Up to 8 percent of people who have psoriasis (a skin condition that is characterized by scaling and swelling) develop psoriatic arthritis. The condition occurs just as often in women as it does in men, and it is most likely to occur between the ages of 30 and 50. In a person with psoriatic arthritis, the joints (and sometimes other areas of the body) become inflamed. Although research scientists do not know the exact cause or causes of psoriatic arthritis, they believe that the condition is caused by a combination of environmental, genetic, and immune system factors. Treating psoriatic arthritis can involve lifestyle changes, medications, and routine monitoring and ongoing care.