This paper reports the preliminary findings of an analysis of the Moray House (verbal IQ) results of a base sample of 4383 Orkney children due to be tested in the schools at age 11±1½ years in 1947–75. Girls enjoy a 3·63% premium relative to phenotypically equivalent boys at test age. Relationships within the sample, which includes some members of the parental generation, are known. The trait is found to be X-linked, with nine phenotypes in the boys, and lyonization is involved.