Influence of Intrauterine Relations on the Intelligence of Twins

An association of a higher IQ for the member of twin sets with the heavier birth weight, already described for highly selected groups of twins, is reported for a numerically larger and more representative sample. For the 374 twin sets studied a higher IQ for the heavier twin is found for like-sex pairs with large birth-weight differences (300 gm or more). When information on zygosity is considered, an association is shown for monozygous sets with large birth-weight differences, but none is demonstrated for like-sex dizygous pairs. Unequal intrauterine relations, perhaps of a circulatory nature, are proposed as an explanation for the IQ differences observed among the monozygous sets.