Van Leeuwen

• What was the background?

  • Argued that 1st degree family not enough to uncover genetic effects on intelligence + separate from environment
  • Can identify genetic influence when doing MZ twin studies - estimate 25 - 50%

  Validity relies on:

  • Phenotypic assortment - mating w/ partners similar intelligence
  • Social homogamy - relationships w/ ppl share similar environment so similar intelligence
  • Assumed parents level of intelligence little or no role in environment provided for child
  • More likely parents provide child w/ environments that match their own genotype - cultural transmission
• What were the aims?

  • If previous 2 assumptions incorrect then results of twin studies could be biased - heritability over or underestimated
  • Twin study can be extended to include parents & siblings of MZ and DZ twins - 2 generations

  1. Carry out extended twin study that considers assumptions usually overlooked
  2. Test which of 2 models is a better explanation of variation in IQ scores - phenotypic assortment or social homogamy
  3. Investigate whether genotype or environment affects intelligence more
  4. Calculate unbiased heritability estimate for IQ
• What was the method?

  • Twin family design
  • Correlation of IQs of twins, siblings and parents
• What was the sample?

  • Letter to 214 Dutch families inviting them to take part
  • Selected from Netherlands Twin Registry - secondary data
  • Contacted by phone after letter
  • 112 families agreed to take part
  • Signed consent forms
  • Children received a gift
  • 48 MZ twins, 64 DZ twins
• What were the materials?

  • Intelligence tests at Uni of Amsterdam - high external reliability and high validity as test already validated

  Children:

  • Raven's Standard Progressive Matrices

  Parents:

  • Advanced Progressive Matrices - more difficult version to avoid ceiling effect - construct validity

  
  

  • Both tests provide score indicating general intelligence (Spearman)
• What was the procedure?

  • Completed tests in groups in separate rooms - control as avoid external influence
  • Took 5 hours including breaks
  • Twin status established by parents taking cheek swabs at home from themselves and their children
  • Used genetic testing technique to determine zygosity using standardised methods - reliability and science as objective
  • 2 twin pairs had a questionnaire instead - ethics - respect
• What were the findings?

  • Rasch model analysed results of IQ tests:
  • Correlations on scores of IQ higher for MZ twins than for siblings, parents and DZ
  • No statistically significant sex differences
  • Correlation of IQ level between couples was significant, suggesting individuals more likely to mate with people of similar intelligence
  • Environmental factors found to be more important for children with genetic disposition of low IQ - genes and environment influence each other
  • Genetics account for 67% of intelligence and remainder by random environmental factors
• What were the conclusions?

  • Biological factors affect intelligence
  • Individual differences in intelligence can be explained by transmission of genes and cultural transmission from parents no effect
  • Genes interact with environmental factors to influence intelligence in significant ways
  • No indication intelligent parents provide stimulating environments to promote children's IQ and children with a predisposed high IQ seek out stimulating environment and low IQ will not