Figure 3.

Path diagram of the fully saturated model fit to the data (Model 3; only one twin shown for clarity). Successive models were fit by fixing parameters to zero and conducting likelihood ratio tests whether adding parameters resulted in a significant improvement in model fit. The (A), (C), and (E) latent variables (represented with circles) are the additive genetic, shared environmental, and non-shared environmental variance components of the Singh Index. The Au and Eu latent variables represent residual additive genetic and nonshared environmental variance in sleep duration. In this model, the main effect of the Singh Index on sleep duration (captured in the dotted single-headed paths from the [A] and [E] components of the Singh Index to sleep duration) is permitted to vary with level of the Singh Index. Similarly, the variance in sleep duration that remains after controlling for the main effect of the Singh Index (double-headed paths from Au, Cu, and Eu to sleep duration) also varies as a function of Singh Index.