Recent methodological advances make possible the use of molecular genetic data to estimate heritability and the degree to which genetic variants are shared between an endophenotype and its associated clinical phenotype, two of our criteria in Table 1, which were previously only possible in twin studies (although family studies can at least provide evidence of familial influence even if they cannot disentangle genes and common environment). Genomic-relatednesss-matrix restricted maximum likelihood (GREML) (Ge et al., 2015; Speed, Hemani, Johnson, & Balding, 2012; Yang et al., 2010), which is implemented in software tools such as GCTA (Yang, Lee, Goddard, & Visscher, 2011) enables researchers to estimate the heritability of a trait (Yang et al., 2010) in a sample of subjects who are unrelated by kinship. Heritability estimates (“SNP heritability”) derived from GREML, or other approaches with a similar aim (So, Li, & Sham, 2011), are based on the degree of genetic relatedness among individuals from different families of origin that is due to measured genetic variants, as opposed to the degree of genetic relatedness implied by different familial relationships (monozygotic twin siblings,
