Assumptions testing
First, a saturated model was fitted, which included separate estimates
for means, prevalence rates, variances, and covariances between ‘twin 1’
and ‘twin 2,’ according to random assignment for order in pairs and
separately between MZ and DZ twin pairs. We then proceeded to fit a
model where we assumed symmetry within each zygosity, i.e., the twin
order was not associated with means/prevalences and variances or with
within-individual covariance between the traits, named “Symmetric”.
Finally, a model where means and variances were additionally assumed to
be the same across zygosities was fitted corresponding to the basic
assumptions needed for a quantitative genetic model, named
“Assumption.”