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

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There is some reason to suppose that the correlation might be lower than the .51 observed among the Shields pairs reared by unrelated families. We have seen that even among these pairs there were substantial similarities in environment. The two remaining studies reported results basically similar to those of Shields. The 19 pairs studied in the United States by Newman, Freeman, and Holzinger (1937) correlated .67, while the 12 pairs studied in Denmark by Juel-Nielsen (1965) correlated .62. These correlations seem substantial, but they cannot be attributed entirely to heredity. The twins in these studies, like those observed by Shields, tended to be reared in quite similar environments and often had considerable contact with each other.

Further, there is reason to believe that the particular IQ tests used in these studies were not accurately standardized for age and for sea (Kamin, 1974). Since a pair of identical twirls is always of the same age and same sex, any tendency for the test to favor a particular age group or sex will tend to make the twins appear more similar in IQ than they really are.

To sum up, the actual studies of separated identical twins have produced results much less conclusive than might have been obtained in an ideal-but in practice impossible-to-perform-experiment. The twins who have been studied do resemble each other in IQ, but-once Burt’s data are rejected-they have also experienced quite similar environments. Thus there is no way of knowing how much of the observed IQ correlation is due to identical genes. and how much to similar environments. There is obviously much room for disagreement in interpreting these data: if there were not, the argument about heredity environment, and IQ would long since have ended.

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