Abstract Performance validity tests (PVT) measure optimal cognitive effort (Sherman et al., 2023). As PVT scores decrease in adults, neuropsychological performance decreases systematically (Green, 2007). Similarly, evidence suggests that even a single error in PVTs influences overall cognitive performance (Erdodi et al., 2017). No studies have investigated this relationship between cognitive performance and PVT performance on multiple PVTs within a large pediatric sample. Youth athletes (n = 174; ages 8–16, M = 12.07) completed the Rey 15-Item test (RFIT), TOMM Trial 1, Reliable Digit Span (RDS), and a thorough neuropsychological battery. T-tests compared neuropsychological performance in youth who scored perfectly on the TOMM (n = 36) and RFIT (n = 90) to youth with single errors (TOMM, n = 28; RFIT, n = 26). A planned contrast analysis of neuropsychological performance was performed for RDS within score levels 6–11. On processing speed, delayed verbal memory, and working memory, the TOMM perfect score group performed better than the single error group (p’s < 0.05). On RFIT, the same relationship was seen on story memory, working memory and simple attention (p’s < 0.05). Perfect score groups scored higher on nearly all subtests (Hedge’s G’s = 0.20–0.60). Scores in perceptual reasoning, cognitive flexibility, processing speed, and visual memory increased as RDS scores increased by one point (p’s < 0.05). This study shows the impact of even subtle differences in effort on cognitive performance in youth. Measuring effort is critical in pediatric neuropsychology, and these results illustrate how important it may be to measure effort on a continuum in pediatrics, as opposed to using pass/fail cutoffs.