Limitations of the present study include a younger cohort, suggesting that these findings may not be assumed to apply for later-life onset depression. Our early-onset sample, however, might be expected to have a recurrent or chronic course of depression (cf. findings with chronic depression, Klein et al., 1999) and thus may generalize to individuals with severe depression later in life. In addition, these results may not be generalizable to depressed individuals with comorbid DSM-IV conditions such as anxiety and substance use disorders, which are common in MDD patients (e.g., Kessler et al., 2003). Furthermore, it is unclear whether these findings may extend to persons with MDD who are being treated with psychotropic medications, but to date no evidence suggests that antidepressants impact frontal EEG asymmetry. Finally, the present study offers a cross-sectional investigation to address a within-subjects question, namely whether an individual's pattern of regional frontal brain activity predicts subsequent development of MDD, so prospective, longitudinal data are needed to firmly establish that EEG asymmetry is a vulnerability marker for depression.
