Genetic correlation between inspection time and IQ.

Danielle Posthuma, Eco JC de Geus, Dorret I Boomsma

Dept. of Biological Psychology, Vrije Universiteit, Amsterdam, The Netherlands

Inspection time is the minimum amount of time a subject needs to make an accurate decision on an obvious stimulus, such as discrimination between two lines. It is part of one of the first, unconscious, stages of information processing. We found a correlation of -0.27 between inspection time and IQ. In order to determine whether this association is mediated by common genes or by common environmental influences, inspection time and IQ were measured in twins and their non-twin siblings. Data from 688 participants from 271 families were collected as part of a large ongoing project on the genetics of adult brain function and cognition. IQ was measured with the Dutch version of the WAIS-3R, inspection time was measured in the so-called П-paradigm. Inspection time was moderately heritable (43%), whereas the heritability estimate for IQ was high (86%). The correlation between inspection time and IQ was entirely due to a common genetic factor which accounted for 14% of the total variance in IQ. Implications of the existence of common genetic factor for inspection time and IQ are discussed in terms of a biological model. This model, derived from established theories on visual information processing, proposes that variation in inspection time is dependent partly upon variation in genetic factors that determine myelination and synaptic efficiency in the central nervous system. Thus, 14% of the interindividual variance in intelligence must be sought in those genetic factors that influence myelination and synaptic efficiency. (The financial support of HFSP grant rg0154/1998-B is gratefully acknowledged)