Observed correlations
We presented correlations from the “Symmetric” model described above. The phenotypic correlations, rph, were based on the within-twin/cross-trait correlations. Intra-class correlations (ICC) are the correlations between the same variable measured in one twin and in his/her co-twin (i.e., cross-twin/within-trait). If the absolute value of ICC is larger in MZ twins than in DZ twins, this indicates that genes are involved in the association. The cross-twin/cross-trait (CTCT) correlations represents the correlations between one variable in one twin and another in the co-twin. If the absolute value of the CTCT is larger in MZ twins than in DZ twins, this implies that genes influencing both traits (at least partly) overlap. Pearson correlations were calculated for the associations between continuous traits, while tetrachoric for binary (asthma) and biserial correlations were calculated for both binary and continuous traits.