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Twin and adoption studies
Twin and adoption studies
Twin studies
Monozygotic twins
Come from one fertilised egg
DNA is 100% shared, therefore always the same sex
Inherited by both twins
No characteristic is entirely genetic as environment is a depending factor
May become less identical over time due to environmental factors
Dizygotic twins
Come from two different fertilised eggs
DNA is not 100% same, similar of any sibling pair
Share an inherited characteristic to an extent, not as much as MZ twins
They compare certain characteristics possessed by MZ and DZ twins to see if genes or environment influence whether they share characteristics.
The extent to which the behaviour is the same between twins is the
concordance rate
.
Adoption studies
The environment is not the same as their biological parents'.
They are useful because they share genetic information with their parents even in a different environment.
If the child grows up to share the parents' traits (or traits of their step-brothers/sisters who are biologically related to the parents), then these traits might be produced by nurture.
Nature
What people are born with
Inheritance
Nurture
What is learned through interactions with the environment
Learned
Evaluation
Twin studies
Strengths
MZ and DZ twins are born at the same time and share the same environment, but MZ twins have identical DNA, whereas DZ twins only share 50% which helps identify how genes could influence characteristics.
There should not be significant environmental differences with regards to the treatment of twins. They aren't treated separately.
Weaknesses
MZ twins are the same sex and identical so they will be treated alike more than DZ twins.
Epigenetic modification (over time different environmental influences affect which genes are switched on and off) can have an effect on the findings.
Adoption studies
Strengths
Separate genes from environment so the two can be tested.
Studies are longitudinal (conducted over a long period of time) so developmental trends can be identified.
Weaknesses
Environment of the adoptive families is not as different as it could be from the biological family's.
Children requiring adoption are often placed with families similar to their own, so differences are minimal.