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Unit 8 Nature and Nurture
Twins, Genes, and Environment
Heredity or environment: which is stronger? The potentials which a person is
born with determine in some way what he will do in life. Therefore heredity is fate,
a kind of predestination. However, genes do not work in a vacuum; as soon as we
begin considering the role that they play in the development of the individual, we
see that there can be no development without the interacting environment. No
characteristic is caused exclusively by either environment or genes.
The relative effects of heredity and environment are most clearly observable in
identical twins. Most identical twins are raised together and are remarkably alike in
both appearance and behavior. These cases demonstrate that individuals with the
same genes, when raised in the same environment, will respond to it in much the
same way. They do not indicate what would happen if these identical individuals
were raised separately.
A number of studies have been made of identical twins raised apart. The twins
who were the subjects of these studies lived in America, were raised in much the
same physical environments, and experienced much the same nutritional histories.
Therefore, as one might expect, they maintained the closest resemblance to each
other in physical appearance, height, and weight. Exceptions occurred when one
twin had developed a rather severe illness and the other had not; but on the whole
everyone is impressed by the great psychological and physical likenesses that exist
between identical twins, even those who have been separated from infancy.
In a study of nineteen sets of twins who had been separated from birth,
investigators found that in approximately two thirds of the sets there were no
more significant differences than existed among unseparated pairs of twins. This
strongly suggests the power of the genes and the limitation of the effect of
environment. However, it must be remembered that, although the identical twins
who were studied lived in different families far removed from each other, the
environments in those families were not, on the whole, substantially different.
Usually every effort would be made to put each child in a home with a background
similar to that of its own family, and therefore it should not be surprising to find
that the twins developed similarly. But in those cases in which there had been a
greater difference in the environments of the separated twins, the differences
between the twins were more substantial. The following case illustrates what
happens to identical twins when they are brought up in contrasting environments.
Gladys and Helen were born in a small Ohio town and were separated at about
eighteen months of age. They did not meet again until they were twenty-eight
years old. Helen had been adopted twice. Her first foster parents had proved to be
unstable, and Helen had been returned to the orphanage after a couple of years;
after several months she was again adopted, by a farmer and his wife who lived in
southeastern Michigan. This was her home for the next twenty-five years. Her
second foster-mother, though she had had few educational advantages herself,
was determined that Helen should receive a good education; Helen eventually
graduated from college, taught school for twelve years, married at twenty-six, and
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