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Bouchard et al.

Bouchard et al. (1990)

Twin study investigating genetic inheritance in intelligence

  • This study used a self-selected sample of MZ twins who had been reared together (MZT) and MZ twins who had been reared apart (MZA) to investigate concordance rates for a number of variables such as IQ.
  • The results showed that for IQ (measured by a standardized intelligence test called WAIS) the concordance rate was 69% for MZA and 88% for MZT.
  • The researchers concluded that environmental factors do play a role in development of intelligence but IQ is to a large extent inherited and that 70% of the observed variation in the sample could be attributed to genetic variation.
  • They claim that the results indicate that in a sample like the one in the study (white, middle-class in an industrialized nation) genetic inheritance in IQ accounts for around two-thirds of the observed variance of IQ.
  • They also said that their findings do not indicate that IQ cannot be increased, that is influenced by environmental factor.

Evaluation

  • Correlational data cannot establish cause-effect relationships.
  • Concordance rates were high in the study but far from 100% so it was difficult to determine the relative influence of genes. Calculation of concordance rates is not always reliable.
  • There was no control for the effect of environmental variables in the study and this affects accurate estimations of a genetic contribution to intelligence.

The findings from this self-selected sample make it difficult to generalize findings