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