Longitudinal Twin Study of Early Literacy Development: Preschool and KindergartenPhases.

Byrne, B., Wadsworth, S., Corley, R., Samuelsson, S. Quain, P., DeFries, J. C., Willcutt, E., and Olson, R. K. (2005). Longitudinal twin study of early literacy development: Preschool and kindergarten phases. Scientific Studies of Reading, 9(3), 219–235.

This study is a behavior–genetic analysis of kindergarten reading, spelling, phonological awareness, rapid naming, and spoken sentence processing involving 172 pairs of monozygotic and 153 pairs of same-sex dizygotic twin kindergarten children from the U.S. and Australia. U.S. sample was obtained using birth records in the state of Colorado. Australian sample were recruited voluntarily indicating an interest in research. Children from Australia were generally younger. U.S. children had parents who generally self-reported higher levels of education than Australian parents.

From the abstract:

[The study] modeled progress from preschool to kindergarten in literacy-related skills, with larger numbers of twins contributing to the preschool phase. Reading, phonological awareness, and rapid naming at kindergarten showed substantial effects of genes and modest effects of shared environment; spelling was influenced by genes and environment equally; and sentence processing was affected primarily by shared environment. Longitudinal analyses indicated that the same genes affect phonological awareness in preschool and kindergarten but that a new genetic factor comes into play in rapid naming as letters and digits are introduced in kindergarten. At preschool, print knowledge and phonological awareness share one source of genetic influence, which in turn affects reading and spelling in kindergarten. Phonological awareness is subject to a second genetic factor, but only the one it shares with print also influences kindergarten reading and spelling. In contrast to the genetic effects, a single source of shared environment affects preschool print knowledge and phonological awareness and kindergarten reading. (p. 219)

Assessments:

Test of Word Reading Efficiency. In the Test of Word Reading Efficiency (TOWRE) source: Torgesen,Wagner, & Rashotte, 1999

Spelling Test (dog, man, one, said, blue, come, plug, went, limp, tree) and 4 nonwords (ig, sut, frot, yilt).  Adapted from Liberman, Rubin, Duquès, and Carlisle (1985)

Rapid naming. 3 versions: colors, digits, and letters, from the Comprehensive Test of Phonological Processing (CTOPP)  Wagner, Torgesen, & Rashotte, 1999.

Phonological awareness tests. CTOPP (a) Elision (b) Blending (c) Sound Matching

Test for the Reception of Grammar. Bishop (1989) tests child’s command of grammatical structures.

Letter-sound knowledge. Children asked to point to the letter in a line of four that represented the letter-sound supplied by the tester. 26 letters tested.

Results and Discussion

Phenotypic Analyses

Australian twins scored higher than U.S. twins on all variables. (Neither gender nor age explain differences.)  Inequalities in effect size may be due to schooling practices: Australian children attend full day kindergarten while Colorado children attend half days. Because of the country differences, all scores were standardized within country prior to further analyses. Variables also adjusted for age and gender.

 Reading, spelling, and phonological awareness accounted for 58.0%variance. Rapid naming tests accounted for 11.6%variance. Grammar Reception test accounted for 7.76% of variance.

Genetic Analyses
For the genetic analyses, components were subdivided into reading, spelling, and phonological awareness. Reading, phonological awareness, and rapid naming latent traits showed heritability and some shared environment effects. Spelling showed a gene and and environment effect, while grammar had only an environment effect. Phonological awareness latent traits among the preschool children were based on several tasks: elision, blending, and matching of rhymes, beginning and final phonemes. Kindergarten measures were the same except the rhyme component. Environmental factors were not reliable for either group, and individual differences in phonological awareness was continuous from K to Gr. 1. 

When analyzing relationships to print knowledge, results indicated that phonological awareness exerts genetic influence on later reading only through genes that it shares with print knowledge. Preschool print knowledge is about half as influenced by genes as is kindergarten reading, but the genetic correlation between these variables is higher (.79) than the genetic correlation between preschool phonological awareness and kindergarten reading (.54).  


Although phonological awareness appears to be an inherited trait in this study, more so than print knowledge, only some of the genetic traits influence variability in reading and spelling at the end of kindergarten. One possible interpretation is that the preschool environmental varies for print familiarity, where children are encouraged to learn the alphabet, resulting in a lower estimate of heritability than would likely be the case in a less explicit environment. Genetic variation becomes more visible when children share a similar classroom environment, learning to associate letters and words with speech. 


Research into effective practices that help those children with a genetic predisposition to develop later reading problems is warranted by this study, with particular emphasis on developing phoneme awareness in young children.