The genetic and environmental etiologies of individual differences in early reading growth in Australia, the United States, and Scandinavia

• First study comparing the etiology of variance in early reading growth between countries.
• Reading data from twins in the U.S., Scandinavia, and Australia.
• Balance of genetic and shared environmental influences varied at post-kindergarten.
• High genetic influences on individual differences by the end of first grade, and on growth, in all countries.

This first cross-country twin study of individual differences in reading growth from post-kindergarten to post-second grade analyzed data from 487 twin pairs from the United States, 267 twin pairs from Australia, and 280 twin pairs from Scandinavia. Data from two reading measures were fit to biometric latent growth models. Individual differences for the reading measures at post-kindergarten in the United States and Australia were due primarily to genetic influences and to both genetic and shared environmental influences in Scandinavia. In contrast, individual differences in growth generally had large genetic influences in all countries. These results suggest that genetic influences are largely responsible for individual differences in early reading development. In addition, the timing of the start of formal literacy instruction may affect the etiology of individual differences in early reading development but have only limited influence on the etiology of individual differences in growth.