“Women become insane," opined the Victorian psychiatrist G. Fielding Blandford, “during pregnancy, after parturition, during lactation; at the age when the catamenia (periods) first appear and when they disappear…" 

Back in the bad old days, it was accepted that women were inherently susceptible to mental illness, due to the imagined intimate connection between brain and reproductive system. For women, read madness. In these sun-lit days of supposed gender-equality, the idea that one sex is more prone to mental illness than the other has become taboo. Wanting to put as much intellectual space between themselves and their nineteenth-century colleagues, mental health professionals don't talk about the issue. And perhaps there is nothing to discuss. 

The World Health Organisation assures us: “Overall rates of psychiatric disorder are almost identical for men and women." But if we don't talk about it, how do we know whether this consensus view is truly valid? We analysed the best evidence currently available: twelve large-scale, national epidemiological surveys from the UK, US, Europe, Australia and New Zealand, South Africa and Chile. These surveys don't simply count the number of hospital admissions or attendances at clinics. They pick the participants at random from the general population and then weight the resulting statistics to ensure that the data is nationally representative. And a remarkably consistent picture emerged: in any given year, women appear to experience higher overall rates of psychological disorder than men. In fact, the difference is typically as much as 20-40%. 

This isn't to say that mental illness is a female problem. Far from it. The rates are alarmingly high for both men and women. Huge numbers of people are struggling with emotional and psychological problems, and from one perspective whether these individuals happen to be male or female is almost immaterial. Moreover, men are more likely than women to develop certain conditions – specifically problems with alcohol and drugs and anti-social personality disorder. 

Nevertheless, when you take into account women's significantly higher rates of anxiety and depression – the most common psychological problems – the figures don't equal out. If the epidemiological data is reliable (and it's the best we have at present), women outnumber men for psychological disorders as a whole. Indeed, the most comprehensive of the national surveys (a study carried out in Germany in the late 1990s), suggests rates are almost 50% higher in women than in men. And these figures don't include sleep or sexual problems – very common conditions that are generally agreed to be more prevalent among women. 

So what do we know about the factors causing this imbalance? Why are women more susceptible to mental illness than men? Some would argue that the statistics simply reflect the fact that men aren't willing to come clean about their psychological problems. And there's probably some truth to this. We know that men are less likely to visit a doctor for a physical ailment, for example. But virtually no research has been done on the issue. And there are plenty of other factors that may help explain the imbalance, meaning that under-reporting probably explains only a small proportion of the difference in the overall rates of psychological problem between men and women.

Women aren't more vulnerable to mental illness because of biology but because of the distinctive pressures with which they often have to cope

The bulk of the evidence points toward the contribution of life events and social roles. Women aren't more vulnerable to mental illness because of biology – take that G. Fielding Blandford and co. – but because of the distinctive pressures with which they often have to cope. Indeed the conditions to which women seem especially prone (such as anxiety and depression) are those for which we know genetic factors are much less significant than environmental influences. Conditions with relatively high heritability – such as schizophrenia and bipolar disorder – tend to occur equally in men and women.

It's certainly plausible that women experience higher levels of stress because of the demands of their social role – with that stress helping to trigger problems like anxiety, depression, eating disorders, and insomnia. Increasingly, women are expected to function as carer, homemaker, and breadwinner – all while being perfectly shaped and impeccably dressed. Given that domestic work is undervalued, and considering that women tend to be paid less, find it harder to advance in a career, have to juggle multiple roles, and are bombarded with images of apparent female “perfection", it would be surprising if there weren't some emotional cost.

These are the kind of pressures that can leave women feeling as if they've somehow failed; as if they don't have what it takes to be successful; as if they've been left behind. Women may also be vulnerable because of the importance they learn to attach to social relationships. Such relationships can be a source of strength, of course. But to some extent we're relying on other people for our happiness: a risky business.

It's worth remembering too that women are also much more likely than men to have experienced childhood sexual abuse, a trauma that all too often results in lasting psychological and emotional damage.

What's striking though is how under-researched an issue this is. Mental health, despite its heightened profile in recent years, is still not sexy. The big research bucks tend to go to scientists working on physical ailments – though it's possible psychological problems might have a higher profile if it were men and not women who were principally affected.

Given the extent of the burden on society and individuals alike, understanding what causes mental illness, and therefore being better placed to prevent and treat it, should need no justification. But we can't do this if we write gender out of the picture. We need to face up to the psychological damage being wrought by entrenched sexual inequality – and get serious about changing it for the better.

Click here to buy The Stressed Sex: Uncovering the Truth About Men, Women, and Mental Health