The estimate for the heritability (73%) is much larger than the one reported earlier based on sum scores (40%, Van den Berg et al. 2006c). In the current sample, twin correlations for sum scores are very much like those reported earlier (MZ:0.45, DZ:0.17). By applying an IRT measurement model the twin correlation estimates for the latent trait are much higher, 0.76 for MZ twins and 0.30 for DZ twins. For comparison, when using a two-step approach, first estimating IRT model parameters in Multilog and then estimating latent scores for each individual (correlation between sum score and IRT estimate: 0.98), the results showed twin correlations nearly identical to those based on sum scores.
