Every dollar spent on better treatment of anxiety and depression produces a fourfold return in better health and ability to work -- a big boost for countries' development and economic growth, the World Health Organisation (WHO) says.
These disorders cost the global economy US$1 trillion (NZ$1.44 trillion) a year, according to a study led by the WHO which estimates for the first time both the health and the economic benefits of spending more on treating the most common forms of mental illness.
Common mental disorders such as anxiety and depression are increasing worldwide, and the number of people suffering from them rose to 615 million in 2013 from 416 million in 1990, the UN agency said in the study.
Last year, world leaders included mental health in an ambitious plan to end poverty and inequality by 2030.
"We know that treatment of depression and anxiety makes good sense for health and wellbeing; this new study confirms that it makes sound economic sense too," said Margaret Chan, director-general of the WHO.
"We must now find ways to make sure that access to mental health services becomes a reality for all men, women and children, wherever they live," she added.
The study, published in the same week as the World Bank and the WHO hold a high-level meeting on mental health, is based on data from 36 low, middle and high-income countries.
Governments spend on average 3 percent of their health budget on mental health, ranging from less than 1 percent in low-income countries to 5 percent in high-income countries, according to the WHO's survey Mental Health Atlas 2014.
"Despite hundreds of millions of people around the world living with mental disorders, mental health has remained in the shadows," said Jim Yong Kim, president of the World Bank Group.
"This is not just a public health issue -- it's a development issue. We need to act now because the lost productivity is something the global economy simply cannot afford," he added.
"Mental health needs to be a global humanitarian and development priority -- and a priority in every country," said Arthur Kleinman, professor of medical anthropology and psychiatry at Harvard University.