IQ: Studies of Separated Identical Twins (Part 2 of 3)

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The second largest study was also done in England by Shields (1962), who managed to test 40 pairs of separated twins. The IQ correlation obtained by Shields was .77 not as high as Burt’s, but still very substantial.
The difficulty, however. is that in the Shields study most of the twins seem to have been reared in quite similar environments. Some were not separated at all until they were seven or eight years old, and 27 of the 40 pairs were actually brought up in related branches of the same family. The twins had usually been born into poor families and the mother had felt unable to take on the burden of two more infants at the same time. The most common single pattern was for the mother to keep one child and to give the other to her sister (or to the father’s sister) to rear. This, of course, tended to result in the ”separated” twins having similar environments.

Thus Shields sap of one pair: “The paternal aunts decided to take one twin each. and they have brought them up amicably, living next-door to one another in the same Midlands colliery village. . . . They are constantly in and out of each other’s houses” (p. 164). This kind of close contact and highly similar environment also occurred even when the twins were brought up by unrelated families. Shields writes of another pair: ”Brought up within a few hundred yards of one another. . . . Told they were twins after girls discovered it for themselves having gravitated to one another at school at the age of 5 . . they were never apart, wanted to sit at the same desk . . .” (p. 189).

For the 27 Shields pairs reared in related branches of the same family, the IQ correlation was 83. For the 13 pairs reared in unrelated families, the correlation was a significantly lower .51. That is clear evidence that “separated” identical twins resemble each other more if the environments in which they have been reared are similar. We cannot deduce what the IQ correlation would be if-as Burt falsely claimed-there were no systematic similarities in the environment of separated pairs. The correlation, if such an ideal experiment could in fact be performed, might conceivably be .00, though few psychologists would expect this outcome.

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