A little less than a third of people who have psoriasis will get psoriatic arthritis. Doctors can't yet tell who before it happens, though. Usually, you'll have the skin symptoms first. But sometimes, arthritis symptoms appear months, or even years, before skin problems do. That makes it hard to diagnose. People with severe psoriasis could have a greater chance of getting psoriatic arthritis. About 40% of people who get psoriatic arthritis have relatives with it or with psoriasis. Scientists don't know which genes are responsible for these conditions. Figuring that out will help doctors predict who is likely to get these conditions. It may lead to new treatments, too.