Investigations that recruit genetically informative samples, like twins, can be used to establish the heritability of an endophenotype. A general population sample can be used for this purpose for dimensional traits that are continuously distributed (which appears to be the case for the vast majority of candidate endophenotypes). Establishing heritability also establishes reliability, another critical endophenotype attribute. Monozygotic twins, who conceptually can be thought of as parallel forms of the same individual, are especially useful for demonstrating the psychometric soundness of an endophenotypic measure because they can be expected to be highly similar for virtually all human traits (Polderman et al., 2015). The relatives of affected individuals with the endophenotype should possess the trait. Affected relatives should be characterized by endophenotype values similar to those of probands. Unaffected relatives are of particular value. Because many, but not all, of the unaffected relatives are presumably at elevated risk for developing a heritable disorder, they should score midway between probands and healthy comparison subjects. These patterns should be evident in all first-degree affected and unaffected relatives - parents, siblings and offspring. Given
