That seems to be true all over the globe. In a 2010 review of 115 studies that spanned 33 countries across the developed and developing worlds, nearly 80 percent of the studies showed that poverty comes with higher rates of mental illness. Among people living in poverty, those studies also found, mental illnesses were more severe, lasted longer and had worse outcomes.
And there's growing evidence that levels of depression are higher in poorer countries than in wealthier ones. Those kinds of findings challenge a long-held myth of the "poor but happy African sitting under a palm tree," says Johannes Haushofer, an economist and neurobiologist who studies interactions between poverty and mental health at Princeton University.
As data builds to connect tough economic circumstances with mental struggles, scientists are still trying to answer a trickier question: Which causes which?
There is no easy answer, says psychologist Crick Lund of the University of Capetown, who studies mental health policy. Mental illness is never caused by just one thing. Poverty can be one factor that interacts with genetics, adverse life events or substance abuse.
But so far, the strongest evidence suggests that poverty can lead to mental illness, especially in cases of disorders like depression.
Because scientists can't experimentally plunge people into poverty to see what happens to their mental health, natural experiments offer one kind of clue. When disasters or tough spells (like losing a job or enduring periods of drought for farmers) destroy financial circumstances, numerous studies show a rise in rates of depression, Haushofer says.
On the flip side, people often get happier after economic windfalls. In a new study, Haushofer and a colleague found that when families in Kenya were given cash grants averaging $700 (nearly twice the amount typically spent per person per year), they reported higher levels of life satisfaction and lower levels of depression than they did before they got the money, which they could spend on anything. The larger the cash transfer, the bigger the mental boost. It didn't matter if the money came in monthly installments or all at once.
Despite the long-held belief that winning the lottery destroys lives as people make bad decisions about how to use the money, Haushofer adds, newer evidence suggests the opposite. In study published this year, researchers in Sweden, reported that lottery winners used fewer anti-anxiety medications and sleeping pills after collecting their payout, suggesting that they became happier.
So how does poverty "get under the skin" or into the brain, Lund asks? Stress is a leading contender. Some studies have found higher levels of the stress hormone cortisol in people living in poverty. In Mexican households that received cash grants, found a 2009 study, young children had lower cortisol levels compared to kids from families that didn't get extra money. Other studies, however, have failed to find any changes in cortisol.
Rates of violence are also higher among people who face economic tension. Living amid violence can exacerbate depression, Lund adds. And studies have found connections between mental illness and poverty-associated conditions, such as not having enough to eat, not making enough money to live on and having a greater chance of developing risks for physical illnesses.
Mental illness may, in some cases, lead people down a road to poverty, Lund says, because of disability, stigma or the need to spend extra money on health care. may play a role, with some evidence suggesting that poverty more often leads to depression while disorders like schizophrenia more often lead to poverty.
Still unclear is how best to break the cycle. Although cash-transfer programs have shown promising improvements to mental health, studies have yet to determine whether those improvements persist in the long-term.
"I think the jury is still out on the extent to which poverty alleviation interventions actually lead to mental health improvements," Lund says. "It hasn't been evaluated rigorously enough."
Data is also lacking on whether mental-health interventions can make a true dent in poverty rates or why some people remain resilient even in extremely challenging circumstances.
"We don't know whether intervening in depression is also a good poverty intervention," Haushofer says. Because depression keeps people out of work, treating it should help, but evidence is still lacking.
Better data may be coming. Lund is in charge of an effort called PRIME, a multinational consortium that aims to implement treatment programs for mental disorders in low-resource settings. One project involves tracking efforts to improve access to mental health services in five countries, including Ethiopia, South Africa and Uganda, with preliminary results expected within the next year.
In 2013, the World Health Organization committed to a mental health action plan, with a goal of increasing access to services for severe mental health disorders by 20 percent and reducing the suicide rate by 10 percent in 135 member countries by 2020.