Van Leeuwen et al (2008) twin study:

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  • Van Leeuwen et al (2008) twin study:
    • Background
      • Previous studies on mono-zygotic twins and dizygotic have estimated contribution of genetics to intelligence to be between 35 and 50 %.
    • Aim
      • Use twins, parents and family members to identify the relationship between family member and IQ.
      • 100% shared genetic material for  monozygotic twins and 50% for dizygotic, between siblings and parents and children.
    • Participants
      • 112 families studied
      • either had MZ or DZ twins plus a sibling aged 9-14 and two parents.
      • Volunteered to take part and passed the screening for disabilities, mental illness and metal materials such as pacemakers.
      • 23 male MZ twins, 25 female MZ twins
      • 20 DZ twins were of opposite sex
      • 23 male DZ, 21 female DZ
    • Method
      • Raven's Standard Progressive Matrices for the children
      • Advanced Progressive Matrices for the parents.
      • This IQ tests covers a range of skills.
      • Families arrived at 9-11 am and had three breaks.
      • Testing period of around 5 hours.
      • Rasch model was applied which takes into account the difficulty of each test question
      • The IQ score they achieved was irrespective of which items were used as difficulty was controlled for.
    • Results
      • No sex differences in the group or in any of the three groups.
      • Higher correlations for MZ twins than first degree relatives.
      • High correlation between IQ of parents suggesting heritability estimate for intelligence wass at 67% with a 95% confidence level.
      • For lower IQ groups, interaction between the genotype and environment was higher.
    • Discussion
      • Individual genetic differences can account for individual differences in intelligence.
      • However, factors such as environment might impact low IQ groups more than high IQ groups.
      • No indication that intelligent parents provide stimulating environments to promote intelligence but children with a predisposed IQ will either seek a stimulating (high IQ) or non-stimulating (low IQ) environments.


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