Genetics of Childhood Disorders: III. Genetics and Intelligence

Abstract

Gen etic research has mad e imp ortant discoveries about intelligence during the past few decades. To outline some of these find ings, I won't spend space on the measurement of intelligen ce except to say that what I mean by intelligence is general cognitive ability defined as g. All reliable and valid tests of cognitive ability intercorrelare at a modest level-g is what they have in common . g is often assessed as a total score across diverse cognitive tests as in intelligence (IQ) tests, although it is more accurately indexed by an unrotated prin cipal component that best reflects what is in common amo ng the tests. Nearly all genetic dat a have been obtained using measures develop ed from th is psychometri c perspective, primarily IQ tests. One new direction for genetic research on intelligence is to investigate other measures such as information-pro cessing and more direc t measures of brain function such as evoked potentials, positron em ission tomograph ic scans, and functional magnet ic resonance imaging and to explain how these measures relate to g. g clearly run s in fam ilies. The correl at ions for first-degree relatives living together average 0.43 for more than 8,0 00 parent-offspring pairs and 0.47 for more than 25,000 pairs of siblings. However, g might run in fam ilies fo r reason s of nu rture or of nature. In studies involving more th an 10,000 pairs of twins, the average g correlations are 0.8 5 for identical twin s and 0.60 for same-sex fraternal twins. These twin dat a suggest a genetic effect size (heritability) that explains about half of the tot al variance in g scores. Adoption stu dies also yield est imates of substantial her itability. For example, identical twins reared apart are almost as similar for g as identical twins reared together. Ado ption stud ies of other first-degree relatives also ind icate substant ial her itab ilit y, as illustra ted below by recent results from the