Within-Individual Variability in Neurocognitive Performance: Age- and Sex-Related Differences in Children and Youths From Ages 8 to 21

Objective: The transition from childhood to adulthood is characterized by improved motor and cognitive performance in many domains. Developmental studies focus on average performance in single domains but ignore consistency of performance across domains. Within-individual variability (WIV) provides an index of that evenness and is a potential marker of development. Method: We gave a computerized battery of 14 neurocognitive tests to 9138 youths ages 8-21 from the Philadelphia Neurodevelopmental Cohort. Results: As expected, performance improved with age, with both accuracy and speed peaking in adulthood. WIV, however, showed a U-shaped course: highest in childhood, declining yearly into mid-adolescence, and increasing again into adulthood. Young females outperformed and were less variable than males, but by early adulthood male performance matched that of females despite being more variable. Conclusion: We conclude that WIV declines from childhood to adolescence as developmental lags are overcome, and then increases into adulthood reflecting the emergence of cognitive specializations related to skill-honing and brain maturation.