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The Cognitive Cost of Being a Twin: Two Whole-Population Surveys

Published online by Cambridge University Press:  21 February 2012

Ian J. Deary*
Affiliation:
Department of Psychology, University of Edinburgh, Edinburgh, United Kingdom. [email protected]
Alison Pattie
Affiliation:
Department of Psychology, University of Edinburgh, Edinburgh, United Kingdom.
Valerie Wilson
Affiliation:
SCRE Centre, Faculty of Education, University of Glasgow, Glasgow, United Kingdom.
Lawrence J. Whalley
Affiliation:
Department of Mental Health, University of Aberdeen, Aberdeen, United Kingdom.
*
*Address for correspondence: Ian J. Deary, Department of Psychology, University of Edinburgh, 7 George Square, Edinburgh EH8 9JZ, UK.

Abstract

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Do twins have a lower mean IQ score than singletons? Previous studies have not examined whole populations and are likely to be biased. Twin data from two whole-population surveys of IQ at age 11 years were examined: the Scottish Mental Surveys of 1932 and 1947. Additional variables from childhood were examined as possible mediating effects. There were 1080 twins from the 1932 survey, and 949 from the 1947 survey. In both surveys twins scored lower on the Moray House Test of verbal reasoning, equivalent to a deficit of about 5 IQ points. In the Scottish Mental Survey of 1947 the whole-population group of twins was compared in detail with a representative population sample. The same mental ability difference of about 5 IQ points was found, and was not accounted for by father's occupation, overcrowding in the childhood home, childhood height, school attendance or the number of people in the family.

Type
Articles
Copyright
Copyright © Cambridge University Press 2005