Genetic and environmental factors affecting intelligence

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        Genetic and environmental factors in

        intelligence test performance


The genetic approach to explaining performance on intelligence tests has controversially claimed that intelligence may be passed from one generation to another.

The main source of evidence for this area has been twin studies which compare the concordance rates for IQ scores for Mz and DZ twins.

Another area of research has been looking at family relatedness and concordance rates on intelligence tests.

MZ twins are identical twins and have the same genetics as each other so if intelligence was genetic both twins should have similar IQ scores.

It is unlikely to be just one that influences intelligence, it is likely to be a group. The first to be identified was IGF2 receptor gene. The second gene was ASPM linked to higher intelligence by effecting the size of the cerebral cortex by expanding it suggesting the larger the size of the cerebral cortex, the greater the processing of information.

APOE gene is associated with Alzheimer's disease, researchers said that there was a variant of it which some have found to have links to intelligence. Once the gene becomes defective, it seems to be linked to Alzheimer's which has sever cognitive dysfunction.

Some genes for example: CHRM2, linked to schizophrenia have been thought to be linked to intelligence. Perhaps dysfunctional genes produce the sever cognitive dysfunction seen in thought disturbance which is a category A symptoms of schizophrenia.

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COMT gene has also been linked to schizophrenia a lower level functioning version has shown poor performance on "stroop" tasks.  

DZ twins are no more genetically similar than brothers and sisters born at different times so there should be less of a relationship between IQ scores of non identical twins.


One study examining identical twins as looked at by Bouchard & McGue (1981) who conducted a review of 111 world wide studies on family IQ ignoring studies which they claimed had mythological deficiencies and came up with the following average correlations of IQ


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