Results from a large-scale twin study

The large-scale twin study described on this slide used data from two Danish databases: the Danish Twin Register and the Danish Psychiatric Research Register.^{[Hilker et al., 2017; Hilker et al., 2018]} From these sources, a sample was constructed comprising twin pairs born between 1951–2000.^{[Hilker et al., 2017; Hilker et al., 2018]}

Interestingly, when the concordance rates of the incidence of schizophrenia were compared between monozygotic and dizygotic twins (i.e., identical and non-identical twins) a distinct difference emerged, from which the authors were able to estimate the hereditability of schizophrenia to be approximately 79%.^{[Hilker et al., 2018]} There are problems with the approach that was taken, however, and the authors are clear that, ideally, twins would be followed over the course of their whole lives.^{[Hilker et al., 2018]} In reality, this approach would be impractical, and very costly. As a compromise position, in this analysis of Danish twin data, a supplementary dataset was constructed, ensuring a minimum follow-up time of 30 years by including only twin pairs born between 1951–1981.^{[Hilker et al., 2018]} The results of the main and the supplementary datasets were almost identical.^{[Hilker et al., 2018]} Furthermore, the result seems closely aligned with a previous attempt to estimate the heritability of schizophrenia, using a collection of twin studies, which reported a genetic heritability value of 81%.^{[Sullivan et al., 2003; Hilker et al., 2018]}

References:
Hilker R, Helenius D, Fagerlund B, et al. Is an early age at illness onset in schizophrenia associated with increased genetic susceptibility? Analysis of data from the nationwide Danish twin register. EBioMedicine 2017; 18: 320–326.

Hilker R, Helenius D, Fagerlund B, et al. Heritability of schizophrenia and schizophrenia spectrum based on the nationwide.

Danish twin register. Biol Psychiatry 2018; 83 (6): 492–498.

Sullivan PF, Kendler KS, Neadle MC. Schizophrenia as a complex trait: evidence from a meta-analysis of twin studies. Arch Gen Psychiatry 2003; 60 (12): 1187–1192.