Eighteen pairs of twins who were registered in the Twin Registry files of the National Research Council with the diagnoses of psoriasis or atopic dermatitis were questioned and examined. Of these pairs, ten had psoriasis, four had atopic dermatitis, and four were found to have neither of the foregoing, but to have other skin diseases. All eight monozygotic twins who showed male pattern alopecia were concordant, and were similar in degree. Discordance, as well as concordance, in monozygotic twins was noted in cases of both psoriasis and atopic dermatitis. Although the number of cases studied was not sufficient to make a statistical conclusion, environmental factors seem to influence expression of these conditions.
Lynfield YL. Skin Diseases in Twins. Arch Dermatol. 1974;110(5):722-724. doi:10.1001/archderm.1974.01630110020002