Mitochondrial DNA and IQ in Europe MARK G. THOMAS University College London KEVIN W.P. MILLER University of California at Santa Barbara C.G.N. MASCIE-TAYLOR University of Cambridge Recently a mitochondrialDNA polymorphism(EST00083) was found at significantly different frequenciesin high IQ and low IQ groupsin two independent studies.We have used mitochondrial sequences from a range of populationsto show that this polymorphism occurred more than once in human history. Furthermore,the polymorphismis particularlycommon in Europe where it is predominantly associatedwith a singlemitochondrial line (lineage)that appearsto date back to the expansion of anatomicallymodernhumans into Europe about 35,000 years ago. Examination of the geneticdiversitywithinthis mitochondrial lineage,together with the knownmigration and set- tlementof Europeansinto the USA, suggeststhat the observedIQ associatedpolymorphism found in Cleveland Ohio is likely to be restricted to only one form (sub-group)of this mitochondrial lin- eage. Family, twin, and adoption data indicate that approximately half of the variance associated with IQ may be due to genetic factors (Bouchard, Lykken, Mcgue, Segal, & Tellegen, 1990; Chipuer, Rovine, & Plomin 1990). In recent years the search for genetic markers (often called quantitative trait loci or QTL; Gelderman, 1975) associated with IQ has been aided by the development of molecular genetic techniques and, in particular, the method of allelic association (Owen & McGuffin, 1993). The IQ QTL project (Plomin et al., 1994) used this approach when looking for differences in the frequency distribution of DNA markers occurring in or near genes that are likely to play a part in neural functioning. Using high- and low-IQ groups of Caucasian children from the greater Cleveland, Ohio metropol- Direct all correspondence to: Mark G. Thomas,The Centerfor GeneticAnthropology,Departmentsof Biology and Anthropology,DarwinBuilding,University College London,Gower Street, LondonWC1E 6BT, England. INTELLIGENCE 26(2): 167-173 Copyright © 1998 by Ablex Publishing Corporation ISSN: 0160-2896 All rights of reproductionin any form reserved. 167