Transfusion syndrome and the heritability of IQ.

The recent analysis of transfusion syndrome effects by Munsinger (1977) suggests that there is literally no effect of social environment on IQ variation in the population. The detailed analysis of Munsinger's report, however, indicates that his conclusions cannot be supported. Though Munsinger assigned birthweights to separated MZ twin pairs described in the literature, there do not in fact exist birthweight data for most pairs. Further, for some pairs, Munsinger also estimated their IQ's. There is no valid basis for Munsinger's claim that he has detected those separated MZ pairs within which transfusion syndrome occurred. When discussing six studies of birthweights of MZ pairs reared together, Munsinger in each case made transcription errors in selecting data from the original studies. Then he misapplied a statistical formula to the erroneous data, and concluded that inclusion of pairs with large birth-weight differences biased the IQ correlation of MZ's downwards. When Munsinger's own data base is correctly analysed, employing his own procedures, inclusion of pairs with large birthweight differences in fact increases the IQ correlation. There is thus no demonstrable basis for Munsinger's claims about transfusion syndrome and the heritability of IQ.