Minnesota Twin Study

To establish the influence of genes or environment on particular traits and characteristics of human beings.

It was expected that monozygotic twins raised together and apart would have a high concordance rate due to the inheritance of genes. It was also expected that monozygotic twins would have a higher concordance rate than dizygotic twins.

Independant variables- monozygotic twins reared together or apart (MZT and MZA), as well as dyzygotic twins reared together and apart (DZT and MZA) Dependant variable: the measurement of IQ using an intelligence test called WAIS

Correlational data

Use of media coverage to recruit sample--self-selected

Over 100 sets of MZT (monozygotic - identical twins raised together) and DZT (dizygotic - fraternal twins raised together) from around the world who were reared/raised together and apart. Researchers gave them approximately 50 hours of psychological and physiological testing using an intelligence test called WAIS

Concordance rate for IQ between monozygotic twins reared together (MZT) was approximately 88%. Similarity rates between monozygotic twins raised apart was at 69%. Bouchard determined a heritability estimate of 70% of intelligence attributed to genetics, and 30% to other factors.

-much research has supported his findings, making them more reliable -large sample of participants, making the study more externally valid than most twin studies

-Bouchard used media coverage to recruit the sample--self selected sample -No control over the frequency of contact between twins before the study -Bouchard assumed that the twins reared together experienced the same environment - equal environment assumption -Correlation data can't establish cause-effect relationship -Only wealthy people can afford to adopt--lack of generalisability -Twins may be different to non-twins--lack of generalisability

These findings (although hard to generalise) can be used to determine the role that genes play in ones intelligence and personality

-"With reference to relevant research studies, discuss the extent to which genetic inheritance influences human behaviour" -"Outline principles that define the biological level of analysis and explain how these principles may be demonstrated in research"

-Twins raised apart may have undergone emotional turmoil in being reunited with their twin with whom they've never made contact -

Bouchard et al.

1990

Scores on the Wechsler Intelligence Scale for Children were analyzed in a sample of 7-year-old twins from the National Col- laborative Perinatal Project. A substantial proportion of the twins were raised in families living near or below the poverty level. Biometric analyses were conducted using models allowing for components attributable to the additive effects of genotype, shared environment, and non- shared environment to interact with socioeconomic status (SES) measured as a continuous variable. Results demonstrate that the proportions of IQ variance attributable to genes and environment vary nonlinearly with SES. The models suggest that in impoverished families, 60% of the variance in IQ is accounted for by the shared environment, and the contribution of genes is close to zero; in affluent families, the result is almost exactly the reverse.