Jang, Livesley, Taylor, Stein, and Moon (2004) studied the factor structure of depression. Using several symptom lists, they identified 14 subfactors. Examples of subfactors included “feeling blue and lonely,” “insomnia,” “positive affect,” “loss of appetite,” and “psychomotor retardation.” Interestingly, intercorrelations among the factors ranged from .00 to .34, and the factors were differentially heritable, with heritability coefficients ranging from .00 to .35. It appears to be the case that (a) some of the dimensions of depression do not covary substantially and (b) some have a heritable basis and others do not, which likely indicates that their etiologies differ. McGrath (2005) provided interesting examples of the heterogeneity of depression symptom items.
