Freeman, whose primary area of research is delusions and hallucinations, has written a handful of self-help books. His latest book, , was co-authored with his brother Jason and will be published in July. "I wrote it because I wanted to find out the answer to that question. It's incredibly important. It's a social issue we should be acting on," he says.

He did the research in his free time, and says it was the gap between men and women that took him by surprise: "In the book, the figure we use is 20 to 40 per cent higher, but it varies depending on which study you're looking at. The German studies suggest it's higher."

Giving his first media interview on the book, Freeman was acutely aware of the need for tact in releasing these findings, as they touch on the sensitive issues of mental health and gender differences. He was keen not to lose sight of the disturbing fact that rates of mental illness are very high among both men and women.

In any given year, one quarter of the population will experience a psychological problem, he says.

If the statistics sound high, it's because psychological problems are far more common than are generally acknowledged. The reason they are not openly discussed is the stigma attached to them.

"It's beginning to change, certainly in the UK, but slowly," he says. "Certainly, in the current economic climate, there are more pressures on men and women, [so] you would be reluctant to admit it in a workplace setting, where you've got concerns about holding onto your job," he says.

The most common psychological problems are anxiety disorders and depression, the two areas where there is the largest discrepancy between men and women. Women are likelier than men to suffer from anxiety disorders, an umbrella term for a range of issues including panic, phobias, compulsive behaviour, post-traumatic stress disorder, chronic disorders , and others.

These aren't new findings, but what is new, and what is detailed in is the size of the gender gap. "People are definitely aware, in the academic world, that within disorders there are sexual differences. But no one has done the maths. No one has totalled it up," Freeman says.

The causes of this gender divide? Environment and social pressures. "At a psychological level, there are pressures going on that affect a woman's self-esteem concept that lead to many women having ideas about vulnerability about the self. They can easily get triggered into feeling miserable about themselves," he says.

Women tend to internalise their problems while men externalise them. This explains why women are more prone to anxiety and depression, and why men are likelier to have issues related to drugs and alcohol.

But this tendency to internalise or externalise is not innate, says Freeman. "Talking about feelings, and dealing with fears and views about yourself involves influences from your environment, from your peers, from your parents, and from the messages you get from the media. But I think there are some differences on a biological level, too," he says.

Society is changing. Freeman points to the increase in heavy drinking by young women, particularly in Britain. "I think what we are seeing in the statistics is that women are catching up with the alcohol problems, but men aren't catching up in anxiety and depression."

The way forward, he says, is a greater focus on well-being. A key indicator is sleep. Sleeping problems, such as insomnia, signal an underlying issue. Working to create a good sleep regime will help bolster mental and physical health.

Diet is also an important factor, along with investment in personal relationships.

Freeman points to the success of Britain's national mental health campaign "Five a Day", which was launched in 2008.

Based on the idea of eating five portions of fruit and vegetables a day to stay healthy, the "Five a Day" campaign advocates doing each of these things daily: connecting with other people; being physically active; noticing what's going on around you; learning new skills and about things happening around you; and giving to someone, whether it be a smile, or a random act of kindness.

"We know there are some really great psychological treatments for anxiety and depression which can have real benefits. There are clearly things we can do. All the 'Five a Day' things help, as well as improving your sleep and exercise and relationships," says Freeman.

Relationships, when they are going well, benefit both genders, but women are likelier to suffer negative effects when they go wrong, he says. This is most apparent in marriages. "You typically get the woman caring more about relationship issues, childcare issues, and all of that.

"There are typically more stresses and strains within those realms, and they are realms that are undervalued. Women's work in the home is less rewarded. Women can also get isolated at home. It's incredibly hard work, and they're not being paid for it."

Freeman is quick to cite studies that show that if men and women are equally invested in the children and household tasks, then they typically suffer the same amount from problems in that area. "If you're in a good relationship, that's great for your mental health and well-being. But when there's imbalance there, that's in favour of men, and men get more benefits from it," he says.

The really surprising find, says Freeman, isn't that women are likelier to have a psychological problem, but that levels of mental illness among both men and women are so high.

When men and women realise that they are not alone with a psychological problem and that mental health issues are very common, then they will begin to feel more at ease about talking about their concerns with friends, family and health professionals, says Freeman.

Many mental health problems, if addressed early, can be resolved without having to resort to medication. Talking is the first step, he says.