Bouchard et al. (1981)

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Last updated 1:09 PM on 4/20/26
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8 Terms

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Aim

To investigate the genetic and environmental influences on intelligence by studying IQ scores between identical or fraternal twins, raised together or apart

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Method

Meta analysis (combining results of multiple studies) of 111 twin studies that looked into the heritability of intelligence. Participants included MZ and DZ twins (reared together and apart), siblings (reared together and apart), parents and their offspring

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Procedure

Researchers selected studies on criteria like: whether twins shared 100% of their genes or 50%, and whether they were reared together or apart. They cleaned the data and calculated median correlations between IQ scores of the individuals. They then applied the falconer model to their data to show variation of intelligence as a result of genetic inheritance.

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Results

When the data was plugged into the formula, a heritability coefficient of 54% was obtained

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Conclusion

The study demonstrates that intelligence is inherited to a considerable extent. At the same time, it is not completely inherited, as even for MZ twins reared together the correlation between their IQ scores is not perfect, which shows that the environment plays a certain role in the development of IQ

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Suggestion in terms of genes

This suggests that while genes play a large part in determining characteristics like intelligence, external factors also play a role

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Strengths

  • unique research design; rare opportunity to separate nature (genes) and nurture (environment) → increases validity of conclusion

  • Large compilation of data → multiple twin studies, greater reliability and broader evidence base.

  • Quantitive, objective data; used IQ tests, standardised measurement → increases scientific credibility

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Limitations

  • correlation data (no causation); shows relationship between genes and IQ → cant prove that genes directly are cause

  • Sampling bias; twins reared apart are rare and not random → limits generalisability

  • Reductionist; focuses mainly on genetics (IQ scores) → ignores emotional intelligence and environmental/cultural influences