the Total Population Register. The latent genetic and environmental correlations were very similar to those obtained using the less strict definitions (results available upon request from the first author). Fourth, we limited our analyses to same-sex pairs, which precluded examination of qualitative sex differences (42), because attempts to include opposite-sex pairs resulted in unstable estimates. Finally, we included all possible same-sex pairs, which may artificially reduce the confidence intervals for the estimates. However, the impact of this is likely minimal given the large sample size.
