"The lack of mental health manpower in rural areas" is especially acute, explained Michael Phillips, one of the lead authors and a professor at Emory University in Atlanta and at Shanghai Jiao Tong University.
More than half of those with full-blown psychotic disorders such as schizophrenia are not diagnosed, much less cared for, he said in a statement.
In India, the percentage of the population with untended mental health problems is equally small. By contrast, treatment rates in rich nations is 70 per cent and up.
The divide between developed countries and these emerging giants is equally sharp when it comes to money spent. Less than one per cent of national healthcare budgets in China and India is allocated to mental health care.
In the United States that figure is nearly six per cent, while in Germany and France it rises to ten per cent or more. Both India and China have recently implemented progressive policies providing for the needs of their mentally ill, but reality on the ground has not yet caught up, the studies showed.
"The treatment gaps, especially in rural areas, are very large," Vikram Patel, a professor at the London School of Hygiene and Tropical Medicine, said of India.
It could take decades for each country's medical systems to fill these gaps, the reports concluded. But the researchers suggest that a large cadre of traditional practitioners - yogis in India, Chinese medicine doctors in China - could be trained to recognise mental health problems and help with treatment.