Structural equation model of social integration and sleep disturbance in adult twin pairs. A = common genetic variance; C = shared environmental variance; E = non-shared environmental variance. ACE components are standardized. Si = social integration; Sl = sleep disturbance. Within MZ twins, the A component correlation is fixed at 1.0; within DZ twins, this correlation is fixed at .5. The C component correlation is 1.0 in all twins, and the E component correlation is fixed to 0.