Project I: Twin Studies

Principal Investigator: John C. DeFries

Co-Investigators: Sally J. Wadsworth and Erik G. Willcutt

Goals of Project I: The long-range goals of this project are the identification, characterization and validation of etiologically distinct subtypes or dimensions of learning disabilities. To accomplish these goals, the twins and their siblings are administered an extensive psychometric test battery that includes tests of general cognitive ability and academic achievement. In collaboration with investigators from Research Projects II, III, and IV, resulting data are used to assess the genetic and environmental etiologies of reading deficits, ADHD, and their comorbidity, as well as their covariation with measures of other psychopathology, reading, language and perceptual processes, mathematics performance and executive functions. The multiple regression analysis of selected twin data (DeFries & Fulker, 1985, 1988) is used to assess the etiology of group deficits in reading performance, as well as the etiology of individual differences. It has also been used to test novel hypotheses of differential etiology of reading difficulties as a function of ADHD dimensions or subtypes, age, gender, and cognitive ability, and to assess the hypothesis that the etiology of deviant scores differs from that of individual differences within the normal range. Bivariate heritability between RD and other disorders, such as ADHD is also assessed, and QTL analyses are used to assess genetic linkage of reading deficits (Fulker et al., 1991; Cardon et al., 1994).

Participants: Since 1982, over 2,500 twins and their families have participated in the CLDRC twin project. Twin pairs in which at least one twin has a school history of reading problems or ADHD symptoms are ascertained systematically through 27 cooperating school districts in Colorado. In addition, a comparison sample of twins without RD or ADHD is recruited. The following table summarizes the total number of twin pairs tested to date.

	Monozygotic Twin Pairs (Identical)	Dizygotic Twin Pairs (fraternal)		Total Twin Pairs
		Same-sex	Opposite-sex
At least one twin has RD	267	215	154	636
At least one twin has ADHD	40	36	41	117
Comparison twins without RD or ADHD	228	150	99	477

The twins are administered an extensive battery of tests in the laboratories of John C. DeFries at the Institute for Behavioral Genetics, Richard K. Olson at the Department of Psychology, University of Colorado, and Bruce F. Pennington at the Department of Psychology, University of Denver. These tests include measures of cognitive ability, reading and mathematics achievement, reading-related language processes, executive functions, and psychopathology.

Results: In approximately two-thirds of the pairs of identical twins, both members of the pair are affected, whereas the corresponding concordance rate for RD in the same-sex fraternal twins is only about a third, a highly significant difference. Although this comparison of concordance rates in identical and fraternal twin pairs clearly indicates that reading disability is due in part to genetic influences, multiple regression analyses of reading performance data provide a statistically more powerful and versatile test (DeFries & Fulker, 1985). When reading performance data were recently subjected to multiple regression analysis (sometimes referred to as DF analysis), we found that over half (0.57) of the reading deficit of the affected members of these twin pairs was due to genetic influences, a very highly significant result.

In addition to obtaining compelling evidence for the heritable nature of reading disability, we have also recently found that the etiology of reading disability differs as a function of IQ and age. Results obtained from DF analyses indicate that heritable influences are substantially more important for twin pairs with an average IQ over 100 than for those with a lower IQ (0.75 versus 0.43, respectively). Also, we have obtained some evidence that the causes of reading and spelling deficits may change differentially as a function of age. Whereas deficits in reading performance are somewhat more heritable in younger children than in older children (0.64 vs. 0.47), the converse pattern occurred for spelling difficulties (0.52 vs. 0.68).

References: (under construction)