Co-written with Peter Singer. This is cross-posted from Project Syndicate, where it original appeared.

If we can prevent great suffering at no cost to ourselves, we ought to do so. That principle is widely accepted and difficult to dispute. Yet Western governments are neglecting an opportunity to reduce the great misery caused by mental illness, even though the net cost would be nil.

The evidence for this claim comes from recent research by a team of economists at the London School of Economics. The team, directed by Richard Layard, drew on data from four major developed countries (Australia, Britain, Germany, and the United States) in which people were asked to indicate, on a 0-10 scale, how satisfied they were with their life.

The researchers refer to those in the bottom 10% of the population in terms of life satisfaction as being in “misery.” Respondents also answered other questions designed to indicate factors affecting life satisfaction.

When Layard’s team analyzed the results, they found that the biggest factors affecting misery were all non-economic: mental health, physical health, and whether someone had a partner. Mental health was the biggest predictor of all; it explained twice as much of the difference between people in terms of life satisfaction as physical health or income inequality did. (This was also true for those in the non-miserable 90% of the population).

Overall, the researchers claim, eliminating depression and anxiety would reduce misery by 20%, whereas eliminating poverty would reduce it by just 5%. If we want to reduce misery in the developed world, then mental health is the biggest challenge we need to overcome.

Many people will find this result surprising. After all, most of us expect that we would become happier if we were richer. So why is mental health, not poverty, the biggest cause of misery?

The answer is that people adapt to higher levels of income over time – a phenomenon known as “hedonic adaptation” – and they compare their income to that of their peers. This gives rise to the so-called Easterlin Paradox, the finding that although richer people are more satisfied with their lives than poorer people, economic growth has often not increased overall life satisfaction in the developed world. If your neighbor becomes richer, you feel poorer. If both of you become richer, neither of you is likely to be significantly happier. In contrast, people do not adapt to poor mental health; nor does your neighbor’s misery make you feel better.

Given that mental health has the greatest impact on life satisfaction, we still need to ask if addressing it is the most cost-effective way for governments to reduce misery. Layard and his colleagues asked how much the British government would have to spend to tackle mental health, physical health, unemployment, or poverty. They concluded that mental health would be the cheapest of the four options: around 18 times more cost-effective in reducing misery and promoting happiness than targeting poverty.

In the United Kingdom, it costs about £650 per patient to provide psychotherapy, which is effective for about 50% of patients. That figure indicates how much governments would need to spend, but does not take into account what they might get back.

Reducing mental illness enables many people to return to work, thereby reducing the cost of unemployment benefits while increasing tax receipts. Hence Layard and his colleagues hypothesized that treating mental health would pay for itself. In effect, the UK government would be able to reduce misery at no cost.

Further economic research, this time by Paul Frijters and his colleagues, also of the LSE, has now assessed the impact of the UK’s Improving Access to Psychological Therapies program, a scheme conceived by Layard and the psychologist David Clark, which was launched in 2008. They conclude that the increase in tax receipts and the reduction in unemployment benefits pay back only about 20% of the cost of treating mental illness. However, they argue that mental health treatment still funds itself because those who receive psychotherapy demand far fewer physical health services.

In reality, the UK didn’t shrink its health budget. The effect of treating mental health therefore was to free up other resources that were used for other patients. But this effect was so substantial that Frijters claims we could expand treatment to all 12% of the UK population who have mild to moderate anxiety or depression and expect the investment to pay for itself in savings in no more than two or three years.

Attitudes toward mental health have changed dramatically in the last few years; even princes and athletes now feel able to open up about it. In the UK, a study showed that mental illness affects one in four people in any year, while research carried out in 30 European countries found that 38% of the population suffered from some kind of mental or neurological illness. What has not been grasped is that this suffering is largely avoidable.

Governments are starting to regard mental health as seriously as they do physical health. But they could do much more. Increasing their spending on mental health could reduce an immense amount of misery – and at no cost in the long run.

Of course, some mental illnesses are more difficult to treat than moderate depression or anxiety, and at some point higher spending may not pay for itself. But until that point has been reached we should all agree on the moral urgency of a radical expansion in funding for mental health.