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Van Leeuwen (2008): A Twin Family Study of General IQ
Aim
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The researchers wanted to separate shared genetic effects from shared
environmental effects on intelligence
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They also wanted to investigate why spouses have similar intelligence scores to
one another
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214 families of twins about to turn 9 years old, who also had an
extra sibling from the ages of 9-14 were used
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They were all invited by letter to participate
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By including this
data from parents and siblings this meant that the effects of heredity and cultural
transmission could also be estimated
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Children were given a present to take part and
families completed a questionnaire or DNA cheek swab at home prior to the
experiment to confirm zygosity (making sure they were twins)
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They were given verbal instructions and completed the test at their own pace
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Adults completed the Ravens Advanced Progressive Matrices test
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Although the number and
arrangement of questions were different the test produced comparable results
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Phenotype Assortment is the idea that associative mating occurs as individuals
choose one another due to having similar intelligence levels
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Social Homogamy is the idea that as people with similar intelligence are put together
in the same environment they are more likely to end up having children together
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Results
IQ measures derived from the tests were estimated using the Rasch model
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Cultural transmission didn’t have a significant effect on genetic variance of IQ
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Spousal correlation for Rasch IQ estimated was moderately high (0
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The statistical modelling showed that the reasons for this were based on
phenotype rather than social circle (social homogamy)
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This suggests a GE interaction wherein genetic and environmental factors affect one
another to determine intelligence levels
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e
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Conclusion
The researchers concluded that:
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How does this piece of research link to the topic area? (Intelligence)

Van Leeuwen (2008) – Evaluation
Strengths

Weaknesses

Individualistic approach – reliant on
individual characteristics of a person
as it found a strong concordance
rate between MZ twins for IQ,
showing individual genetics have an
impact on intelligence
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Can also be situational – looks at the
impact of environment on an
individual and whether spousal
resemblance could be better
explained by phenotype assortment
or social homogamy
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