Tuesday, 28 November 2017

Van Leeuwen's twin study of general IQ (2008)


Context/background

Individual differences in intelligence tend to cluster in families, resemblance in intelligence between relatives can be due to genetic similarity, environmental similarities, cultural transmission between generations, social interaction in the family, or all of the above

Environment stimulated intelligence = bright parents giving children extra schoolwork or providing intelligence boosting toys may result in more intelligent children/adults

Phenotypic assortment = mating pattern based on individuals with similar genetics/intelligence level being attracted to each other. Spouses usually resemble each other in IQ scores


Aim

To separate genetic and environmental effects on IQ and to also investigate why spouses have similar intelligence scores


Method

Research article, used twins, siblings and parents from 112 families
2 hypotheses were compared, social homogamy and phenotypic assortment

could also be seen as a collection of mini case studies

Associations between twins from shared environments were tested as well as a collection of mini case studies, or a correlational study


Procedure

Sample-

Twins were recruited from the Netherlands twin registry, and twin families with an extra sibling aged 9-14 were also collected

Twins and siblings took part in an MRI study

214 families invited by letter, 112 participated

There was no significant difference between educational level of mothers who participated

103/112 had full siblings participating

23 MZ males, 23 DZ females, 25 MZ males and 21 DZ females, and 20 SZ of opposite sexes

Parents signed consent forms for themselves and kids also signed consent forms


Testing- 

Collected cognitive behaviour and hormonal data, pubertal status and MRI brain data on 2 different days, then cheek swabs for DNA collected at home

Children tested for cognitive ability in separate rooms with a cognitive test battery with the Raven's standard progressive matrices

Parents used Raven's advanced progressive Matrices and the whole procedure lasts 5 hours

Kids completed at their own pace, 60 problems divided by 12 = 5 sets, the problems became progressively harder and provided an index of general intelligence

The 1st set of APM for adults used 12 practice items and the second used a set of 36 items


Findings

Descriptive statistics of the Raven IQ scores=
No significant sex differences observed, correlations were higher in MZ twins than the other relatives

The mean IQ score was higher in older siblings and there was also more variance in siblings than twins

There was high spousal correlation due to phenotypic assortment, and inherited genetic factors influenced children's intelligence

The environment was more important in explaining individual differences for low IQ groups than for high IQ groups


Conclusions

Variability in fluid intelligence is largely explained by additive genetic effects that were genetically transmitted

Cultural transmission from parents doesn't influence the childrens' IQ

Individual differences largely accounted for by genetic differences, parental influence on children's IQ explained by gene transmissions, not cultural transmission

Environmental factors were more important in kids with a genetic predisposition for low IQ


Evaluation

Suggests men and women are more skilled at different kinds of cognitive activities

Twin studies represent determinism, and cross cultural research is politically sensitive

This was not a representative sample, it was ethnocentric

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