An investigation of the difference in measured intelligence between twins and single births*

SUMMARY Mean V.R. (verbal reasoning) scores recorded in the eleven‐plus examination for Birmingham multiple births in the years 1950‐57 were 95‐7 for 2164 twins and 91‐6 for 33 triplets. The mean for 48,913 single children born in the years 1950‐54 was 100‐1. The low scores of twins are not explained by differences from single births in their distributions by maternal age and birth order or by birth weight and duration of gestation. They are also not accounted for by the increased risks associated with monozygosity (assessed by comparison of like‐ and unlike‐sex twins) or with delivery of the second twin. Taken together these observations, like the previous ones on single births, suggest that variation in experience before and during birth has little influence on measured intelligence and that the explanation of the large difference between twins and single children must be sought in the postnatal environment. There were 148 twins whose co‐twins were stillborn or died within 4 weeks after birth; their mean score was 98‐8, only a little lower than that of single births (99‐5) standardized to the maternal age and birth rank distribution of twins. From this evidence it is concluded that the handicapping of twins, reflected in their low verbal reasoning scores, is due to postnatal rather than prenatal influences. These conclusions are of course based on children who took the eleven‐plus examination and cannot be accepted without reservations for those who did not.