1) Women seek help more readily than men do. They use healthcare services more often and their depression is more likely to be diagnosed. One study showed that men are more likely to “forget” depression – that is, not report symptoms.

“A lot of people out there are mildly depressed and don’t seek treatment,” she says. “Labels like that don’t sit well with some men. There’s a stigma, a challenge to masculinity.

“Some men may say they’ve been feeling really irritable, angry. I had a man who wanted to throttle his wife. Loved his wife dearly. He was so angry.

“A depressed woman is more likely to come in, weep and cry, tell you how low she feels. A depressed male is more likely to be as grumpy as hell, hard to live with. A man is more likely to tell you that irritability is causing problems at home, talk about trouble at work, or problems at home with his wife.”

2) It’s in the genes. But, says Carter, studies show that major depression is equally heritable in men and women, and that both are equally at genetic risk of depression. She concedes that genetic factors might increase women’s vulnerability through other means, eg, temperament.

Professor Peter Joyce, from the psychological medicine department, has undertaken a separate study of the genetics of depression and personality. “We’re trying to find what genes may increase risk for depression. We think part of understanding depression is going to be linked to understanding personality. Certain personality styles are more vulnerable – the anxious person, for example.” The study has involved some 200 people plus their parents and siblings. Joyce expects to publish results soon.

A Dunedin study concentrated on one particular gene, the serotonin transporter (drugs like Prozac work on the serotonin transporter), which comes in two forms, short and long. That study, and others around the world since, showed that if you have the short form of the gene, then even if you experience adverse life events, your rates of depression don’t go up much. But if you’ve got the long form, and you become stressed, your rate of depression goes up markedly. “So it’s saying some are genetically vulnerable to the impact of life events and others are not,” says Joyce.

3) Hormonal fluctuations impact on women’s moods, says Carter, especially during the premenstrual and postpartum periods and menopause. Hormonal changes during puberty may predispose women to depression. Women are less prone to depression before hormones kick in at adolescence and after 55 following their reproductive years, which supports the hormonal theory. It is usually agreed that hormones play a part in gender differences, but how much of a part is uncertain.

Says Joyce: “Rates of depression increase dramatically in teenage years and it’s tied into biological puberty. It’s all the hormones turning on, and that’s when women start doubling men’s rates of depression. So one of the factors has to be something to do with female hormones.”