Twins and Siblings: Concordance for School-Age Mental Development
Ronald S. Wilson
Child Development
Vol. 48, No. 1 (Mar., 1977), pp. 211-216
Within each dizygotic pair, the full-scale IQ scores ranged from exactly the same to differences of more than 30 points. The median within-pair difference for dizygotic twins was eight points [...] The concordance for full-scale IQ and verbal IQ showed no significant change from dizygotic twins to sibling pairs to twin-sibling sets, so the unique experiences of being born and raised as twins did not promote significantly greater similarity in IQ.
However, they also did some more in-depth analysis of twin-sibling sets (comparing a pair of twins with a non-twin sibling).
In most of the sets (88%) the differences were less than 15 points, and among these sets the twins scored above the sibling nearly as often as below. But in the few remaining sets where larger differences were found, the sibling typically had the higher score, and these sets involved the brightest siblings in the sample with a median verbal IQ of 130.