Twin Studies/adoption studies
And correlation
Below are the correlation coefficients for the IQ s of mz and dz twins.
Mz 0.86
Dz 0.45
Explain the implications of these results in the context of the nature nurture
debate (5)
Students tend to do very badly in exams on questions similar to the one
above.
This handout might help you understand how correlation coefficients and
twin studies provide evidence to support both the learn view and the innate
view. It will also give you evaluation points against the use of family studies
in the nature nurture debate.
You should be very confident answering the above question if you
understand how psychologists use twin studies, adoption studies and
correlation to establish whether behavior is mainly inborn as the biological
approach suggests or mainly learnt as the learning theorists believe.
It is usually the physiological approach that uses twin studies to support their
arguments but no twin or adoption study supports the view that behavior is
inborn completely.
Correlation coefficients are used to show the closeness of a relationship
between two sets of data
They are expressed as a number between plus or minus one and so look like
the decimal numbers shown above ie 0.something. As we are tending to look
for a positive relationship between two sets of data they tend to be positive
numbers.
A psychologist can use twin studies in two ways ether comparing monozygotic with dy-zygotic or comparing mono-zygotic twins reared apart with
those reared together. When you look at mono-zygotic twins reared apart
you have isolated the variable of genetic influence because they have
identical genetic make up but differing environments.
The correlation coefficient expresses how similar they are.
Can you see that if they are still similar in something like I.Q.despite being
reared in totally different environments then we can assume that genetics
have been an important factor in their I.Q.?
Do not make the mistake of thinking that the correlation coefficient is
the I.Q. score! Many students in exam situations do this.
With Mono- zygotic and dy-zygotic we make the assumption that the twins
have similar environments to each other in their upbringing. so the dy-
Twin Studies/adoption studies
And correlation
zygotic twins who are not identical will still be raised in a very similar way
to each other. Therefore any difference in the correlation coefficient is likely
to be due to genetics. Generally speaking psychologists have found that
there are differences and that monozygotic twins are more similar as shown
in the above figures. However clearly if I.Q. or other behaviors were
100% inborn the correlation coefficient would be plus 1 and it never is
so we know that things such as I.Q. are not completely inborn.
Strengths
 Produced a great deal of data to support biological arguments for
causes of mental health problems enabling psychologists to find
biological ways of helping people.
Weakness
 Monozygotic twins are rarely separated from birth and raised in a
totally different environment. This is really what is required to make
the claims of the researchers truly valid.
 Mz twins reared together probably share a more similar rearing than
dz twins reared together. (eg if you are not as good looking or
charming as your none identical twin then your life and psychological
development could be very different indeed)
With regard to adoption studies we conduct the correlation on:Adopted child and biological mother
Adopted child and adoptive mother
If the biological view is to be supported, which pair would have the higher
correlation coefficient?
Strengths
 Adoption studies are a very powerful resource supporting the
physiological view because you have isolated the environmental
factors. If a biological mother has had no contact with the baby since
birth and still matches I.Q. more than the adoptive mother this is
powerful evidence that genes are more important than environment.
Weakness
 Selective placement is a problem. Babies tend to be matched with a
similar background to the one of the natural parents.
 Adopted children are not representative of all children. If an adoptive
child has a lower I.Q. than the adoptive mother and matches the birth
mother more in this respect it maybe that the trauma of adoption has
lowered the child’s IQ not genetic factors. (Obviously this would only
be the case if birth mother was lower than adoptive mother).