Findings of the present study also extend findings from the twin literature which provide more information on the liability for comorbidity (as they examine genetic risk within and between twins), and capture all inherited genetic variants shared by monozygotic twins. Thus, the source of any shared genetic effects cannot be discerned. Molecular genetic investigations in samples of unrelated individuals demonstrate genetic correlation, from common variants, and separate from phenotypic comorbidity. As such, molecular genetic studies provide insight into shared genes that increase risk for both conditions, help clarify the genetic architecture, and lay the groundwork for risk prediction and functional examination of common variation underlying and shared between PTSD and AD (see review by Martin, Taylor, & Lichtenstein, 2017).
