The cognitive cost of being a twin: Two whole-population surveys

I J Deary, A Pattie, V Wilson, L J Whalley

Research output: Contribution to journalArticlepeer-review

Abstract / Description of output

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.

Original languageEnglish
Pages (from-to)376-383
Number of pages8
JournalTwin Research and Human Genetics
Issue number4
Publication statusPublished - Aug 2005

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