"This is because people who are mentally ill become seriously unproductive. So when they are successfully treated, there are substantial gains in output. And these gains exceed the cost of therapy and medication," GHC researchers said.

The report, published Saturday, said that while mental illness was one of the main causes of unhappiness in the world, the net cost of treating it was actually negative.

Reducing mental illness is one of the key ways to increase happiness worldwide, according to a study by the Global Happiness Council (GHC).

The most common conditions associated with mental illness are depression and anxiety disorders, the study said. And at least a quarter of the global population were thought to experience these conditions over the course of their lifetime.

Researchers at the GHC also said that mental illness was a "major block" on the global economy as it was found to be the main illness among people of a working age. Therefore, treating the conditions, it said, would save national income per head by 5 percent — that equates to billions worldwide.

The study estimated that for every $1 spent on treating depression, production would be restored by the equivalent of $2.50. And while physical healthcare costs were thought to balance out, the GHC claimed net savings when treating anxiety disorders was greatest of all — with production restored by the equivalent of $3 for every $1 spent.

In the U.K., the National Health Service (NHS) estimates that around 10 to 15 percent of people are considered to have had a mental illness at some stage of their lives.

There are many types of mental illness but most conditions fit into either a neurotic or psychotic category, according to the NHS.