A population-based heritability estimate of bipolar disorder – In a Swedish twin sample

• The heritability for bipolar disorder was 60% using a novel statistical approach taking into account available time of follow-up, and controlling for sex and year of birth. • The heritability was similar to previous family studies performed on the Swedish register data, using traditional twin-modeling methods, indicating that previous methodology have been sufficient. • We detected no influence from common environmental factors in our heritability analysis, due to lack of power. Future studies using transnational twin cohorts to expand the sample-size are needed. • The prevalence for bipolar disorder was higher in females, but genetic factors did not contribute to those sex-differences. Twin- and family studies have shown variations in the heritability estimates of bipolar disorder (BPD). The current study uses an updated statistical methodology for heritability estimation in BPD by taking available time of follow-up into account while controlling for co-variates. We identified monozygotic and dizygotic same and different sex twins with BPD ( n = 804) or unaffected from BPD ( n = 91,604) from the Swedish Twin Register and the National Patient Register. We applied structural equational modeling with inversed probability weighting to estimate the heritability, taking into account censoring and truncation of data. Sex-limitation models were constructed to analyze qualitative or quantitative sex-differences in BPD. Heritability for BPD was 60.4% (95% Confidence Interval: 50.3–70.5) after age, sex, left-hand truncation and censoring of the data was taken into account. A larger proportion of females were affected from BPD (females 62.2%; males 37.8%, p < 0.001), but no sex-difference in BPD heritability was found, nor any sex-specific genetic effects. We demonstrated a robust 60% heritability for BPD with no evidence of sex-specific genetic effects on disease liability.