Date of Award

Spring 1-1-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Psychology & Neuroscience

First Advisor

John K. Hewitt

Second Advisor

Matthew C. Keller

Third Advisor

Matthew B. McQueen

Fourth Advisor

Yuko Munakata

Fifth Advisor

Michael C. Stallings

Abstract

The promotion of high intellectual ability is of huge and increasing societal interest with accelerating technological advances. Distinct trajectories of structural brain development according to IQ score are hypothesized to reflect an extended sensitive period during childhood and early adolescence in individuals of higher IQ. A thorough explanation of this question requires a synthesis of individual differences research from behavior genetics and the knowledge of species-general hallmarks of normal development from developmental cognitive neuroscience; two fields that, for the most part, have proceeded independently. This dissertation investigates this question from several angles. First, developmental structural equation modeling is applied to longitudinal twin data to examine the typical patterns of continuity and change in genetic and environmental influences on intelligence between age 1 and 16, demonstrating a pattern of increasing heritability and decreasing shared environmental influence that is not qualitatively different in individuals of higher IQ. Study 2 uses a DeFries-Fulker regression framework in a large cross-sectional study of twins and a smaller longitudinal replication sample of twins, biological siblings and adoptive siblings to examine whether the change in the magnitude of genetic and environmental influences on IQ throughout development is consistent with a sensitive period in IQ development that is extended in individuals of higher IQ, treating score as a continuous predictor of environmental sensitivity. Study 3 seeks to demonstrate this pattern for a measurable environmental variable in a longitudinal sample of Phenylketonuria patients, and to test causal models for individual differences in sensitive period length. Finally, Study 4 outlines motivation for and considerations in designing a computational model to examine the mechanistic role of neurobiological events underlying sensitive periods in determining outcome and why an extended sensitivity to the environment may be beneficial in the development of high intelligence. Results are supportive of an extended sensitive period for cognitive development in individuals of higher IQ that is driven by actualized intellectual capacity in late childhood. These findings may have implications for the search for genetic variants underlying individual differences in intelligence and successful interventions for promoting high cognitive ability.

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