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The authors investigated closeness and other variables measuring depression in 22 identical and 13 fraternal twin pairs. Each twin rated him/herself on a two-part questionnaire; part 1 included questions on demographic characteristics and the twin relationship, and part 2, the Zung Self-Rating Depression Scale, measured depressive symptoms. There was a high degree of agreement across all variables for the total sample, and degrees of depression were comparable to that in the general population. Closeness was found to be inversely correlated with depression. The authors suggest that future studies include other groups (siblings, spouses, etc.) and follow-up data.
Paluszny et al. (Thu,) studied this question.