On the heritability of inspection time and its covariance with IQ: a twin study

Using the classical twin design, this study investigates the influence of g enetic factors on the large phenotypic variance in inspection time (IT), an d whether the well established IT-IQ association can be explained by a comm on genetic factor. Three hundred ninety pairs of twins (184 monozygotic, MZ ; 206 dizygotic, DZ) with a mean age of 16 years participated, and 49 pairs returned approximately 3 months, later for retesting. As in many IT studie s, the pi figure stimulus was used and IT was estimated from the cumulative normal ogive. IT ranged from 39.4 to 774.1 ms (159 +/- 110.1 ms) with fast er ITs (by an average of 26.9 ms) found in the retest session from which a reliability of .69 was estimated. Full-scale IQ (FIQ) was assessed by the M ultidimensional Aptitude Battery (MAB) and ranged from 79 to 145 (111 +/- 1 3). The phenotypic association between IT and FIQ was confirmed (- .35) and bivariate results showed that a common genetic factor accounted for 36% of the variance in IT and 32% of the variance in FIQ. The maximum likelihood estimate of the genetic correlation was - .63. When performance and verbal IQ (PIQ & VIQ) were analysed with IT, a stronger phenotypic and genetic rel ationship was found between PIQ and IT than with VIQ. A large part of the I T variance (64%) was accounted for by a unique environmental factor. Furthe r genetic factors were needed to explain the remaining variance in IQ with a small component of unique environmental variance present. The separabilit y of a shared genetic factor influencing IT and IQ from the total genetic v ariance in IQ suggests that IT affects a specific subcomponent of intellige nce rather than a generalised efficiency. (C) 2001 Elsevier Science Inc. Al l rights reserved.