Most individuals diagnosed with a psychiatric disorder will later receive at least one additional diagnosis during their lifetime, complicating both diagnosis and treatment. Although life experiences and environmental factors shape this risk, inherited genetic factors also contribute substantially.

This correlates with something my wife said to me the other day: people who suffer from one bad disease often also suffer from other bad diseases. It's like a small portion of the population will be carry most of the problems, kinda lika a pareto distribution of stuff: 20% of people suffer from 80% of the diseases.

Of course, if this was genetic or environmental, we could not say, but this study (published in Nature) seems to show that the former plays an important role in this occurrence of additional diagnosis for psychiatric disorders.