The history of psoriasis in 95 twin individuals was compared with that of a matched control group of singleton (nontwin) psoriatics. The findings indicate that twin members with psoriasis do not differ from psoriatic singletons in the general population, with respect to clinical manifestations of the disease.
Psoriasis in concordant monozygotic twin pairs tended to be similar, with respect to age of onset, distribution pattern, severity, and course. This pattern was not found in concordant dizygotic twin pairs.
The striking difference in rate of concordance for psoriasis between monozygotic and dizygotic twin pairs indicates that there is a heritable contribution to the etiology of psoriasis. The fact that not all monozygotic twin pairs are concordant suggests that environmental factors are also important and supports a theory of a multifactorial mode of inheritance for psoriasis.
Farber EM, Nall ML, Watson W. Natural History of Psoriasis in 61 Twin Pairs. Arch Dermatol. 1974;109(2):207-211. doi:10.1001/archderm.1974.01630020023005