Differentiation of general and specific abilities in intelligence. A bifactor study of age and gender differentiation in 8- to 19-year-olds

The structure of intelligence is not static. Differentiation hypotheses suggest that the contribution of g and specific abilities on tasks performance varies with age, but most research focused solely on g. Here we sought to test intelligence differentiation in both its general and specific components using a bifactor modelling approach that – previously ignored in literature – should be better suited to this aim. In parallel, the possible differences in intelligence differentiation between males and females are explored for the first time. A population of 8866 youths (8–19 years old) of the Philadelphia Neurodevelopment Cohort completed 12 tasks measuring four components of cognition (complex cognition, executive control, episodic memory, and social cognition) and the WRAT-4. Using bootstrap and meta-regression analysis, the bifactor-(S·I - 1) model shows that the variance explained differently changes over time according to peculiar development patterns of specific components. Executive functions lose specificity with age and their variance is explained only by g at the end of adolescence, episodic memory is increasingly explained by g, and complex cognition is explained more by g in males – and less in females – when age increases, also suggesting a possible role of gender in intelligence differentiation. We conclude discussing the importance of using adequate statistical models and we proposed studying differentiation also at the level of specific abilities to account for the complexity of the developmental phenomenon that could be better described by development priority theories.