Descriptive statistics were obtained using JMP Pro 10.0.2 (Cary, NC) and SAS 9.2 (Cary, NC). Twin modeling was conducted with OpenMx (Boker et al., 2011), using full information maximum likelihood. We tested three plausible models of how additive genetic (A), shared environmental (C), and unique environmental (E) factors might explain covariation between SELF and PEER across time. These models have been described previously (Kendler et al., 2008) and are provided in the Supplementary Material. Briefly, model 1 (Supplementary Figure 1A) is a simple model representing a causal mechanism, with three key components: i) latent factors (A1self-A8peer) loading (via La1–La8) onto each of 8 observed variables (4 measures each of SELF and PEER); ii) forward transmission (e.g., B12 from SELF1 to PEER1) of genetic/environmental influences across time within a phenotype (SELF or PEER); and iii) bidirectional causation within time across phenotypes, such that SELF1 has a causal effect on PEER1 (via B15), and vice versa (via B51). Model 2, a correlated factors model, postulates that the within- and cross-time correlations between SELF and PEER result from two correlated latent factors, the
