PURPOSE. Numerous genetic diseases and environmental stimuli affect optic nerve morphology. The purpose of this study was to identify the principal heritable components of visible optic nerve head structures in a population-based sample of twins.METHODS. Fifteen optic nerve specialists viewed stereoscopic optic nerve head photographs (Stereo Viewer-II; Pentax Corp., Tokyo, Japan) from 50 randomly selected monozygotic or dizygotic twin pairs. Before viewing, each expert was questioned about which optic nerve head traits they believed were inherited. After viewing a standardized reaching set, the experts indicated which twin pairs they thought were monozygotic. Participants were then questioned about how their decisions were reached. A rank-ordered Rasch analysis was used to determine the relative weighting and value applied to specific optic nerve head traits.RESULTS. The proportion of twin pairs for which zygosity was correctly identified ranged from 74% to 90% (median, 82%) across the panel. Experts who correctly identified the zygosity in more than 85% of cases placed most weighting on shape and size of the optic disc and cup, whereas experts with the lowest scores placed greater weighting on the optic nerve head vasculature in reaching their decisions.CONCLUSIONS. In determining the genetic components of the optic nerve head, the results of this study suggest that the shape and size of the optic disc and cup are more heritable and should receive a greater priority for quantification than should vascular features.