recall method necessitates that expert athletes recall via questionnaires or interviews the number of hours they have spent in practice and other sport activities since they began in their sport. Multiple researchers have shown that hours accumulated by expert adult athletes in practice activities since starting in their sport are greater compared to control groups of lesser-skilled athletes (for recent reviews, see Baker & Young, 2014; Ford, Coughlan, Hodges, & Williams, 2015). In a few of these stud- ies, expert adult or late adolescent athletes reported that they experienced the main characteristics of early specialization in childhood (Ford et al., 2012; Law et al., 2007). Law et al. (2007) used this method to demonstrate that Olympic and international standard adult female rhythmic gymnasts started training in the sport at 6–8 years of age, competing at regional level at age 7–8 years, and started spending all their leisure time training in the sport from 11–12 years of age. During childhood, the Olympic gymnasts accumulated around 2,000 hours of sport-specific training and engaged in one other sport activity. It is likely gymnasts specialized early because expert performance in this sport is required in mid-adolescence, as opposed to in late adolescence or early adulthood in most other sports. Similarly, Ford et al. (2012) showed that late adolescent elite soccer players in Mexico and Sweden started training in the sport around 5 years of age, participated in competitions at age 8 years, and started at an elite academy at 12–13 years of age. During childhood, the elite players accumulated over 1,600 hours of sport-specific training and engaged in one to three other sport activities. However, there was large variation in training hours between players across this period (e.g., min = 224 hrs; max = 5,680 hrs) and both groups accumulated around 1,200 to 1,400 hours in informal, playful soccer activities during childhood, albeit with large variation in hours between players (e.g., SD = 800 hrs). Moreover, late adolescent elite players in England, Brazil, and Portugal demonstrated fewer of the characteristics of childhood specialization in the sport.

Retrospective recall studies have two methodological shortcomings that limit their findings in relation to the question at hand. First, these studies only describe the current youth devel- opment system in place in that sport and country and those who stayed in the system. It does not follow that the system is optimal and it may be that many other potential athletes dropped out of the system during their youth or were not selected. Second, retrospective recall studies only show the activities that preceded the attainment of expert status in adulthood or late adoles- cence. They do not show that the early activities caused the attainment of expert status. In the next section, we review retrospective recall studies that show how the childhood sports activi- ties that preceded the attainment of expert status in adulthood or late adolescence do not meet the criteria of early specialization. In addition, we review one of the strongest pieces of evidence against early specialization, namely the negative consequences, costs and outcomes of engaging in this pathway.

Assessing evidence against early specialization

Early specialization has been associated with a number of negative outcomes, costs or conse- quences for youths, such as increased incidence of overuse injuries, burnout, dropout, overtraining syndrome and decreased social development. Olympic rhythmic gymnasts reported more overuse injuries and lower health across their development when compared to lesser-skilled international gymnasts (Law et al., 2007). Both groups reported more injuries in

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adolescence compared to childhood. In addition, Olympic gymnasts significantly increased training hours across adolescence, suggesting a positive relationship between training load and injury incidence in this period. Higher workloads and some other aspects of early specializa- tion (e.g., being elite level) have been associated with greater incidence of upper limb overuse injuries in youth gymnasts (DiFiori, Puffer, Mandelbaum, & Mar, 1996) and baseball pitchers (Lyman et al., 2001, 2002; Olsen et al., 2006; for a review, see DiFiori et al., 2014). Similarly, higher training loads and other aspects of early specialization (e.g., frequent intense competi- tion activity, sole-focus) has been associated with greater incidence of burnout, dropout, and overtraining in youth tennis (Gould, Tuffey, Udry, & Lowhr, 1996), gymnastics (Dubuc, Schinke, Eys, Battochio, & Zaichkowsky, 2010), swimming (Fraser-Thomas, Côté, & Deakin, 2008), and golf (Cohn, 1990). These examples of the negative consequences of early special- ization led several researchers to recommend early diversification as an alternative pathway (Baker, 2003; Baker et al., 2009; Bergeron et al., 2016; Côté et al., 2009; Côté & Hancock, 2016; Jayanthi et al., 2012; Mostafavifar et al., 2013; Wojtys, 2013).

Further evidence against early specialization can surprisingly be found in deliberate practice theory (Ericsson et al., 1993). The 1993 paper is often misquoted, hence the need for us to use direct quotes from it in this section. In the 1993 paper they outline ‘three phases of develop- ment toward adult expertise’ (p. 369). Of particular interest to this chapter is the first phase of development detailed in the paper. Ericsson et al. state they ‘rely on Bloom’s (1985) character- ization of this (first) period of preparation’ (p. 369). They further state that ‘from many interviews with international-level performers in several domains, Bloom (1985) found that these individuals start out as children by engaging in playful activities in the domain. After some period of playful and enjoyable experience they reveal ‘talent’ or promise. At this point, parents typically suggest the start of instruction by a teacher and limited amounts of deliberate practice’ (Ericsson et al. 1993, p. 369). They summarize this as follows: ‘the first phase of partic- ipation in a domain begins with an individual’s introduction to activities in the domain and ends with the start of instruction and deliberate practice’ (p. 369). The inclusion of this early, pre-deliberate practice stage is often overlooked in reviews of Ericsson et al. (1993). Elsewhere, Ericsson (2003; 2007) uses a version of the ‘power law of practice’ (Newell & Rosenbloom, 1981) to demonstrate how performance improvement is rapid in the earlier stages of experi- ence in a domain, whereas the rate begins to slow or plateau later in the process. Ericsson clearly states that individuals should begin to engage in deliberate practice once further improvements to performance in the domain start to slow or plateau, suggesting a pre-period of informal activities. We hypothesize that a positive relationship exists in certain domains between the complexity of the domain and the duration of this early period of playful activi- ties. For example, more complex fields, such as certain sport domains, will require longer durations of this first stage of playful activities before performance improvement begins to slow or plateau when compared to less complex domains. In the next section, we review several retrospective recall studies with professional athletes that show they did not engage in the early specialization pathway in childhood, but that they did engage in informal, playful activities during this period.

Assessing evidence for early diversification

Several researchers have advocated early diversification as the pathway that does not lead to the negative consequences of early specialization, but does precede the attainment of professional status in adulthood (Baker, 2003; Baker et al., 2009; Côté et al., 2009; Côté & Hancock, 2016; Fraser-Thomas et al., 2005; Vierimaa et al., 2012; Jayanthi et al., 2012; Mostafavifar et al., 2013;

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Wojtys, 2013). The main evidence for early diversification as the activities that precede the attainment of professional status in adulthood is twofold. First, the negative consequences of early specialization support the idea that children should not engage in that pathway. Moreover, as shown in Table 9.2, early diversification is predicted to protect against overuse injures and dropout or burnout and to have other positive outcomes, such as improving social skills. Second, retrospective recall studies examining the developmental activities of athletes are predicted to show early diversification precedes the attainment of later expert status. In the following section we provide a critical review of these retrospective recall studies.

In order to ascertain the sport activities engaged in during childhood, it is necessary to have data on start age in the primary sport and other sports; start age in a talent development programme in the primary sport; the amount of play, practice and competition activity in the primary sport during childhood; and the amount of play, practice and competition activity in other sports during childhood. Table 9.3 (below) uses these criteria and summarizes some stud- ies that have been cited elsewhere as support for early diversification. The criteria for inclusion in Table 9.3 are that the participants in the study are adults playing at the highest level of their professional sport. As such, many papers examining adolescent athletes including those who participate in talent development programmes were excluded because a number of those participants will not achieve the highest professional level and, therefore, their activities create a confounding variable in these data sets that limits their findings (Bridge & Toms, 2013; Côté, 1999; Ford, Ward, Hodges, & Williams, 2009; Ford et al., 2012; Hayman, Borkoles, Taylor, Hemmings, & Polman, 2014; Henriksen, Stambulova, & Roessler, 2010; Leite & Sampaio, 2012; San & Li, 2014; Wall & Côté, 2007; Ward, Hodges, Starkes, & Williams, 2007). In addition, a few studies contain adult participants who did not play at the highest level of their professional sport (Hill, 1993; Ford & Williams, 2008, 2012; Roca et al., 2012; Surya, Bruner, MacDonald, & Côté, 2012) and that were excluded from Table 9.3 because their data sets are confounded by this fact. All of these studies have added to knowledge in this area, but for the purposes of this chapter the childhood activities of athletes at the highest level of their professional sport are considered.

We found that no studies provided full data sets on the four aspects of start age in the primary sport; start age in a talent development programme in the primary sport; the amount of play, practice and competition activity in the primary sport during childhood; and the amount of play, practice and competition activity in other sports during childhood. Some stud- ies with professional-level athletes that we wished to include in Table 9.3 had insufficient sets of this data (Coutinho, Mesquita, Fonseca, & De Martin-Silva, 2014; Haugaasen, Toering, & Jordet, 2014; Hodges, Kerr, Starkes, Weir, & Nananidou, 2004; Weissensteiner, Abernethy, Farrow, & Muller, 2008; Young & Salmela, 2010). Moreover, the level of detail required to complete Table 9.3 and answer the key question in this section appeared – on construction of the table – to be available only from studies on athletes from the highest level of single sports. Studies that present combined data from professional athletes across multiple sports were excluded from Table 9.3 because they did not contain sufficient amounts of the data we required to be included (Baker, Côté, & Abernethy, 2003a, 2003b; Barreiros & Fonseca, 2012; Hopwood, MacMahon, Farrow, & Baker, 2015; Leite, Baker, & Sampaio, 2009; Memmert, Baker, & Bertsch, 2010; Moesch, Elbe, Hauge, & Wikman, 2011; Moesch, Hauge, Wikman, & Elbe, 2013). These multi-sport studies are addressed in a separate section later in the chapter. Similarly, qualitative studies with professional athletes lacked the details we required for Table 9.3, so were excluded from it and are reviewed in their own section below (Carlson, 1988; Côté, 1999; Durand-Bush & Salmela, 2002; Johnson, Tenenbaum, Edmonds, & Castillo, 2008; Monsaas, 1985; Phillips, Davids, Renshaw, & Portus, 2010; Storm, Kristoffer, & Krog, 2012).

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Table 9.3 shows that the most common pathway in the studies examining the develop- mental activities of team sport athletes at the highest level of their professional sport is a combination of the early diversification and early engagement pathways (Berry, Abernethy, & Côté, 2008; Hornig, Aust, & Güllich, 2016; Soberlak & Côté, 2003). That is, athletes at the highest level of these team sports started their engagement in childhood and during that stage they engaged in varying amounts of activity in their primary sport. In addition, during child- hood they played other sports. Moreover, in most of these studies the most common other sports were other team sports, suggesting the possibility of some transfer of skill occurring between sports (e.g., Causer & Ford, 2014; Smeeton, Ward, & Williams, 2004). A variable that appears to link these team sports is that professional status is attained in late adolescence or early adulthood, probably enabling less specialized engagement in childhood. Australian Football League (AFL) players had the lowest amount of time spent in primary sport activity, albeit the amount of time spent in other sport hours by these players during childhood was low to medium (Berry et al., 2008). In contrast, in childhood, the ice hockey players had the highest amount of time spent in primary sport activity (Soberlak & Côté, 2003), whereas the German soccer players had the lowest amount of time spent in other sports (Hornig et al., 2016).

Table 9.3 shows less coherent findings and less published data for adult athletes at the high- est level of individual sports. Triathletes in Canada started in their primary sport in adulthood and in running in mid-adolescence, although experts started in these activities earlier compared to lesser-skilled triathletes, with all this preceded in time by some unspecified involvement in other sports (Baker, Côté, & Deakin, 2005). In contrast, as described earlier, Olympic and inter- national standard female rhythmic gymnasts in Canada engaged in an early specialization pathway (Law et al., 2007). It is highly likely that differences in the age that peak performance is required in these sports led to these differences in activities between studies, with triathletes requiring peak performance in adulthood (Baker et al., 2005) and gymnasts in early or mid- adolescence (Law et al., 2007). There is a relatively low amount of data published on the developmental activities of athletes at the highest level of professional individual sports, partic- ularly those meeting the criteria for inclusion in Table 9.3. Athletes at the highest levels of individual sports have been participants in both multi-sport quantitative studies and qualitative studies investigating their developmental activities, which are reviewed in the next sections.

Multi-sport samples

Some researchers have published combined data from professional-level athletes across various sports in single countries (Baker et al., 2003a, 2003b; Barreiros & Fonseca, 2012; Durand-Bush & Salmela, 2002; Leite et al., 2009; Memmert et al., 2010; Moesch et al., 2011, 2013; Storm et al., 2012) or two countries (Hopwood et al., 2015). Some of these studies present insufficient data sets to reliably answer the question at hand, either focusing on the later, elite stage (Barreiros & Fonseca, 2012; Durand-Bush & Salmela, 2002) or by not measuring start age in – or amount of – informal activity (Leite et al., 2009; Moesch et al., 2011). Most of the other studies support the data presented in Table 9.3 showing the most common pattern of childhood

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sport activities is a combination of the early diversification and early engagement pathways (Memmert et al., 2010; Moesch et al., 2013; Storm et al., 2012). Two studies (Hopwood et al., 2015; Storm et al., 2012) present exceptions to this synthesis of these data sets. Hopwood et al. did not measure other sport involvement in elite athletes across multiple sports in Canada and Australia and found that they had an average start age in the primary sport later than 12 years of age. Storm et al. found a gymnast engaged in the early specialization pathway, supporting the findings for this sport of Law et al. (2007), whereas a rower reported a start age in the sport in mid-adolescence and another rower in adulthood. The findings of Storm et al. and Hopwood et al. may highlight some of the factors that mediate the sport activities engaged in during childhood and early adolescence. These factors include the age at which peak performance is required, the size of the sport in terms of participant numbers, and the athlete attributes required for peak performance.

Of particular importance to this discourse is the Baker et al. (2003a) study because it is often cited as support for the early diversification pathway. Participants were 15 Australian national team players who were selected by coaches as the best decision makers in the sports of female netball (n = 3), female field hockey (n = 4), male basketball (n = 4) and male field hockey (n = 4). It appears the players started in their primary sport in childhood, as their current mean age was 27.6 years and they ‘had been playing their primary sport for an average of 20.7 years’ (p. 15). However, their first involvement in formal activities in their primary sport was an aver- age of 12.9 years prior to their current age, suggesting they engaged in informal activities in their primary sport prior to this during childhood and early adolescence. However, the start age and amount of informal activities in their primary sport in this period is not recorded. The authors do state that the

… athletes reported participating in a wide range of sport activities that were not organized by an adult (i.e., activities that might constitute deliberate play). These deliberate play activities (such as backyard games, pick-up games) were performed in informal atmospheres that were not part of an organized sporting system. The most commonly reported deliberate play activities were forms of football, cricket and cycling.

(Baker et al., 2003a, p. 19)

The key finding presented in this study was a medium-sized negative correlation (r = –.54) between the number of sports played during youth and the amount of hours accumulated in practice in the primary sport prior to being selected to the national team. Athletes who had engaged in more other sports in their youth demonstrated less practice hours in their primary sport prior to selection, and vice versa, providing support for early diversification.

The Baker et al. (2003a) study contains participants from three sports and both genders. The published paper contains mean, standard deviation, minimum and maximum descriptive statis- tics as function of these two variables. At one end of this data set, the male basketball players demonstrated high sport-specific hours and a lower number of other sports, whereas at the other end, the female netball players demonstrated low sport-specific hours and a high number of other sports. In a correlation these differences between these two groups matter when their data are combined because together they contribute to the significant negative correlation. That is, the male basketball players would cause the ‘more training hours associated with fewer other sports’ and the female netball players would cause the ‘fewer training hours associated with more other sports’. In this data set, the hockey players’ data is more similar to the netball players than the basketball players, but that group contains both male and female hockey players.

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If you were to remove the male basketball players from the data set and re-run the correlation on the more similar data from netball and hockey players, we believe there would be at best a small-sized negative correlation. Moreover, in this data set the female participants demonstrated ‘fewer training hours and more other sports’ in comparison to male participants who demon- strated ‘greater training hours and fewer other sports’. This difference between genders may have fully contributed to the aforementioned significant negative correlations when their data is combined. Therefore, the Baker et al. study appears to show that between-sport and -gender differences exists in the developmental activities of professional-level athletes, rather than providing all-out support for early diversification. It appears that females diversified more than males in this study, possibly because female sports have a lower number of total participants compared to male sports. However, gender differences have not been examined extensively in the literature subsequently.

Qualitative studies

A few researchers have published qualitative studies examining the developmental activities of professional-level athletes (Carlson, 1988; Johnson et al., 2008; Monsaas, 1985; Phillips et al., 2010). All of these studies support the data presented in Table 9.3 that the most common pattern of childhood sport activities was a combination of the early diversification and early engagement pathways (Carlson, 1988; Johnson et al., 2008; Monsaas, 1985; Phillips et al., 2010). Tennis players in Sweden (Carlson, 1988), swimmers (Johnson et al., 2008) and tennis players in North America (Monsaas, 1985) and cricket bowlers in Australia (Phillips et al., 2010) at the highest level of these sports started in their primary sport in childhood. In addition, during childhood they played various other sports. Moreover, the primary sport and other sport activ- ity, certainly in the earliest stage, was mainly either in informal settings or in more recreational-like formal settings. In the main, these athletes specialized in early to mid-adoles- cence so that they engaged in increasing amounts of more professionalized formal activity solely in their primary sport. The amount of activity is not quantified in these studies, but these athletes do appear to have engaged in a lot of activity in the primary and other sports during childhood and early adolescence.

Assessing evidence against early diversification

The main hypothesis against early diversification is the positive relationship that exists between amount of time spent in domain- or task-specific activity and level of competence in that domain or task. Higher amounts of domain- or task-specific activity are associated with higher levels of competence in that domain or task, suggesting earlier start ages lead to a greater accu- mulation of activity and the highest levels of competence. In partial support of this hypothesis, Table 9.3 shows that professional athletes in team sports engaged in meaningful amounts of their sport during childhood. However, these athletes engaged in other sports during child- hood, which provides partial support for the early diversification pathway. Early diversification is thought to influence skill acquisition in the primary sport through transfer of learning. Transfer of learning refers to skill that is acquired in one sport leading to improved performance in another sport. Its occurrence is thought to depend upon the amount of similarity in percep- tual, cognitive and/or motor elements between the two sports or tasks, with greater similarity leading to greater transfer (Schmidt & Lee, 2011). In most of the studies shown in Table 9.3, the majority of other sports engaged in had similar elements to the primary sport, suggesting some transfer of learning could have occurred to the primary sport. For example, expert AFL

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players spent more time during their youth in other invasion sports compared to time spent in other types of sports, such as net/wall or field/run sports, suggesting the possibility that some transfer of learning could have occurred (Berry et al., 2008).

Engaging in a sport that has different elements to the primary sport is hypothesized to lead to low amounts of transfer of learning. Researchers have suggested that engaging in these other sports may benefit the child in other ways, such as through increasing social skills or protect- ing against the negative consequences of engaging in the primary sport, perhaps through physical adaptations that protect against overuse injuries (for reviews, see Côté et al., 2009; Côté & Hancock, 2016; Fraser-Thomas et al., 2005; Vierimaa et al., 2012). For example, greater involvement in extracurricular activities including sport was positively associated to academic adjustment, psychological competencies, and a positive peer context in young middle-class adolescents in North America (Fredricks & Eccles, 2006; see also Busseri, Rose-Krasnor, Willoughby, & Chalmers, 2006; Rose-Krasnor, Busseri, Willoughby, & Chalmers, 2006). Further research is required to determine how engaging in other sports enhances these skills in talented youth athletes and protects against overuse injuries as predicted in Table 9.2.

Recommendations for resolving research issues

We are keen for research on this topic to progress and remain open to future findings chang- ing any conclusions we draw from the current literature base. In order for this to occur, we believe four main research issues need to be addressed. These are an increase in the number of research studies conducted; the collection of comprehensive data sets; greater control of inde- pendent variables; and the use of newer, more robust research methodologies.

First, several researchers have published multiple review articles on this topic forwarding new hypotheses to be tested (Côté et al., 2009; Côté & Hancock, 2016; Fraser-Thomas et al., 2005; Vierimaa et al., 2012). However, the amount of data on this topic and these hypotheses is relatively low. Therefore, researchers should concentrate future efforts on collecting data to test these hypotheses in order to address the main question of early specialization, engagement and/or diversification.

Second, in order to test these hypotheses and answer this question, researchers using retro- spective recall methodologies must ensure they collect comprehensive data sets that include: (i) start age in the primary sport and other sports; (ii) start age in a talent development programme in the primary sport; (iii) the amount of play, practice and competition activity in the primary sport during childhood; and (iv) the amount of play, practice and competition activity in other sports during childhood. It is not possible to draw firm conclusions from data sets that do not include these measures.

Third, researchers must gain greater control over the independent variables that affect data collected in these studies. The main independent variable in these studies is the skill level of the participants. Researchers should ensure that participants in the study are adults playing at the highest level of their professional sport. Data collected from youth or lesser-skilled adults limits the conclusions that can be drawn. Another key independent variable is gender, with more stud- ies being conducted to date on male as opposed to female participants. A third independent variable is the country from which the sample is recruited. Some countries contain adult athletes playing at the highest level of a professional sport, whereas for the same sport the best adult athletes in other countries play at a lower level, with data collected from these lesser-skilled groups again limiting conclusions that can be drawn. Our preferred method in retrospective recall studies is to collect data from adults playing at the highest level of a professional sport in multiple key countries (e.g., for a cross-country example, see Ford et al., 2012).

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A fourth key independent variable is the sport from which the sample is recruited. First, there is a low amount of data from athletes in individual sports, with the majority of studies being conducted on athletes from team sports. Second, the sport influences the data and conclusions through at least three other variables. These are the age at which peak perform- ance is required in the sport, the size of the sport in terms of participant numbers, and the athlete attributes required for peak performance in the sport. We hypothesize that sports with higher ages at which peak performance is required, smaller numbers of participants, and/or lower numbers of mainly physical attributes required would be preceded by greater early diversification (e.g., triathlon, rowing, wrestling). For example, Bullock et al. (2009) report how athletes with a background in sprinting sports were able to start in the minor winter sport of skeleton in young adulthood and compete at World Cups and Olympic Games one year later, providing support for our hypothesis. In contrast, we hypothesize that sports with lower ages at which peak performance is required, high numbers of participants, and/or a high number of more complex technical attributes required would be preceded by greater early engagement (e.g., tennis, soccer). Researchers must gain control over all of these contextual and independent variables and report their results with reference to this larger framework, rather than overstating claims from small data sets sampled from this larger context.

Finally, the retrospective recall methodology introduced by Ericsson et al. (1993) is over 20 years old. There is a need for researchers to develop and use new research methodologies to examine this question and produce higher quality evidence, such as cohort studies or random- ized control trials (Petticrew & Roberts, 2006; for an example, see Coughlan, Williams, McRobert, & Ford, 2013).

Recommendations for resolving practical issues

The translation of research into evidence-based practice that positively affects the youths participating in sport and the adults working or volunteering in it is one of our key aims. We believe that three issues in this area must be addressed in children’s sport for this to occur. These are: ensuring all children have the opportunity to engage in informal playful activities in safe environments; ensuring adult-led formal activity does not contain the characteristics that constitute the early specialization pathway; and ensuring sports do not function independently of one another. First, adults should ensure that all children have the opportunity to engage in informal playful sports activities in safe environments by creating multiple accessible areas and providing equipment to do so in school playgrounds, parks, and communities. Second, no child should engage in sport activities that constitute the early specialization pathway given the nega- tive consequences associated with this engagement. Adult-led formal sport activity with children should contain the characteristics of informal-playful activity because of its many posi- tive benefits. Educators must equip coaches, physical education teachers and any adults working with children in sport with the knowledge and skills required for them to lead this activity. Educators should consult skill acquisition researchers to help design the content of this educa- tional activity. The recommendations in this chapter only address macro-level activities, whereas adult/child interactions in sport occur as micro-level activities that require an addi- tional set of evidence-based recommendations (e.g., Ford, 2016). Third, formal sports tend to function independently from one another in many countries, such that funding, resources, and expertise are not combined. However, sports must work more closely together to fund, resource, and share expertise at youth level in order for child sport to meet the recommenda- tions outlined in this section.

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In summary, the practical recommendations for adults working with children are listed below.

1 No child should engage in sport activities that constitute the early specialization pathway. 2 Children who are highly motivated to play a single sport more so than other sports should

be given the opportunity and facilitated to do so through informal playful activities and formal playful-like activities. Those children should be facilitated to engage in some other sports through informal playful activities and formal playful-like activities.

3 Children who are not highly motivated to play a single sport should be given the oppor- tunity and facilitated to sample a variety of sports through informal playful activities and formal playful-like activities.

4 For most sports, across adolescence, the amount of formal training and competition should increase linearly with age until 18 years of age where the athletes should engage full-time in their sport. The amount of informal sport activity across adolescence should decrease linearly with age until 18 years of age where such activity may be ceased.

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