To Örebro University

Background: Given close genetic and emotional connections between a twin pair, the death of a co-twin sibling may considerably affect the mental health of the surviving twin. However, evidence from population-based cohort studies is currently lacking.

Methods: Based on the Swedish health registers, we identified 4528 exposed twins whose twin sibling died between 1973 and 2013. For comparison, 22,640 matched unexposed twins (i.e., had twin sibling but didn’t experience such a loss) and 4939 full siblings of these exposed twins were included. Controlling for multiple confounders, we used Cox models to estimate hazard ratios (HRs) with 95% confidence intervals (CIs) of clinical ascertained psychiatric disorders after loss of a co-twin.

Results: The median age at a co-twin loss was 59 years. Compared to the unexposed twins, exposed twins were at increased risk of psychiatric disorders (HR = 1.65 [95% CI 1.48–1.83]). The association seemed stronger for loss of a monozygotic twin (HR = 2.02 [95% CI 1.56–2.61]) than loss of a dizygotic twin (HR = 1.46 [95% CI1.27–1.67]), and was evident after loss by natural causes (HR = 1.49 [95% CI 1.32–1.69]). Additionally, such risk was most pronounced during the first year after loss and when loss occurred at young age. The HR was 1.55 (95% CI 1.31–1.82) when compared exposed twins to their full siblings who also exposed to loss of a normal full sibling due to the death of the deceased twin. 

Conclusions: Losing a co-twin is a strong life stressor indicated by the increased subsequent risks of psychiatric disorders among the surviving twins.