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1、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 t
2、hat 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. 2 The relative effects of heredity and environment are most clearly observable in identical twins.
3、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 ind
4、ividuals were raised separately. 3 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 N. Therefore, as one might
5、 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 exi
6、st between identical twins, even those who have been separated from infancy N. N 4 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
7、of twins. N 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 N from each other, the environments in those families were not, on the
8、 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 i
9、n 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. 5 Gladys and Helen were born in a small Ohio town and were separated at about eight
10、een 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 N 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
11、 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 N, was determined that Helen should receive a good education; Helen eventually graduated from college, taught school for twelve
12、years, married at twenty-six, and had a daughter. 6 Gladys was adopted by a Canadian railroad conductor and his wife. When she was in the third grade, the family moved to a rather isolated part of the Canadian Rockies N, where there were no schools, and Gladys formal education came to an end, and wa
13、s not resumed until the family moved to Ontario N. She stayed at home and did housework until she was seventeen, and then went to work in a knitting mill. She went to Detroit N at nineteen, got a job, and married when she was twenty-one. 7 Helen had been healthier than Gladys, in childhood and adult
14、hood, but other than that, their environments had been very similar except N for their education. Their weight, height, hair color, and teeth were very similar. The differences that distinguished them were obviously associated with the different social lives they had led. 8 Helen was confident, grac
15、eful, made the most of her personal appearance, and showed considerable polish and ease in social relationships. Gladys was shy, self-conscious, quiet and without charming or graceful manners. A scientist who studied them remarked, As an advertisement for a college education the contrast between the
16、se two twins should be quite effective N. 9 Considering the nature of their environmental experiences, the differences in Helen and Gladys are not surprising. Since psychological traits depend so much upon experience, it is to be expected that they will reflect it. On the other hand, traits that are not liable to be influenced by the environment are more likely to exhibit a high degree of similarity in identical twins. Important as the
