Several methodological and sample-specific factors might explain the inconsistent findings, including small patient samples, diagnostic heterogeneity and comorbidity, sexual dimorphism in EEG asymmetry and/or depressive illness, choice of EEG reference, the measure of depression used (e.g., DSM-IV diagnosis versus questionnaire indexing current symptom severity) and the reliability and stability of EEG asymmetry within and across sessions (e.g., Allen et al., 2004a, 2004b; Coan, Allen, & McKnight, 2006; Davidson, 1998; Hagemann, 2004). To date, however, the role of these factors has not been systematically examined due primarily to relatively small patient samples. The present study utilized a large sample of depressed and non-depressed individuals to address these and other factors in order to evaluate the utility of frontal EEG asymmetry as an endophenotype of risk for depression.
