In their recent article, Hong et al1 reported heritabilities of auditory sensory gating. As they defined it in their article, heritability reflects the proportion of overall variability in a trait in a population that results from additive genetic effects. Within human populations, such estimates have traditionally relied on special relationships that can (with some well-understood limitations) disentangle genetic from familial-environmental effects. The most popular of these methods have been twin studies comparing monozygotic and dizygotic twin pairs and various adoption designs.