Multivariate genetic analysis of psychometric tests

Robert Plomin* F.M. Spinath Social, Genetic and Developmental Psychiatry Research Centre, Institute of Psychiatry, King’s College London, Denmark Hill, London, UK SE5 8AF. *e-mail: [email protected] Inter-individual differences in performance on diverse psychometric tests of cognitive abilities – such as verbal, spatial, memory and processing speed – intercorrelate about 0.30 on average and a general factor (an unrotated first principal component) accounts for about 40% of the total variance of these psychometric tests [1]. g is not the whole story of cognitive abilities– group factors representing specific abilities also represent an important level of analysis – but trying to tell the story of cognitive abilities without g loses the plot entirely. Arecent article in TICS made the case for the existence and importance of g, which is sometimes called ‘intelligence’ [2]. Acompanion paper described the cognitive and psychophysical correlates of psychometric g [3]. In this review, we consider g and its cognitive correlates from a genetic perspective. By ‘genetic’, we mean ‘heritable’ (inherited DNAdifferences among individuals) rather than ‘innate’ (evolutionarily constrained differences among species). Astronger case has been made for substantial genetic influence on g than for any other human characteristic. Dozens of studies including more than 8 000 parent-offspring pairs, 25 000 pairs of siblings, 10 000 twin pairs, and hundreds of adoptive families all converge towards the conclusion that genetic factors contribute substantially to g [4]. Estimates of the effect size, called heritability (see Ref. 4 for explanation), vary from 40 to 80% but estimates based on the entire body of data are about 50%, indicating that genetic variation accounts for about half of the variance in g. Breaking down the research by age, heritability can be seen to increase almost linearly from infancy (about 20%) to childhood (about 40%) to adulthood (about 60%) [5]. Genetic research has moved beyond merely estimating heritability. One new direction that is the focus of the present review is multivariate genetic analysis, which provides important new insights into the structure of cognitive processes linked to g. Multivariate genetic analysis of psychometric tests Atechnique called multivariate genetic analysis examines the extent to which genetic and environmental factors mediate the phenotypic covariance between variables. It also yields a statistic called the ‘genetic correlation’, which is the extent to which genetic effects on one trait correlate with genetic effects on another trait independent of the heritability of the two traits. That is, although all psychometrically assessed cognitive abilities are moderately heritable, the genetic correlations between cognitive abilities could be anywhere from 0.0, indicating complete independence, to 1.0, indicating that the same genes influence different cognitive abilities. Multivariate genetic analysis is described in Box 1. Multivariate genetic analyses of psychometric tests consistently find that genetic correlations are very high – close to 1.0 – in adolescence and adulthood [6,7]. That is, if a gene were identified that is associated with a particular psychometrically assessed cognitive ability, the same gene would be expected to be associated with other cognitive abilities as well. In other words, although psychometric g accounts for about 40% of the total phenotypic variance of diverse cognitive tests, genetic g accounts for nearly all of the genetic variance.