Perceptual speed and IQ are associated through common genetic factors.
Individual differences in inspection time explain about 20% of IQ test variance. To determine whether the association between inspection time and IQ is mediated by common genes or by a common environmental factor, inspection time and IQ were assessed in an extended twin design. 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. The sample consisted of a young adult cohort (mean age 26.2 years) and an older adult cohort (mean age 50.4 years). IQ was assessed with the Dutch version of the WAIS-3R. Inspection time was measured in the so-called II-paradigm, in which a subject is asked to decide which leg of the II-figure is longest at varying display times of the II-figure. The number of correct inspections per second (i.e., the reciprocal of inspection time) was used to index perceptual speed. For Verbal IQ and Performance IQ, heritabilities were 85% and 69%, respectively. For perceptual speed, 46% of the total variance was explained by genetic variance. No differences in heritability estimates across age cohorts or sexes were found. Across the whole sample, a significant phenotypic correlation was found between perceptual speed and Verbal IQ (0.19) and between perceptual speed and Performance IQ (0.27). These correlations were entirely due to a common genetic factor that accounted for 10% of the genetic variance in verbal IQ and for 22% of the genetic variance in performance IQ. This factor is hypothesized to reflect the influence of genetic factors that determine axonal myelination in the central nervous system.
Posthuma
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Department of Biological Psychology, Vrije Universiteit, Amsterdam, The Netherlands. danielle@psy.vu.nl
de Geus
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Boomsma
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D000328:Adult; D015331:Cohort Studies; D005260:Female; D006801:Humans; D007360:Intelligence / Q000235:genetics*; D008297:Male; D008875:Middle Aged; D009426:Netherlands; D011930:Reaction Time / Q000235:genetics*; D012931:Social Environment; D014427:Twins / Q000235:genetics*