Figure 1.

Cross-lagged longitudinal monozygotic twin difference model at two ages. All variables are intrapair difference scores. Significant values on the paths would indicate the following: A, covariance between intelligence and reading at Time 1; B, within-trait, across-time association for intelligence; C, between-trait, across-time intelligence–reading association; D, between-trait, across-time reading–intelligence association; E, within-trait, across-time association for reading; F, covariance between intelligence and reading at Time 2.