Mental illness is the main sickness of the working age population with economic costs around 8% of GDP. This column, based on the authors’ recent book, discusses the effectiveness of a large programme of psychological therapy, launched in England in 2008. The savings due to welfare benefits, extra taxes, and physical healthcare outweigh the costs of the programme. In this case, psychological therapy costs nothing.

In rich countries, 38% of all illness is mental illness.1 It particularly affects people of working age where it accounts for 50% of the total (see Fig 1). The overall economic cost has been estimated at 8% of GDP, not to mention the massive suffering involved. Policymakers increasingly wonder what they can do about it.

A central issue has always been cost, and one appeal of the programme to policymakers has been the cost savings which it generates. Averaged over all patients from mild to severe and all lengths of treatment (from those who drop out after 2 sessions to those who get up to 20 sessions), the cost per patient is £650. Against this, we have to set the savings on, first -- welfare benefits -- and then -- physical healthcare.

As we have mentioned, mental illness is the main illness of working age. In most rich countries about 1% of the working age population is on disability benefits due to depression or anxiety disorders. In Britain, one such person costs the government £650 a month more than if they were not on benefit. (This includes both the benefits and reduced tax payments.) So, suppose we treat a representative group of people with depression or anxiety disorders. If, as a result of the treatment, 4% of those treated worked an extra 25 months, the average patient would be working one month more than otherwise. This would be enough to repay the cost of the treatment.

Is 4% a realistic estimate? A number of randomised trials in the US and Britain come up with much larger numbers (Layard and Clark 2014).