Heredity plays a strong role in psoriasis, especially in those who develop it early on. The majority of these people report having a sibling or other family member who has psoriasis as well. That said, one might assume that identical twins would both have psoriasis if it was in their genetic cards. But this is not always the case, meaning that there is likely an environmental influence that factors in, too.
Many diseases (including psoriasis) have been linked to specific HLA types, which are genetically encoded into our DNA. These HLA types produce markers that appear on the surface of white blood cells and that can be easily determined with routine lab tests. Different groups of psoriasis patients seem to be more likely to carry a few specific types of genetic HLA associations. In one group, psoriasis that appears early in life as well as in another family member is common. In another, those with psoriasis don't develop it until they're in their fifties and none of their relatives have it.
Keeping in mind these two very different groups, it becomes clearer to see that it is likely that several different genes -- including so called "psoriasis susceptibility and severity genes" -- play a role in this complex disease.
Still, environmental and other trigger factors seem to play a role in the severity or onset of the disease, too. For example, some patients may have a psoriasis breakout after suffering a strep throat infection. Surgical cuts or wounds may turn into psoriasis (a phenomenon known as Koebnerization) in certain people. Many medications are even associated with causing or worsening psoriasis. Stress, although overemphasized as a factor in most psoriasis cases, may play in role in select patients. Some accountants may see a worsening of their psoriasis around tax season, for example, while others may not.