ABSTRACT
Identical and same-sex fraternal twins reared together in their shared family environment can provide a way to separate the average balance of genetic and family environment influences on reading disability and on individual differences in reading across the normal range. Both types of twins have equal shared environments in their homes, schools, and communities (we clarify the meaning of this assumption in the concluding section). However, identical twins share all their genes, whereas fraternal twins share half of their segregating genes (genes that differ between individuals) on average (see Plomin, DeFries, McClearn, & McGuffin, 2000, for additional assumptions of twin studies). Thus, there is evidence for genetic influence on dyslexia if identical twins are more likely to share the disorder compared with fraternal twins. Similarly, for individual differences in reading across the normal range, a significantly higher correlation for identical twins compared with fraternal twins can provide evidence for genetic influence. The average effects of shared family environ-ment and nonshared environmental influences can also be estimated from twin data in ways that are described later.
