Recently, emphasis has shifted from T cells to innate (natural) immunity as the possible major culprit in psoriasis. All known elements of innate immune responses are up-regulated in psoriasis lesions, which must have a polygenetic origin. We hypothesize that urbanized populations have been under evolutionary pressure that selects for increased innate immunity responses because those offer relative but immediate protection from epidemic infections. That would have resulted in a changing gene pool, in which alleles of polymorphisms associated with increased innate immunity responses have amplified in these populations. Having too many of these genes together in one individual would result in a relatively low number of infections. On the other hand, it would also result in a higher prevalence of diseases related to increased innate immunity, such as psoriasis, and perhaps also multiple sclerosis and rheumatoid arthritis. Indeed, in indigenous people (Inuit, Aborigines, Ami) who have not been under this selection pressure, morbidity due to infections is high and the prevalence of psoriasis is low or even absent.