Our results are consistent with prior observations of the heterogeneous nature of MDD, particularly with regard to ascertainment. Individuals who meet MDD criteria from community or primary care sources may have a more inclusive and less comorbid form of MDD (98) whereas tertiary ascertainment may yield subjects with greater comorbidity and perhaps distinctive etiology (99). In particular, it is formally possible (but unproven) that the PCLO results are accurate – genetic variation in PCLO might be causal to the types of MDD seen in community samples but other loci contribute to a distinctive type of MDD seen in tertiary care samples.
