Are Socioeconomic Status and Time in Preschool Mediators of Shared Environment Variance in Reading Ability?

While socioeconomic status (SES), time in preschool (TiP) and their interaction are related to reading ability, the results have been mixed. This study assessed whether the shared environment mediated these associations using a longitudinal behavior genetic design. Further, this study contrasted the source of variance in reading accounted for by (a) SES and TiP, against that of (b) genes, the shared environment, and the unique environment. Participants were 511 monozygotic twin pairs, and 515 dizygotic same-sex twin pairs from Australia, the United States and Scandinavia. They were assessed at preschool, kindergarten, grade 1, and grade 2. The interaction between TiP and SES did not explain variance in reading at any stage. At preschool, SES and TiP combined accounted for 9% of the total variance in reading, or 17.6% of the shared environment variance. Of that 9%, SES explained 8.1% independent of TiP. By Grade 2, SES and TiP explained 4% of the total variance, or 57% of the shared environment variance. Of that 4%, SES accounted for 3% with TiP accounting for just 1%. It was found that SES, but not TiP, had a small but significant influence on reading over time, and is a substantial mediator of the shared environment influence of reading by Grade 2.