The objective of this study is to determine which pre-existing athlete characteristics, if any, are associated with greater deficits in functioning following sports-related concussion, after controlling for factors previously shown to moderate this effect (e.g., time since injury). Ninety-one independent samples of concussion were included in a fixed+systematic effects meta-analysis (n = 3,801 concussed athletes; 5,631 controls). Moderating variables were assessed using analogue-to-ANOVA and meta-regression analyses. Post-injury assessments first conducted 1–10 days following sports-related concussion revealed significant neuropsychological dysfunction, postural instability and post-concussion symptom reporting (d = −0.54, −1.10, and −1.14, respectively). During this interval, females (d = −0.87), adolescent athletes competing in high school competitions (d = −0.60), and those with 10 years of education (d = −1.32) demonstrated larger post-concussion neuropsychological deficits than males (d = −0.42), adults (d = −0.25), athletes competing at other levels of competition (d = −0.43 to −0.41), or those with 16 years of education (d = −0.15), respectively. However, these sub-groups’ differential impairment/recovery beyond 10 days could not be reliably quantified from available literature. Pre-existing athlete characteristics, particularly age, sex and education, were demonstrated to be significant modifiers of neuropsychological outcomes within 10 days of a sports-related concussion. Implications for return-to-play decision-making and future research directions are discussed. (JINS, 2013, 19, 1–17).