Talent identification programmes are to identify talented players based on current observational performance attributes (Vaeyens et al., 2009). Coaches unlock the future potential of attaining professional contracts by honing talented players technical, tactical, and physical skills, (Till, et al, 2010; Parsonage et al, 2014; Höner, & Feichtinger, 2016). In particularly a greater focus goes towards older players, who exacerbates the physiological advantages of early maturation (i.e., greater height, weight, and physicality) which are desirable and important for successful game performance (Kelly et al., 2021). However, the recent emergence of the underdog hypothesis suggests that the negative impact of relative age and selection bias benefits younger players, as they develop superior psychological skills from the adverse environments (Gibbs et al., 2012; Kelly et al., 2021). Despite researchers presenting theories that psychological factors are the cause of younger players triumphs (Jones et al., 2018), there is little to no evidence to support the notion and the literature surrounding the psychological predictors of age grade rugby union is weak. With this in mind, this thesis aims to examine the pervasiveness of relative age in age grade rugby union whilst identifying the physiological and psychological differences between regional and club players in order to examine the psychological factors that may arise during the reversal of relative age. This thesis is comprised of a general introduction, three experimental chapters and a general discussion. Chapter 1 investigates the physiological and psychological differences between regional and club rugby union players. Experimental chapter 2, builds on the findings of chapter 1, and aims to identify the predominance of relative age, and its repercussions between birth distributions in regards to physiological and psychological characteristics of regional and club players. Finally, experimental chapter 3 tracks the retained regional players over a season to examine the changes in physiological and psychological development in regards to relative age. A cross-sectional design was utilised for study 1 and 2 and a longitudinal approach for study 3. All examined data was collected at Rygbi Gogledd Cymru, regional age grade talent camps. Experimental chapter 1 examines the physiological and psychological characteristics that differentiate regional and club players. Our physiological and anthropometric findings added to the profound talent identification literature where regional players have greater anthropometric characteristics and a more robust physical abilities than club players. The psychological findings indicated unexpected differences where club players presented greater coping skills, and integrated motivation than regional players, however regional players are more optimistic and tend to strive for perfectionism more so than lower playing standards. Experimental chapter 2 used the same data from study 1, players were categorised by birthdate and playing level. The aim of the study was to examine the existence of relative age effect and the physiological and psychological differences between playing level and age grade categories. There was an overrepresentation of older players compared to younger players in age grade rugby union, particularly within backs positions. Relatively older players were taller, heavier, and performed better in physical performance tests within the under 16s cohort. Whereas, in the elite under 18s relatively younger players showed more favourable psychological characteristics for performance such as, extraversion, openness to new experiences, emotional stability, and commitment to training than older elite under 18s players. Experimental chapter 3 was a longitudinal study to track the development of anthropometric, physical, and psychological characteristics whilst considering the impact relative age has on the development of retained regional players aged between 16 and 18 years. The results consider the importance of balancing physical performance improvements with anthropometric development amongst age grade players as physical performance is a common outcome for anthropometric growth and development. Interestingly younger players develop more stable personality traits associated with successful career attainments than older players who were developing symptoms of athlete burnout. Findings offer a potential experimental explanation towards the reversal effect proposed by Jones et al., 2018. Based on the findings presented in chapter 1-3 of the thesis, it provides a foundation for future research to further examine psychological differences between playing standards and their importance in regards to talent development. The implementation of coaches offering a psychological curriculum within a holistic talent development programmes can potentially develop psychological skills as well as the mental health of players which may lead to improved performances and well-being.