Background:Sport specialization has been associated with increased injury and negative psychosocial effects on young athletes. With the continuing trend toward specialization, studies have begun to examine what motivates this decision (eg, building a skill, getting a scholarship). No study has directly assessed the personal characteristics underlying these stated reasons.Purpose/Hypothesis:This study examined the role of athlete competitiveness (enjoyment of competition and competitive contentiousness) as a characteristic associated with propensity to specialize in the United States. We hypothesized that, at the high school level, athletes would be more likely to engage in sport specialization owing to enjoyment of competition versus competitive contentiousness.Study Design:Cross-sectional study; Level of evidence, 3.Methods:We conducted an online survey of 975 high school athletes in the United States who were recruited via the Dynata research panel. Measures included a previously published sport specialization categorization (low, medium, high) and the 2 dimensions of the Revised Competitiveness Index (enjoyment of competition and competitive contentiousness). Also collected were athlete characteristics, sports played by the athletes, level of competition, and whether they planned to play sports in college. Analytical methods employed included cross-tabulations, multinomial logit, and ordinary least squares regression.Results:Overall, 22.4% of the athletes reported a high, 34.8% reported a medium, and 42.9% reported a small level of specialization. No differences in the distribution of sport specialization by sex or age were observed; however, athletes who definitely planned to play in college were significantly more likely to have a high level of specialization (P < .001). Enjoyment of competition was associated with greater specialization (beta = .196; P < .001), whereas competitive contentiousness was associated with lower levels of specialization (beta = −.299; P < .001). These findings were robust to all 3 different analytical methods we employed.Conclusion:Study findings indicated that, while athlete competitiveness is associated with sport specialization, the nature of that competitiveness determined the association. Being an argumentative contrarian may predispose athletes to lower levels of sport specialization, whereas enjoying competition may encourage higher levels of specialization.