With reference to relevant research studies, to what extent does genetic inheritance influence behaviour?

Behaviour - intelligence.

Two factors affect intelligence - genetics and environment (nature vs nurture). To what extent does genetics influence intelligence? This can be investigated through the use of twin and adoption studies, as you are able to observe and compare genetics and environment in both situations.

Twin Studies - investigating genetics (compare intelligence of twins, correlation can be attributed to genetics, as they share genetic material).

Bouchard and McGue (1981)

• Conducted a meta-analysis of 111 studies of siblings.
• Investigated IQ correlations between siblings.
• Found that there was a positive correlation between kinship and IQ correlation - the closer the siblings were, the more similar their IQ.
• Evaluation of study: 
• Large study, so generalisable.
• However, siblings are raised in the same environment, so influence may not be purely genetic, and it is difficult to differentiate between influence of genetics and environment.
• Also, age - the further apart the siblings in age, the less correlated their IQs are expected to be.

In order to investigate the role of genetics by itself, identical twins that are raised separately from birth must be studied. Identical twins have a 100% genetic relationship, but if raised in separate environments, any similarity (beyond that expected by chance) in IQ must be due to similarity in genetics.

Bouchard et al. (1990) - Minnesota Twin Study

• Conducted a longitudinal study since 1979, comparing MZA twins to MZT twins in terms of intelligence (MZA - monozygotic raised apart, MZT - monozygotic raised together)
• Each twin completed approximately 50 hours of testing and interviews.
• It was a cross cultural study, using participants from all over the world.
• Results - concordance rates of intelligence from the study

Same person	87%
MZA	86%
MZT	76%
DZT	55%
Siblings reared together	47%

Bouchard et al. determined a heritability estimate of 70% - 70% of intelligence can be attributed to genetic inheritance. 30% attributed to other factors.
Evaluation of study:

Strengths	Weaknesses
- Size of the study means it is more generalisable - Nature of sample - cross cultural - Mean age of participants 41 years, as opposed to previous studies with adolescents	- Relied on media coverage to recruit participants - Ethical concerns with the way twins were reunited - Frequency of contact between twins prior to study not controlled. - 'Equal environment assumption' - twins reared together may not have experienced the same environment.

Adoption Studies - investigating environment (no biological/genetic link between parents and child, so correlation in IQ is purely due to environment)
Horn et al. (1979)

• Conducted a study on parents who raised BOTH adopted and natural children.
• All children have the same upbringing, therefore same environment, but some were similar in genetics and some were not.
• Found no significant differences in correlation of parent-child IQs between natural and adopted children.
• Adoptive parents were wealthy, white, middle class and children were poor, lower class backgrounds, with lower IQ parents.
• Conclusion that genetics has little impact - mostly environment.

Wahlstein (1997)

• Well controlled adoption studies in France
• Transferred infant from family with low socio-economic status to with high economic status parents.
• Increased IQ of children by 12-16 points.
• Suggests that environment has a lot to do with intelligence, as well genetics - enriched environment = good.

Both genetics and environment influence intelligence equally?