analyses were 1408 SNPs from the genomewide scan. The program implements a model-based clustering method for inferring population structure using genotype data. Individuals in the sample are assigned (probabilistically) to populations, or jointly to two or more populations if their genotypes indicate that they are admixed. We allowed for up to five populations, but found the best fit at two: one mostly AA and one mostly EA. We found that only 1% of the subjects self-reporting to be of AA descent clustered in the EA group, and of the subjects self-reported to be EA, only 1.5% clustered in the AA group. Of subjects identifying themselves as White Hispanic, 45% clustered in the AA group versus 55% of subjects who identified themselves as Black Hispanic. Structure reports the posterior probability that individual i is from population k with a value between 0 and 1. A large majority of subjects had a posterior probability of about 0.9 and only a small minority, mostly mixed-ethnicity families, clustered near the middle. The threshold we used in those cases was 0.5. Individuals with a clinical diagnosis of a major psychotic illness (schizophrenia or schizoaffective disorder) were excluded as probands. When an affected sibling pair was
