UQx PSYC1030.3x 6-1-3 Research into causes of personality


One way we can investigate what causes personality
is through twin studies. These studies get at the whole issue of nature
versus nurture, which has been big in developmental psychology. Twin studies are useful because they compare
the personalities of twins who are identical twins or monozygotic twins, to twins who are
fraternal twins or dizygotic twins. Now, monozygotic twins share a lot more genetic
material than dizygotic twins. That’s because monozygotic twins come about
from a single egg that’s fertilised by a single sperm, whereas, dizygotic twins are
from two eggs and two sperm. What happens is, at some point early on, when
the cells are separating, monozygotic twins come about because the group of cells split
and two separate fetuses develop, whereas, dizygotic twins are always two separate groups
of cells developing into two fetuses in the placenta. Monozygotic twins often share a placenta,
while dizygotic twins often have separate placentas. Now, it’s not to say that monozygotic twins
share a hundred percent of genetic material, because there are some mutations that happen
after cell separation. But, they share a lot more genetic material
than dizygotic twins. Dizygotic twins share the same amount of material
as any two siblings who were born at any point in time. They are just two siblings who happen to be
born very close in time. So, we can compare the personalities of people
who come from identical genetic backgrounds versus those who come from similar but not
identical genetic backgrounds. It turns out, sometimes twins are raised in
the same family environment, and sometimes, for a whole variety of reasons, twins are
separated and raised in different family environments. Tellegen and colleagues conducted a study
in 1988 where they looked at the similarities in personality between twins who were raised
in the same family environment and those who were raised in different family environments. So, they recruited twins who were reared together
in the same family environment, and those who were reared apart. For those who were reared together, they recruited
siblings who were either identical or fraternal twins. The same from those who were reared apart. These were the personality dimensions that
the twins were compared on… anxiety proneness, aggression, alienation, impulse control, emotional
well-being, traditionalism, and achievement orientation. Tellegen and colleagues measured the correlations
between the twins’ scores on these dimensions. A larger number means that as one of the twins
scored higher on a personality dimension, the more the other would score higher as well. And likewise, as one of the twins scored lower,
the other would also tend to score lower. So, the higher the correlation, the more similar
twins were on that personality dimension. A correlation of .52 is moderate to high,
and suggests, as one twin scored higher on anxiety proneness, their twin sibling scored
high on anxiety proneness as well. Looking at those correlations, what do you
notice? It seems fairly obvious that the identical
twins were more similar to one another than the fraternal twins. The correlations for the identical twins are
larger than the correlations for the fraternal twins, and this is the same whether they were
reared together or apart. So, the identical twins were similar to each
other whether they were reared together or apart. These results suggest that GENES are a relatively
bigger contributor to personality than shared environment. Another thing to take notice here is that,
none of the correlations here are perfect. A perfect correlation is ONE. The strongest correlation from these results
is .61. If we work out the amount of VARIANCE explained
by a correlation, by squaring the correlation, it works out to be 0.37, which means that genetics
explains about a third of the shared variance in personality between identical twins. This leaves two thirds of the variance unexplained––and
it doesn’t seem to be the shared environment. I would like to tell you what it is, but nobody
really knows for sure, although the best guess at the moment is unshared environmental factors,
such as going to different schools and so on. The point is that genes are a bigger contributor
than a shared environment, but they’re not the only factor contributing to personality. Part of it might also come down to the idea
that personality is really a function of the person and their environment––like the
situation they’re in. So the situation contributes to how personality
is expressed. Here is another set of data that supports
the same conclusion, which was published by Scarr and colleagues in 1981. In this study, Scarr and colleagues compared
the personality of mothers and their children, as well as the personalities between children. For some of the mother and child pairs, the
children were biologically related to the mother, while others were adoptively related. For the paris of children, some of the children
were biologically related to the other children, while others were adoptively related. So, the pairs who were biologically related
were high in genetic similarity, while those who were adoptively related were low in genetic
similarity. And in terms of environmental similarity––it
was the opposite. Remember that the children in this study were
adopted, so if they were biologically related to their mother or sibling, they grew up in
a different environment. And if they were not biologically related,
they were adopted into the family and they grew up in a similar environment to their
adoptive mother or sibling. If we look at the correlation values between
the pairs who were biologically related, the mother and child pairs scored a correlation
of 0.21, while the sibling pairs scored 0.28. For those who were adoptively related, the
mother and child pairs scored 0.12, and the sibling pairs scored only 0.05. The results from this study support the results
from the twin studies, suggesting that genes contribute to our personality more than the
environment.

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