Cognitive resemblance in reading-disabled twins.

This study examined the cognitive abilities of 60 pairs of reading-disabled twins aged between eight and 18 years (at least one member of each twin pair had been diagnosed as having a reading disability). Principal component analysis of nine cognitive tests yielded three readily interpretable composites of cognitive abilities--reading, symbol-processing speed and sequential memory. Scores on these three cognitive composites were significantly lower for the disabled readers than for normal readers. Individual differences in these cognitive composites within the reading-disabled twin sample were found to be due to genetic differences. Furthermore, deficits in the reading composite for the reading-disabled sample appear to be heritable.