The study can't show why LGB people are at greater risk, but there are lots of possible reasons.

"The stress that results from being a member of a minority is one of the main reasons we see mental disorder in LGB people," said Semlyen. "We know that social support is a protective factor in mental health. So is feeling included, feeling safe, not feeling rejected by your family." Gay people may be more isolated, more lonely, less likely to have family and social support, she says.

Bhugra said that "there is increasing evidence that the way we're brought up affects our brain structures, and therefore our brain functioning".

He says that the situation is probably getting better, as Britain (and the world) gets more gay-friendly. "There've been some fascinating studies done in America that show that gay-friendly social policies lead to rates of mental health problems dropping," he said. "One study looked at a state before changes in the law and afterwards, and there was a clear drop in mental health problems afterwards. It would appear that there is a causal effect." The implication is that as LGB people feel more welcome and included in society, they are less likely to suffer mental health problems.