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Comparing the similarities of MZ's and DZ's, who have been reared together and
share the same environment.
If genes control the characteristics, then MZ's who share all genetic material should
be more similar than DZ's.
If characteristics were just down to genes we would expect MZ's to have all the
same characteristics but this isn't the case. The degree of similarity can be assessed
by looking at the average of a large group of twins. We would use correlation to do
If genetic factors determine characteristics the score for one twin would be close to
You can also study the similarity of twins (effect of genes/environment) by studying
If the twins are similar (despite being separated) it supports the role of genes.
However, if the twins are different, this supports the role of the environment.
The most direct way to isolate the effect of genes and the environment involves
If a child is adopted they are in the situation where they are in a different
environment to their birth home, but have the genetic information from their
Similarities between the child and their biological parents would suggest a role for
However, similarities between the adoptive parents and the child, would suggest a
role for the environment.
To asses a characteristic is chosen (personality or mental health). The child and the
adoptive parents are compared.
The child is often followed throughout adult hood, to see if once out of the adoptive
home, they still possess similar characteristics to the adoptive home.
Also, they can be compared to their biological parents.