Most people who commit suicide have a mental disorder - anorexia, major depressive disorder, bipolar disorder, schizophrenia and borderline personality disorder are the most common, but an elevated suicide risk is part and parcel of many of the others, too. People who kill themselves also generally feel deeply depressed and hopeless at the time.

What the statistics do not tell us - and what psychologists most want to know - is exactly which people are most at risk. The vast majority of depressed, hopeless people do not commit suicide, so why do some do it?

In 2005, psychologist Thomas Joiner, a suicide specialist at Florida State University in Tallahassee whose own father committed suicide, set out to answer that question. By studying suicide statistics and paying particular attention to the groups with above average rates, Joiner believes he has found a common thread others have missed. "It was the first grand theory of suicide in quite a while," says Nock.