Chronic depression and dysthymia are even more likely to co-exist with anxiety disorders; indeed, some authors argue that dysthymic disorder (DD) is less common in a pure form than in a mixed depression, anxiety co-morbidity, for which the term cothymia has been proposed. Attention deficit hyperactivity disorder (ADHD) is an under-recognized problem in adults and is a common complicating factor in depression. People with depression have more Axis II disorders (most commonly dependent, avoidant, histrionic, or borderline personality disorder) than the general population. Individuals with chronic depression have more co-morbid Axis II disorders than do individuals with episodic major depression. Chronic depression, regardless of severity level, is associated with considerable functional impairment and morbidity. Chronic low-grade depression causes significant impairment and suffering. Dysthymia can be conceptualized as a prodromal disorder, since nearly all individuals with DD will at some point experience a full-blown major depressive episode.

The structured clinical interview for DSM-III personality disorders (SIDP) was the first semi-structured interview schedule developed to test and improve DSM-III Axis II validity and reliability. The SIDP-R is the revised edition for DSM-III-R. Both versions of this interview have been tested for reliability and validity, as well as presence and comorbidity of Axis II disorders. The validity of the SIDP has also been tested in a number of studies, usually in terms of concurrent/convergent validity with self-report questionnaires. The schedule for schizotypal personalities (SSP) provides information on SPD and achieves high reliability; however, it does not allow for determination of comorbidity with other Axis II disorders. One important feature is that the self-report measures of the schizotypal signs have been validated against clinicians' observation. The SSP also demonstrated strength specifically in the population it was designed on, that is relatives of schizophrenics, who constitute an important research group.