The QIMR group, led by Professor Nick Martin, has identified specific locations on Chromosomes 2 and 6 as being highly influential in determining IQ. To do this, they applied multipoint linkage analysis to data from 634 sibling pairs (including non-identical twins) from Australia and the Netherlands who were genetically scanned for the study.

Although earlier twin studies had revealed the existence of genes that dictated human intelligence, usual genetic association methods had not been effective in identifying them. Where association analysis may overlook closely spaced genes that act together to affect a trait, linkage analysis is more sensitive to such combined effects.

Traditional IQ tests are designed to assess abilities across different areas - memory, vocabulary, semantics, symbolic reasoning - collectively grouped into higher orders such as verbal and performance intelligence. The region on Chromosome 2 shows significant links to performance IQ, also overlapping a region associated with autism. The region on Chromosome 6 showed strong links with both full-scale and verbal IQ with a marginal overlap to an area implicated in reading disability and dyslexia.

During the study, where the non-identical twins and other sibling pairs had significant differences in IQ, they also had significant variation in these regions on Chromosomes 2 and 6.