Relatives of Creative People More Prone to Schizophrenia

"Great wits are sure to madness near allied, and thin partitions do their bounds divide" (Dryden). There often seems to be a link between creativity and mental illness. Many great poets, playwrights, artists and composers suffered from depression, alcoholism, obsessionality, bipolar or psychotic disorders at some time in their lives. How strong is the link and what might account for it?