Figure 3.

A schematic illustration of the endophenotype concept in psychiatry. Endophenotypes are measurable traits, invisible to the unaided eye, along the pathway between disease and genotype (see Figure 7). They are simpler manifestations of the genetic underpinnings of a disorder than the syndrome itself. Shaded areas indicate the expected presence of an endophenotype (e.g., sleep spindle deficits) in individuals with schizophrenia, those with spectrum disorders, syndromally unaffected family members, and the general population. Existing evidence shows that sleep spindle deficits meet criteria 1, 2, and 5. Although spindles are trait-like across nights in studies of healthy individuals (e.g., Cox et al. 2017), stability of the deficit in longitudinal studies of individuals with schizophrenia across prodromal, psychotic, and remitted states would satisfy criterion 3. No study to date has addressed criterion 4. Definition and criteria adapted from Gottesman & Gould (2003).