The heritable nature of reading disability has been well documented (DeFrie
s & Alarcon, 1996), and possible abnormalities of brain structures have bee
n associated with the disorder (Filipek, 1995). However, the etiology of in
dividual differences in morphological brain measures has not been examined
extensively. The purpose of this study was to apply behavioral genetic meth
ods to assess the etiology of individual differences in neuroanatomical str
uctures. Measures of reading performance, cognitive ability, and magnetic r
esonance imaging scans were obtained from 25 monozygotic (MZ) and 23 same-s
ex dizygotic (DZ) twin pairs with reading disability, and 9 MZ and 9 DZ con
trol twin pairs participating in the Colorado Learning Dis-abilities Resear
ch Center. Results obtained from multiple regression analyses (DeFries & Fu
lker, 1985, 1988) of these twin data indicated that individual differences
in the size of most cortical and subcortical structures were largely due to
heritable influences. Moreover, estimates of heritability did not change a
ppreciably after controlling for IQ and total brain size.