Similarity of monozygotic and dizygotic twins in level and lability of subclinically depressed mood.

Ninety-two adult twin-pairs were recruited through advertisements in the local media. Twin zygosity was determined by use of the self-report inventory developed by Nichols and Bilbro (1966). The level and lability of subclinical depression were assessed. Mood level was assessed by the Beck Depression Inventory and by the MMPI D, D30, and DR Scales. Mood lability was assessed by daily administrations over a 2-week period of the Depression Adjective Check List (DACL) and the Wessman-Ricks Elation-Depression Mood Scales. Monozygotic twins resembled one another more than dizygotic twins in most measures of both level and lability of mood, which provides modest evidence for a genetic influence on subclinical levels of depression. Implications of these findings for the continuity hypothesis of depression are discussed.