1. Key research: Van Leeuwen et al
Key research: Van Leeuwen et al. (2008) A twin-family study of general IQ

2. Aim
There have been many studies that have investigated whether twins either identical (monozygotic) or not identical (dizygotic) share genetic intelligence traits.
The aim of this study was to not only study identical twins (who share 100% of genetic material) but also non-identical twins and other siblings (who share 50% of genetic material) as well as parents (who share 50% of genetic material with their children).
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3. Sample
112 families who were screened for mental illness and materials like pacemakers (which affect scans) volunteered. This was made up of:
families with identical male twins (23)
identical female twins (25)
non-identical male twins (23)
non-identical female twins (21)
non-identical wins of different genders (20)
Every family had other siblings in addition to the twins with two parents.
The mean age of the twins was 9.1.
The mean age of the other siblings was 11.9.
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4. Procedure
Children and adults both completed cognitive tests to assess their intelligence.
Families arrived at a research lab and were tested over five hours.
Adults (parents) completed Raven’s advanced progressive matrices. Children completed Raven’s standard progressive matrices.
The difficulty of each question was controlled for by a model that predicted how likely it is that the participant would get the question correct based on their ability.
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5. Results
There was a higher correlation of results between the identical twins than any of the other participants (non-identical twins, siblings and parents).
There was also a high correlation between parents, which suggests that the effect of genes on intelligence (the heritability) is 67% while the remaining 33% comes from environmental factors.
However, low IQ scores were explained by the interaction of the environment on the genetic make-up.
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6. Conclusion
Individual genetic differences account for differences between the intelligence of people. However, other factors like the environment can have more of an impact on low IQ scores.
Participants with high IQ are more genetically predisposed to seek out intellectually stimulating environments, while those with low IQ are more likely to seek out non-intellectual environments.
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