title: The cognitive cost of being a twin: evidence from comparisons within families in the Aberdeen children of the 1950s cohort study.
abstract: OBJECTIVE
  To determine whether twins have lower IQ scores in childhood than singletons in the same family and, if so, whether differences in fetal growth explain this deficit.
  
  
  METHODS
  Cohort study.
  
  
  METHODS
  Scotland.
  
  
  METHODS
  9832 singletons and 236 twins born in Aberdeen between 1950 and 1956.
  
  
  RESULTS
  At age 7, the mean IQ score of twins was 5.3 points lower (95% confidence interval 1.5 to 9.1) and at age 9, 6.0 points lower (1.7 to 10.2) than that of singletons in the same family. Adjustment for sex, mother's age, and number of older siblings had little effect on these differences. Further adjustment for birth weight and gestational age attenuated the IQ difference between twins and singletons: the difference in mean IQ was 2.6 points (-1.5 to 6.7) at age 7 and 4.1 points (-0.5 to 8.8) at age 9.
  
  
  CONCLUSIONS
  Twins have substantially lower IQ in childhood than singletons in the same family. This effect cannot be explained by confounding due to socioeconomic, maternal, or other family characteristics, or by recruitment bias. The reduced prenatal growth and shorter gestations of twins may explain an important part of their lower IQ in childhood.
authors.affiliation: Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, London WC1E 7HT.
authors.forename: Georgina A
authors.initials: GA
authors.lastname: Ronalds
authors.forename: Bianca L
authors.initials: BL
authors.lastname: De Stavola
authors.forename: David A
authors.initials: DA
authors.lastname: Leon
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D001724:Birth Weight; D002648:Child; D003072:Cognition Disorders / Q000523:psychology*; D015331:Cohort Studies; D004522:Educational Status; D005191:Family Characteristics; D005260:Female; D005317:Fetal Growth Retardation / Q000523:psychology*; D006801:Humans; D007360:Intelligence; D008297:Male; D008875:Middle Aged; D012044:Regression Analysis; D012606:Scotland; D014427:Twins / Q000523:psychology*